SBI4U
Grade 12, University Preparation
Biology
Unit 4 – Homeostasis
SBI4U – Biology
Unit 4 - Introduction
Introduction
In the last unit, you examined the role of DNA in the living organism and studied the
effects of mutations. A change in the DNA code can alter protein function which can
reduce or completely void the cell’s ability to function properly. This, in turn, can affect
whole organs and even the whole organism. This unit will focus on the systems that
allow the body to maintain itself in proper working order. To work appropriately,
conditions within the body must be maintained at certain levels. As the outside
environment changes constantly, the body must be able to counteract these changes to
maintain the internal environment. This is homeostasis. Some of these systems allow
for almost instant change while others ebb and flow as required. When these systems
work in concert, the organism operates at peak efficiency, but, if something goes wrong,
it can have far reaching consequences.
Overall Expectations
By the end of this unit, you will be able to:
•
•
•
Evaluate the impact on the human body of selected chemical substances and of
environmental factors related to human activity
Investigate the feedback mechanisms that maintain homeostasis in living organisms
Demonstrate an understanding of the anatomy and physiology of human body
systems, and explain the mechanisms that enable the body to maintain homeostasis
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Grade 12, University Preparation
Biology
Lesson 13 – Maintaining Balance
SBI4U – Biology
Lesson 13
Lesson 13 – Maintaining Balance
Introduction
In order to maintain optimal conditions in the internal environment, the body must be
able to monitor that environment and detect any changes that occur. If a change does
take place, it then must be able to effect change to counteract the imbalance. No
matter the system or systems involved, there is a basic mechanism in place that allows
this to occur. In this lesson, these mechanisms will be examined, along with one of the
homeostatic systems, the excretory system.
What You Will Learn
By the end of this lesson, you will be able to:
•
•
•
•
Use appropriate terminology related to homeostasis including nephron, positive
feedback, negative feedback, thermoregulation, and dialysis
Plan and construct a model to illustrate the essential components of the homeostatic
process by creating a flow chart that illustrate representative feedback mechanisms
in living things
Describe the anatomy and physiology of the excretory system and explain how the
system interacts to maintain homeostasis
Describe the homeostatic processes involved in maintaining water, thermal and
acid-base equilibrium and explain how these processes help body systems respond
to both a change in the environment and the effects of medical treatments
Thought Questions
Use the following questions to begin thinking about homeostasis.
A. How is the body able to maintain internal body conditions when the environment is
always changing?
B. What dangers exist if the body is unable to make the proper changes?
C. What is a feedback loop?
D. How does the excretory system help maintain internal balance?
Homeostasis and Feedback Loops
Homeostasis is the process which allows the body to adjust internal conditions to
variations in the external environment. Some of the optimal conditions include
maintaining body temperature at 37°C, a blood pH of 7.35 and a blood sugar level of
0.1%. These conditions are maintained via a control system that monitors and
responds to any change. Each control system has three components: a monitor, a
coordinating center and a regulator. The monitor is a series of special sensors located
in the appropriate body organ that can detect when the body is moving outside its
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Lesson 13
normal parameters. When this happens, a signal is sent to the coordinating center
which is usually located in the brain. The coordinating center will process the
information received about the change and send the appropriate response signal to the
regulator. The regulator is that portion of the system that is able to induce a change
back to normal conditions.
Figure 13-1 The General Control System
Source: Di Giuseppe et al. Supplement p. 190
One example of this control system in action would be the control of carbon dioxide
levels in the body. The monitor is a series of chemical sensors in the brain that monitor
the carbon dioxide levels in the blood. During exercise when cellular respiration
increases to provide energy, there is a subsequent increase in carbon dioxide release.
The monitors would detect this and send a signal to another part of the brain relaying
this information. The brain then sends a signal to the regulator. In this case the signal
tells the chest muscles and diaphragm to increase the rate and depth of breathing. This
will remove the excess carbon dioxide from the system.
When a mechanism or control system is responsible for keeping body conditions within
specific parameters, it is referred to as a negative feedback system. Any change in
the variable being monitored triggers the control mechanism to counteract any further
changes in that direction. It prevents small changes from becoming large changes.
This system works in two directions. It prevents levels from getting too high, but, it also
prevents levels from getting too low. A perfect example of this type of feedback system
is the case of thermoregulation or maintenance of body temperature. If temperatures
get too high, the complex organic molecules, more specifically the proteins, can
denature and stop functioning which will lead to the death of the organism. Conversely,
if temperatures get too low, the proteins will again shut down and the body will
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Lesson 13
eventually shut down as well. Therefore, the system controlling temperature must have
the ability to prevent it from increasing and decreasing beyond certain limits.
Figure 13-2 Thermoregulation (Negative Feedback)
Source: Di Giuseppe et al. 339
Though not present very often, there are a small number of positive feedback systems.
In this case, positive feedback serves to reinforce a change that is temporarily
occurring. One example would be childbirth. In this positive feedback system, the
mechanism is moving the system away from balance and stability. As shown in the
diagram below, as the baby’s head pushes against the woman’s cervix, signals are sent
to release more oxytocin, a chemical substance that stimulates contractions of the
uterus which further pushes the baby’s head against the cervix which causes another
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Lesson 13
signal to be sent which releases more oxytocin and so on and so on until the baby is
delivered. The mechanism shuts down when the initiating signal (the baby’s head) is no
longer present.
Figure 13-3 Positive Feedback Loop
Source: Blake et al. 111
Support Questions
(do not send for evaluation)
1.
Heat exhaustion caused by a person’s exposure to heat can result in weakness
or collapse. It usually involves a decrease in blood pressure. Explain why the
homeostatic adjustment to heat can cause a drop in blood pressure.
2.
During lactation (milk production) the suckling by the baby stimulates the
production of oxytocin, which in turn causes contraction of smooth muscle
surrounding the milk duct, causing milk to flow. The flow of milk increases the
suckling by the baby and more oxytocin is produced.
a. Identify the type of feedback system described above.
b. What would end the feedback loop?
3.
Drugs such as ecstasy interfere with the feedback mechanism that helps
maintain a constant body temperature. Explain why these drugs are dangerous.
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Lesson 13
The Excretory System
The job of the excretory system is to remove nitrogenous waste from the body. As
organisms ingest protein, it is broken down into its component amino acids and used to
synthesize the proteins the body requires. Any excess protein undergoes metabolic
breakdown in the liver and is converted into carbohydrates. This is accomplished
through the removal of an amino group (a nitrogen and two hydrogens). This process of
deamination creates nitrogen rich wastes in the form of ammonia. Ammonia is
extremely toxic and can quickly damage cells and tissues so organisms must have a
system in place to deal with it before damage can occur. The manner in which the body
deals with this waste is dependent upon the environment in which the organism lives.
Fish, living in a watery environment, are able to release the ammonia directly into their
surroundings. Because this is accomplished as soon as the ammonia is produced, it
does not have time to cause any damage. Land based animals have to deal with these
wastes in other ways because water, which is normally released with ammonia, needs
to be conserved. Amphibians and mammals have access to or are able to store a fair
amount of water so some amounts of water can be lost while eliminating waste. They
are able to convert the ammonia to a less toxic form called urea. This lesser toxicity
allows the urea to be stored in the body and concentrated so not as much water is lost
upon its release. Urea is the yellow part of urine that we see. Birds and insects require
a system that allows for as little water loss as possible. They are able to convert
ammonia into the least toxic substance uric acid. In this form, uric acid can be
concentrated to a semi-solid form requiring very little water. This is the whitish
substance visible in bird droppings.
Figure 13-4 Forms of Nitrogenous Waste
Source: Di Giuseppe et al. Supplement p. 192
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The Human Kidney
The kidney is a fist sized organ, found in the lower back on each side of the spine. Its
job is to filter the nitrogenous waste products that are present in the blood and remove
them for elimination from the body. Each kidney sends the filtered waste through a tube
called the ureter which connects to the bladder. The bladder will hold the waste as
urine until full. The opening of the bladder is controlled by a sphincter which, when
relaxed, allows the urine to drain out through the urethra and out of the body. Because
the kidney is so important to proper functioning, there is a great deal of redundancy in
the system. Only one kidney is required to keep the body working properly but two are
present to perform the job. The kidney not only removes waste but is also responsible
for controlling water balance, pH and the levels of sodium, potassium, bicarbonate and
calcium in the blood. It also secretes a chemical substance called erythropoietin that
stimulates red blood cell production and activates vitamin D production in the skin.
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Figure 13-5 The Human Excretory System
Lesson 13
Source: Di Giuseppe et al. 346
Each kidney is composed of three areas- the outer cortex, the medulla and the hollow
inner pelvis where the urine collects before entering the ureter. Contained within the
cortex and medulla is the functional unit of the kidney, the nephron. It is the nephron
that is responsible for filtering the waste products from the blood. Each kidney contains
approximately one million nephrons.
Figure 13-6 The Human Kidney
Source: Di Giuseppe et al. 346
The nephron can be divided into five main parts: Bowman’s capsule, the proximal
tubule, the loop of Henle, the distal tubule and the collecting duct. The upper portions of
the nephron rest in the cortex while the lower portions are contained within the medulla.
Small branches of the renal artery called the afferent arterioles supply the nephron with
blood. The arterioles branch into a capillary bed of tiny blood vessels called the
glomerulus which is surrounded by the cap-like Bowman’s capsule. The blood then
leaves the glomerulus through the efferent arterioles which branch into another capillary
bed called the peritubular capillaries that surround the tubule portion of the nephron.
The waste present in the blood moves from the afferent arteriole into the glomerulus. It
then enters the nephron through Bowman’s capsule. It then moves through the tubules
from the proximal tubule, through the loop of Henle, through the distal tubule and finally
into the collecting ducts. From there, the urine moves into larger and larger ducts until it
collects in the ureter and drains into the bladder. All other useful substances that are
filtered such as glucose, sodium and many other ions are returned to the surrounding
capillary beds and continue through the body.
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Lesson 13
Figure 13-7 The Nephron
Source: Di Giuseppe et al. 347
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Lesson 13
Formation of Urine
Urine formation involves three processes. Filtration moves the fluids from the blood into
Bowman’s capsule. Reabsorption transfers the essential solutes such as ions, as well
as water, back into the blood. Secretion uses active transport to move materials from
the blood back into the nephron.
Filtration
As blood in the afferent arteriole enters the glomerulus, it enters a region of high
pressure which is approximately 2½ times normal capillary pressure. It is this increased
pressure that pushes the dissolved solutes out of the blood and into Bowman’s capsule.
The solutes include: salts, glucose, amino acids, hydrogen ions, vitamins, minerals and
urea. Plasma protein, blood cells and platelets are too large to move through the semipermeable membrane. As the solutes move, water moves as well by the process of
osmosis, moving from an area of high concentration to low. Because of the high
pressure in this region, it is important for individuals to maintain proper blood pressure.
If body blood pressure is high, pressure in the glomerulus will be 2½ times beyond it.
This can cause damage to the glomerulus and permanently damage the filtration
system. High blood pressure can be detected, especially in pregnant women, when
proteins are detected in the urine. Under normal circumstances, proteins are too large
to pass through the membrane but, if pressure is above normal, they can be forced
through and detected.
Reabsorption
This process moves all the necessary solutes back into the blood after filtration at the
glomerulus. Sodium ions are moved by active transport causing negative ions to follow
but, there is a maximum amount or threshold level of material that can be moved. Any
excess is excreted as sodium chloride (salt) in the urine. Glucose and amino acids are
moved out of the proximal tubule. Again, there are limits as to how much glucose can
be moved back across the membrane so any excess leaves the body in the urine. This
movement of solutes out of the filtrate creates an osmotic gradient which prompts the
movement of water back across the membrane. As this happens, the solutes become
more concentrated. Approximately 120 mL of filtrate move into the nephron per minute
and of that amount 119 mL are reabsorbed so, only 1 mL of excreted wastes is
produced. Some of the urea also moves back but more stays in the nephron than
actually returns to the blood.
Secretion
This is the movement of materials from the blood to the nephron. Urea, excess
hydrogen ions and other substances like antibiotics are actively moved into the distal
tubule of the nephron.
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It is because of the way the nephron works that so many substances present in the
body can be detected in the urine. Excess salt and glucose can be detected and may
suggest underlying health issues. Artificial substances such as antibiotics, steroids and
other drugs and medications can be detected in urine as well as they are secreted from
the blood into the distal tubule.
Figure 13-8 Urine Formation
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Source: Di Giuseppe et al. 350
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Table 13-1 Nephron Parts and Functions
Lesson 13
Source: Di Giuseppe et al. 351
Please visit the following website(s)
http://biology-animations.blogspot.com/2007/12/nephron-animation.html
http://www.sumanasinc.com/webcontent/animations/content/kidney.html
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Lesson 13
Support Questions
(do not send for evaluation)
4.
Describe the three main processes that are involved in urine formation.
5.
The following is a random list of processes that occur in the formation and
excretion of urine once the blood has entered the kidney. Place these processes
in the correct order.
a.
b.
c.
d.
e.
urine is stored in the bladder
blood enters the afferent arteriole
fluids pass from the glomerulus into Bowman’s capsule
urine is excreted by the urethra
sodium ions, glucose and amino acids are actively transported from the
nephron
f. urine passes from the kidneys into the ureters
Water Balance-Osmoregulation
As part of a homeostatic system, the kidney helps to regulate the amount of water
present in the body. Aside from oxygen, water is the most important substance for
proper body functioning. An individual can go for weeks without food but will only last 23 days without water. The kidney can be used to help retain water in the body when
levels are low and it can release excess water when levels are high. A chemical
substance called antidiuretic hormone (ADH) works with the kidney to help regulate
these levels. (Hormones will be discussed in the next lesson). Osmoreceptors located
in the hypothalamus region of the brain detect changes in osmotic pressure. When fluid
levels are low, the blood solutes become more concentrated, increasing the osmotic
pressure. Water moves from the cells into the bloodstream to try to decrease the solute
concentration. This causes the cells in the hypothalamus to shrink. The shrinking
causes a message to be sent to the pituitary gland to release ADH. Once released,
ADH causes the semi-permeable membranes of the nephron tubules to become more
permeable to water prompting it to move into the capillary beds and back into the
bloodstream. A second message is sent to create the feeling of thirst, prompting the
ingestion of water to increase fluid levels to normal. Once this occurs ADH is no longer
released and the permeability of the membrane decreases so more water is released as
urine. We are all physically aware of this water balancing. We know that if we drink a
lot of liquids, we will need to urinate frequently. Conversely, when we haven’t been
drinking a lot, the urine we do release is much a much darker yellow colour indicating a
higher concentration of urea.
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Lesson 13
Figure 13-9 Osmoregulation
Source: Di Giuseppe et al. 353
pH Balance
A pH of 7.3-7.5 is required for proper body functioning. This balance is maintained by
the presence of buffering systems such as the bicarbonate-carbon dioxide buffer
system. Buffers are able to resist changes in pH by taking up or releasing hydrogen
ions or hydroxide ions. The main buffer in the blood is carbonic acid (H2CO3), a weak
acid that releases H+ and the bicarbonate ion (HCO3-). If the blood is too acidic, excess
H+ combined with the HCO3- to form H2CO3.
H+
+
HCO3-
↔
H2CO3
If the blood is too basic, the carbonic acid dissociates to form H+ and HCO3-.
H2CO3
↔
H+
+
HCO3-
The H+ can be excreted by the nephron and the bicarbonate ion will diffuse back into
the bloodstream to be used again in the future.
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Blood Pressure Balance
The kidneys also play a role in maintaining proper blood pressure. Conditions that lead
to an increase in fluid loss can lead to a decrease in blood pressure which reduces the
delivery of oxygen and nutrients to tissues. Blood pressure receptors near the
glomerulus in the kidney detect low blood pressure. Specialized cells release a
substance called renin, an enzyme which helps to activate angiotensin. This substance
causes the blood vessels to constrict, reducing their diameter and increasing pressure.
Aldosterone is also released and acts on the cells of the distal tubules and collecting
ducts of the nephron causing an increase in sodium transport back into the blood. As
sodium reabsorption increases, more water moves out of the nephron and back into the
bloodstream increasing blood volume and therefore, blood pressure.
Kidney Disease
As mentioned earlier, because of the way kidneys function to filter the blood, many
dysfunctions and diseases can be detected by examining the products in urine.
Diabetes Mellitus
Insulin is a product of the pancreas that acts to convert excess glucose into glycogen
that can be stored temporarily in the liver. When more glucose is required for cellular
respiration, the glycogen can be converted back and used. Therefore, a certain level of
glucose is required in the blood at all times to be carried to the body cells that require it
as an energy source. When the control of insulin is not working properly, the levels of
glucose in the blood can spike after a carbohydrate rich meal or plunge when glucose is
not immediately available. The distal tubule of the nephrons is designed to be able to
maintain 0.1% blood glucose levels. If there is a higher concentration due to lack of
insulin to convert glucose to glycogen, the excess is released in the urine. This excess
can be detected through urine analysis. Another clue to this disease is the release of
copious amounts of urine. As glucose is a solute, water is released along with it
following the osmotic pressure gradient. The individual is often left thirsty because of
the large amount of water loss.
The use of injected insulin was first discovered by two Canadians Drs. Banting and Best
who worked at the University of Toronto. The were able to determine that the lack of
functioning insulin was the cause of the disease and showed that insulin could be
injected into the body to help control blood sugar levels.
Diabetes Insipidus
This form is caused by the destruction of the ADH-producing cells or the pathways that
signal its release. If ADH is not present to regulate water reabsorption, much of the
filtered water is not recovered and lost as urine. An affected individual may produce 1020 L of urine per day creating a strong thirst response. Large quantities of water have
to be ingested to maintain proper fluid levels.
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Bright’s Disease
This disease is also known as nephritis which is not one single disease but many
causes that are characterized by inflammation of the nephrons. If the nephron is
damaged, it alters the permeability of the membrane allowing larger solutes to pass
through. Since there is no mechanism in place to reabsorb these larger solutes, they
cannot be recovered and are lost in the urine. Again, because of the higher solute
concentration in the waste, more water is lost as well.
Kidney Stones
Kidney stones are caused by the hardening of mineral solutes from the blood. They can
be alkaline or acidic in nature. The stones can become lodged in the ureter, blocking
the path of the urine to the bladder and it can also get caught in the urethra. The
delicate tissues of the tube can be damaged as pressure pushes the stone down the
passageway. In many cases, the stone is left to pass through on its own, which can
cause great pain. Some have likened the pain to the pain of childbirth. If the stone is
too large, it may have to be removed surgically. Newer methods are using high energy
shock waves to break up the stone so the pieces can be passed. Use of this method
depends on the location of the stone and its composition but it does provide an
alternative to surgery.
Disease Technology
Kidney Dialysis
When the kidney is damaged for whatever reason and cannot filter the wastes out of the
blood, dialysis can accomplish the task. Dialysis uses the principles of diffusion and
blood pressure to exchange substances across a semi-permeable membrane. It can
remove the waste products but it cannot perform active transport to move the wanted
solutes back into the bloodstream.
In hemodialysis, a machine is connected to the circulatory system through a vein.
Blood is pumped through a series of tubes that are submerged in a bath of various
solutes. Because urea is not present it will diffuse into the solution. The chemical
substances that a functional kidney would normally produce can be added into the
solution and they will diffuse back into the blood. This form of dialysis was invented and
first used by Dr. Gordon Murray in 1946 at Toronto General Hospital. A female patient
with non functioning kidneys was connected to the 46 m of tubing required at that time.
After six hours, the level of toxins in her blood decreased and she regained
consciousness. Today, thanks to Dr. Murray, patients can live relatively normal lives,
having their blood filtered three times a week, six hours per session. This may seem
labour intensive but in the past, the alternative was death.
Recently, a second form of dialysis has been developed called continuous ambulatory
peritoneal dialysis (CAPD). Using this method, 2 L of dialysis fluid is pumped into the
abdominal cavity and the membranes in the body selectively filter wastes from the
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Lesson 13
blood. Urea and other wastes enter the fluid by diffusion. The waste filled fluid can be
drained off and replaced several times a day. This allows patients to continue with
moderate activity over the day and provides them with independence as they can learn
to perform the procedure themselves in their own home rather than having to go to the
hospital for treatment.
Kidney Transplant
When the kidneys are damaged beyond repair, dialysis will work in the short term, but,
the only real solution at this point, is a transplant. Kidney transplants are 85%
successful. If the transplant works, proper kidney function is restored including its blood
filtering abilities and the formation of the chemical substances that the kidney produces
that help with normal body functioning. The main disadvantage is the possibility of
rejection of the kidney as foreign tissue by the immune system. Immune suppression
drugs have to be taken to prevent this from happening. The kidney is placed in the
lower abdomen near the groin and connected to the blood vessels. After a few days,
the kidney should be fully functional and the patient should no longer need dialysis.
Support Question
(do not send for evaluation)
6.
Complete the table of kidney diseases and their treatments.
Kidney Disease
Diabetes mellitus
Cause
Lack of insulin
production
Effects
Glucose in urine will
cause dehydration
Diabetes insipidius
Treatment
ADH provided by
injection
Bright’s disease
Kidney stones
Key Question #13
1.
Draw and label a negative feedback system for a home heating and cooling
system. Include the following in your diagram: furnace, air conditioning unit,
thermostat, coordinating centre, and regulators. (6 marks)
2.
Fluids were drawn from different areas of the nephron. The solutes in the fluid
were measured and the results are presented in the chart below. Analyse the
results following and answer the accompanying questions. (10 marks)
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Solute
Bowman’s
Capsule
Protein
0
Urea
0.05
Glucose
0.10
Chloride
0.37
Ammonia
0.0001
Substance X
0
Quantities are in g/100 mL.
Lesson 13
Glomerulus
0.8
0.05
No data
No data
0.0001
9.15
Loop of Henle
0
1.50
0
No data
0.0001
0
Collecting Duct
0
2.00
0
0.6
0.04
0
a. Which of the solutes was not filtered into the nephron? Explain your answer.
(2 marks)
b. Predict whether glucose would be found in the glomerulus and provide
reasons for your prediction. (2 marks)
c. Why do urea and ammonia levels increase after filtration occurs? (2 marks)
d. Is it correct to say that veins carry blood with high concentrations of waste
products and arteries carry blood with high concentrations of nutrients?
Explain. (2 marks)
e. Compare the blood found in the renal artery and renal vein with respect to
urea and glucose. (2 marks)
3.
Athletes now undergo random urine testing for drugs. From your knowledge of
excretion, describe the pathway of substances such as drugs through the
excretory system from the time they enter the blood stream until they are
excreted in the urine. Be sure to include mention of the blood vessels, the parts
of the nephron and the urinary structures that they pass through.
(12 x ½ mark = 6 marks)
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Grade 12, University Preparation
Biology
Lesson 14 – The Endocrine System
SBI4U – Biology
Lesson 14
Lesson 14: The Endocrine System
Introduction
In the first lesson, you were introduced to the ideas concerning homeostasis and the
feedback systems that help the body counteract any changes. The focus was on the
kidney and how it works to maintain balance of body fluids and waste products. Though
mentioned briefly, chemical substances in the body called hormones play a major role in
maintaining homeostasis in many areas of body functioning. These hormones affect
various glands and cell types in the body, controlling their actions to keep the body
working under normal parameters. This lesson will focus on the endocrine system of
hormones and describe their effects on the body.
What You Will Learn
By the end of this lesson, you will be able to:
•
•
•
•
•
Assess the effects on the human body of taking chemical substances to enhance
performance or improve health
Evaluate some of the human health issues that arise from the impact of human
activities on the environment
Use appropriate terminology related to the endocrine system including: insulin,
testosterone, estrogen and pituitary
Explain how reproductive hormones act in human feedback mechanisms to maintain
homeostasis
Describe the anatomy and physiology of the endocrine system and explain how this
system interacts to maintain homeostasis
Thought Questions
Use the following questions to begin thinking about the endocrine system.
A.
B.
C.
D.
How can hormones help the body maintain normal conditions?
How can hormones help the body adapt to stress?
What are the male and female reproductive hormones and how do they work?
How can chemical substances like steroids improve athletic performance and why
are they dangerous?
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Lesson 14
The Chemical Signals
The endocrine system is composed of a number of glands that produce chemical
substances called hormones that act on different tissues in the body. Hormones can
act on one specific tissue or organ or they can act on many of the tissues, it simply
depends on the role of that hormone. Hormones are only able to affect the cells that
they are designed to affect because those cells carry receptors on their membrane
surface that the hormone needs to bind to in order to enter the cell and affect the
change. There are two main groups of hormones: steroid hormones and protein
hormones.
Steroid hormones – These hormones are made from the cholesterol molecule and
include the sex hormones as examples. As they move to their target cells, these
hormones enter the cytoplasm and combine with the receptor molecules located there.
The complex then moves into the nucleus where it combines with the appropriate
section of DNA which prompts transcription and then translation of the appropriate
protein. This protein will then change the cell in some way re-establishing homeostasis.
Protein hormones – These hormones contain chains of amino acids and are soluble in
water. They combine with receptors on the cell membrane at very specific sites.
Sometimes the formation of the protein-receptor complex prompts the formation of a
secondary messenger known as cyclic AMP which activates enzymes in the cytoplasm.
This cascade of enzyme action will give rise to a product that will alter the cell to reestablish homeostasis.
Figure 14-1 The Action of Steroid and Protein Hormones
Copyright © 2009, Durham Continuing Education
Source: Di Giuseppe et al. 374-5
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Lesson 14
The Endocrine System
There are seven major glands that produce the hormones of the endocrine system as
shown in the diagram. Each of these glands will be studied, focusing on which
hormones they produce, the tissues the hormones affect, and the result of hormone
action. Some homeostatic mechanisms require the combined effect of many hormones
in order to maintain the body within normal parameters. This is what can make it so
difficult for medical science to mimic the body’s responses when the system is not
working properly. Any upset in this system can cause a great many disorders or
diseases that need to be controlled.
Figure 14-2 The Location of the Endocrine Glands
Source: Di Giuseppe et al. 373
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The Master Endocrine Gland – The Pituitary
This gland is often referred to as the master gland since it regulates the activity of the
endocrine glands. It is located in the brain and connected to the hypothalamus. This
provides a link between the endocrine system and the nervous system. The pituitary
gland is actually composed of two glands, the anterior pituitary and the posterior
pituitary. The anterior portion produces and releases a number of hormones while the
posterior portion stores and releases hormones that are produced by the hypothalamus.
The anterior pituitary produces a total of six different hormones. Four of them act on
other endocrine glands. Thyroid-stimulating hormone (TSH) acts on the thyroid gland,
stimulating it to release thyroxine which affects metabolism. Both follicle-stimulating
hormone (FSH) and leutenizing hormone (LH) act on the reproductive organs
stimulating development and the ability to reproduce. Adrenocorticoptropic hormone
(ATCH) stimulates the adrenal gland, producing hormones in response to stress. The
other two hormones act directly on target cells causing a change. Growth hormone,
also known as somatotropin, acts on the majority of cells in the body during the growth
phase of development. It spurs growth by increasing intestinal absorption of calcium,
increasing cell division and stimulating protein synthesis and lipid metabolism.
Prolactin, the last hormone of the anterior pituitary, stimulates the production of
mammary gland tissue and milk production. Its regulation is unusual in that the
hypothalamus secretes a chemical transmitter called dopamine that inhibits its
production. After birth, stimulation of the nerve endings in the nipples during feeding
stimulates its production.
The posterior pituitary stores and secretes two hormones when signalled by the
hypothalamus. ADH or anti-diuretic hormone has already been discussed in the
previous lesson. It helps control salt and water balance in the kidneys making the
tubule membranes more or less permeable to water. Oxytocin plays an important role
in childbirth. This is the hormone that stimulates the muscles of the uterus to contract
during labour and the release of milk from the breasts.
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Lesson 14
Figure 14-3 The Pituitary and its Hormones
Source: Di Giuseppe et al. 376
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Lesson 14
Table 14-1 The Pituitary Hormones
Hormone
Anterior Lobe
Target
Thyroid-stimulating
hormone (TSH)
Adrenocorticotropic
hormone (ATCH)
Growth Hormone
(GH)
Follicle-stimulating
hormone (FSH)
Leutenizing
hormone (LH)
Thyroid gland
Prolactin (PRL)
Primary Function
Adrenal cortex
•
•
•
Most cells
•
Ovaries, testes
•
Ovaries, testes
•
Mammary glands
•
•
Uterus, mammary
glands
kidneys
•
•
•
Stimulates release of thyroxine from thyroid
Thyroxine regulates cell metabolism
Stimulates release of hormones involved in stress
responses
Promotes growth
Stimulates follicle development in the ovaries and
sperm development in the testes
Stimulates ovulation and formation of the corpus
luteum in females
Stimulates the production of testosterone in males
Stimulates and maintains milk production in
lactating females
Posterior Lobe
Oxytocin
Anti-diuretic
hormone (ADH)
Initiates strong contractions
Triggers milk release in lactating females
Increases water reabsorption by the kidneys
The Hormones of Metabolism
There are three endocrine glands involved with regulating body metabolism. The
anterior pituitary produces growth hormone. The thyroid gland helps regulate the rate at
which glucose is oxidized and the parathyroid gland regulates calcium and phosphate
levels.
The Thyroid Gland
The thyroid gland is located at the base of the neck, in front of the trachea or windpipe.
When stimulated by TSH it releases the hormones thyroxine and triiodothyronine.
Iodine is an important component of both these hormones and must be ingested for
proper formation. These hormone increases sugar consumption and energy production
especially in the heart, skeletal muscle, liver and kidney. People with higher thyroxine
levels use up glucose and other nutrients more quickly so they tend not to gain weight.
In more extreme cases it is called hyperthyroidism. If thyroxine levels are naturally low,
people tend to gain weight more easily as it takes longer to use up the glucose in the
blood so any excess is converted to fat. This is called hypothyroidism. The levels of
the hormone are controlled by a negative feedback loop. Receptors in the
hypothalamus recognize when thyroxine levels fall and secrete thyroid-releasing
hormone which stimulates the pituitary to release thyroid-stimulating hormone. TSH
then stimulates the thyroid to release more thyroxine.
This gland also releases another hormone called calcitonin which is responsible for
promoting the movement of calcium from the blood into bone tissue. This hormone is
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released when a calcium rich meal is digested. The majority of all calcium is stored in
the skeletal bones of the body. This hormone works in concert with a hormone that is
produced by the parathyroid gland.
The Parathyroid Gland
These four small glands are embedded within the thyroid gland. These glands maintain
homeostasis by responding directly to chemical changes in their surroundings. When
calcium levels are low the glands release parathyroid hormone (PTH) which raises
calcium levels. It does this by acting on the kidney, the intestines and the bones. The
hormone induces the kidney to retain calcium while promoting calcium release from the
bones. Bone cells break down and the calcium and phosphate separate. The calcium
enters the blood and the phosphate is excreted in the urine. The intestine also adds to
calcium level by prompting better absorption of calcium from digested foods. Once
calcium levels increase, PTH is no longer released.
Due to the actions of calcitonin from the thyroid gland and PTH from the parathyroid
gland, calcium levels in the blood are maintained. These hormones are called
antagonistic hormones because they have opposite effects on the same substance.
The body uses many of these antagonistic systems to maintain balance.
Table 14-2 The Hormones that Affect Metabolism
Gland
Thyroid
Thyroid
Parathyroid
Anterior pituitary
Hormone
Thyroxine and
triiodothyronine
Calcitonin
Parathyroid hormone (PTH)
Growth hormone
Effect on Metabolism
Regulates the rate at which glucose is
oxidized
Lowers calcium levels in the blood
Raises calcium levels in the blood
Promotes protein synthesis by
increasing uptake of amino acids by
cells
Causes a switch in cellular fuels from
glucose to fatty acids
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Support Question
(do not send for evaluation)
1.
A laboratory experiment was conducted to determine the effect of thyroxine on
metabolic rate. Four groups of adult, male rats were used. All the groups were
maintained in similar environments, designed to provide maximum physical
activity. Each group was supplied with adequate water and one of the following
diets.
Diet A: food containing all essential nutrients
Diet B: food containing all essential nutrients and an extract of thyroxine
Diet C: food containing all essential nutrients and a chemical that counteracts
the effects of thyroxine
Diet D: food containing all essential nutrients, except iodine
The results of the experiment are found in the table below.
Group
Average initial mass
(g)
I (diet A)
II (diet ?)
III (diet ?)
IV (diet ?)
Average mass 2
weeks after
treatment (g)
310
320
318
315
312
309
340
400
Final average
oxygen
consumption
(mL/kg/min)
4.0
10.1
2.7
2.0
a. Formulate a hypothesis for this experiment.
b. Which group was most likely used as the control. Explain your answer.
c. Diet B was most likely fed to which group? Explain your answer.
d. Diet D was most likely fed to which group? Explain your answer.
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The Hormones that Respond to Stress
Though other glands are involved, it is the adrenal gland that produces hormones that
respond to stress in an organism. The adrenal glands are located on the top of each
kidney. It is composed of two layers: an outer cortex and an inner medulla. Each layer
acts independently of the other and provides different hormones depending on the
situation. The medulla portion release hormones that provide a rapid but short lived
response to stress while the cortex provides a more sustained stress response.
The Adrenal Cortex
The cortex portion of this gland produces the hormones cortisol which stimulates
carbohydrate synthesis and aldosterone which regulates blood pressure by altering the
salt and water balance in the body. Both also contribute to the long-term stimulation of
the immune system when the body is under stress. Their release is controlled by ACTH
from the pituitary gland.
Cortisol causes a large increase in the production of carbohydrates from amino acids
and other substances. This increase in the conversion of organic molecules into
glucose leads to increased glycogen stores in the liver. This allows for quick conversion
back into glucose when the body needs it. It also stimulates the breakdown of lipids for
use as an alternate energy source, inhibits metabolism and suppresses protein
synthesis in the body except for the brain and muscles. Cortisol also has antiinflammatory properties helping to decrease the build-up of fluids in tissues by
decreasing permeability of the blood vessels. During stress, more energy is available to
focus on the problem, slowing down cellular processes that are non-essential.
Aldosterone affects the kidneys, regulating water and salt levels. It prompts the kidneys
to reabsorb more sodium which causes more water to be retained. If more fluid is
present, blood pressure will be higher.
The Adrenal Medulla
This portion of the adrenal gland secretes two very similar hormones called adrenaline
(epinephrine) and noradrenaline (norepinephrine). These are the major hormones
released in response to stress. They both act to increase heart rate and blood pressure
and cause the blood vessels to dilate in the heart and respiratory system. The liver is
also stimulated to break down glycogen into glucose and subsequently raise blood
sugar levels. This is all part of the “flight or fight” response to a sudden, unexpected
stressful stimulus. We have all felt this response when surprised or think we are in
danger. Your heart pounds, your breathing increases and your awareness increases.
This mechanism probably helped our primitive ancestors survive when danger
approached. These mechanisms work well in the short term but, if the body is
subjected to long term stress, it can be very detrimental to overall health. The body will
adjust to this perceived threat but the new normal creates further problems. We see
this today more and more as people deal with the stress of day to day living.
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Lesson 14
Table 14-3 Problems Associated with Long-Term Stress
New Normal
Higher blood sugar
Increased blood
pressure
Increased heart rate
Problems Created
Alters the balance between blood and extracellular fluids and
can lead to increased fluid uptake by the blood and increased
blood pressure (possible kidney damage)
Increased water loss from the nephron
Possible rupture of blood vessels due to higher pressure
(possible aneurysm)
Increased blood clotting
Can lead to higher blood pressure (possible stroke)
Possible destruction of heart muscle (possible heart attack)
There are other hormones that do change during stress that involve the regulation of
blood sugar levels. They will be discussed in the next section.
Figure 14-4 The Adrenal Gland And Stress
Source: Blake et al. 184
Please visit the following website(s)
http://trc.ucdavis.edu/biosci10v/bis10v/media/ch26/cortisol.swf
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Lesson 14
Support Question
(do not send for evaluation)
2.
Cortisone is often prescribed as an anti-inflammatory drug. Why are doctors
hesitant to provide this drug over a long duration?
Hormones that Affect Blood Sugar
The pancreas is responsible for secreting the hormones that maintain a blood sugar
level of 0.1%. These hormones are produced by specialized cells scattered throughout
the pancreas called the Islets of Langerhans. They produce the antagonistic hormones
of insulin and glucagon. When the blood sugar level increases, usually after the
ingestion of a meal, insulin is released. It promotes the conversion of glucose into
glycogen and causes the body cells to become more permeable to glucose. This
moves the glucose out of the blood returning the levels to normal. When levels are
low, glucagon with promote the conversion of glycogen back into glucose. This allows
levels to be kept relatively stable.
Both adrenaline and noradrenaline affect blood sugar levels as well by increasing the
amount of glucose that is present in the blood. Cortisol also helps create more glucose
by promoting the conversion of other molecules such as proteins and lipids into glucose.
Table 14-4 Hormones Affecting Blood Sugar
Hormone
Insulin
Hormone Location
Islets of
Langerhans
Glucagon
Islets of
Langerhans
Adrenal medulla
Adrenaline
and
noradrenaline
Cortisol
Adrenal cortex
Effect
Increases permeability of cells to glucose and
increases glucose uptake
Allows for the conversion of glucose to glycogen
Brings about a decrease in blood sugar
Promotes the conversion of glycogen to glucose
Brings about an increase in blood sugar
Promotes the conversion of glycogen to glucose
Brings about an increase in blood sugar
Brings about an increase in heart rate and cell
metabolism
Promotes the conversion of amino acids to
glucose
Promotes the breakdown of fats to fatty acids
Decreases glucose uptake by the muscles
Brings about in increase in blood sugar in
response to stress
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Lesson 14
Figure 14-5 Control of Blood Glucose
Source: Di Giuseppe et al. 378
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Lesson 14
Support Question
(do not send for evaluation)
3.
Explain what advantage is gained by elevating blood sugar and blood pressure in
times of stress.
The Reproductive Hormones
Males and females produce both androgens (male-associated hormones) and
estrogens (female-associated hormones). The difference is in the amount of each type
of hormone produced and the changes they cause in the body. The adrenal cortex
produces both types. In males, the androgens produced by the cortex are negligible
compared to the amount produced by the testes but in females the cortex is responsible
for producing 50% of the androgens. These hormones promote muscle and skeletal
development. The estrogens produced here become more important to females after
menopause when the ovaries stop producing.
At puberty, gonadotropin releasing hormone (GnRH) produced by the hypothalamus
stimulates the pituitary gland to release both FSH and LH. In males, LH acts on the
interstitial cells which in turn produce testosterone. FSH acts on the Serteoli cells and
initiates sperm production. Once testosterone reaches the appropriate level, it acts as a
negative inhibitor for the hypothalamus. This stops the stimulation of the pituitary which
stops the release of FSH and LH. The Serteoli cells also produce a substance called
inhibin which also acts as an inhibitor for the hypothalamus. When testosterone levels
begin to drop, the feedback loop is broken and levels are stimulated to increase. This
will occur continuously to maintain the proper level of the hormone. Once males reach
puberty and become sexually mature, sperm production will occur constantly for the rest
of their lives.
Testosterone, when produced at the beginning of puberty acts to influence the
development of the secondary sexual characteristics. This includes stimulating the
development of the penis and testes and increasing sex drive. It promotes the
development of facial and body hair, the growth of the larynx creating a deeper voice,
and inhibits fat development while promoting muscle development.
In females, the onset of puberty starts the same way as in males. GnRH from the
hypothalamus stimulates the release of FSH and LH. FSH causes the follicle to grow
which releases estrogen. This stimulates the development of the endometrium (lining of
the uterus). LH stimulates the development of the corpus luteum which also releases
estrogen. As estrogen levels rise it acts as an inhibitor, shutting down FSH release by
the pituitary. At the same time, the increased estrogen levels stimulate the release of
more LH. As these levels increase, ovulation occurs. The corpus luteum is stimulated
to release both estrogen and progesterone. This further develops the endometrium.
As levels increase, they inhibit both FSH and LH. If fertilization does not occur, the
corpus luteum deteriorates and the estrogen and progesterone levels fall causing
menstruation. This cycle will continue to repeat itself from puberty until menopause.
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At the beginning of puberty, these hormones stimulate the production of the secondary
sexual characteristics in females. These include the development of breasts, the
growth of hair around the genitals and under the arms, the widening of the hips and an
increase in body fat.
Figure 14-6 Feedback Loops of the Male and Female Reproductive Systems
Source: Di Giuseppe et al. Supplement p. 231
Endocrine System Disorders
A large number of disorders affecting human health can be traced back to improper
hormone function. Because many of these hormones have multiple functions, it can be
difficult to pin point the exact cause of the disorder. Luckily, new treatments are
constantly being created to overcome the symptoms of the various disorders. The
following table provides you with some examples of some of the more common
disorders of the endocrine system.
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Lesson 14
Table 14-5 Endocrine System Disorders
Disorder
Diabetes
Endocrine Gland
pancreas
Goiter
Thyroid gland
Pituitary
dwarfism
Pituitary gland
Gigantism
Pituitary gland
Acromegaly
Pituitary gland
Cushing’s
disease
Adrenal gland
Addison’s
disease
Adrenal gland
Symptoms
Insufficient insulin production causes excess
glucose in the blood and urine
Excretion of large volumes of urine, extreme thirst,
low energy
If left untreated it can lead to blindness, kidney
failure, nerve damage and limb amputation due to
dying tissue
Lack of iodine causes overstimulation of the thyroid
causing continuous growth
The thyroid enlarges creating a large swelling of the
neck area
Insufficient growth hormone production resulting in
abnormally short stature
Puberty may be delayed or not occur
An excess of growth hormone prior to puberty
resulting in abnormal growth of the long bones
Excess growth hormone production during
adulthood causing excessive thickening of bone
tissue
Leads to abnormal growth of the head, hands and
feet and spinal deformities
Excess secretion of cortisol due to elevated levels
of ACTH or adrenal gland tumour
Causes high blood pressure, high blood sugar,
muscle weakness and accumulation of fluid in the
tissues
Insufficient secretion of cortisol and Aldosterone
Causes low blood pressure, low blood sugar, weight
loss, and muscle weakness
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Lesson 14
Support Question
(do not send for evaluation)
4.
A number of laboratory experiments were conducted on mice. The endocrine
system of mice is similar to that of humans. Brief summaries of the procedures
are provided in the table below.
#
1
Procedure
Gland removed
2
3
Hormone injected
Blood flow from the
posterior pituitary reduced
Hormone injected
4
Observation
Urine output increased
Sodium ion concentration in urine increased
ACTH level increased in blood
Blood glucose levels decreased
Urine production increased
Glycogen converted to glucose in the liver
Blood glucose increased
a. In procedure 1, identify the gland that was removed and explain why the
levels of ACTH increased.
b. In procedure 2, identify the hormone that was injected and explain why blood
sugar levels decreased.
c. In procedure 3, identify the hormone that was affected and explain why urine
production increased.
d. In procedure 4, identify the hormone that was injected and explain why blood
glucose levels increased.
Performance Enhancing Drugs
People have experimented with different natural substances to study the effect they
have on the human body. Caffeine was found to mimic adrenaline, raising heart rate
and blood pressure and increasing alertness. In sports competition, it was not until the
1950’s that anabolic steroids were introduced and used by weightlifters. These steroids
are designed to mimic testosterone which naturally stimulates the body to reduce fat
storage and build muscle. Anabolic actually means growing or building. These steroids
have been found to increase muscle mass, reduce body fat, increase strength and
improve endurance. For athletes in sports that require short bursts of strength, it gave
them an advantage. Some athletes claim that the use of steroids helps them recover
from injury at a faster rate which allows them to train more rigorously. Steroid users
typically use levels that are 10 to 10 000 times higher than any dose prescribed by any
doctor for medical purposes. There are a number of long term health risks that have
been linked to extended use of these drugs. In younger individuals who have not fully
grown, steroids cause the premature fusing of the growth plates in the long bones of the
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Lesson 14
body which can reduce their potential height. Some people experience mood swings
and uncontrollable rages. On a more physical level, altering the levels of sex hormones
can affect the secondary sexual characteristics. Both males and females can
experience acne, bad breath, high blood pressure, liver disease, kidney disease and
certain cancers. Males may experience premature baldness, shrinking testes, reduced
sperm count and the development of breasts. Females may develop facial hair, have a
reduction in breast tissue and experience changes or cessation of their reproductive
cycle.
It was in the 1980’s when the use of steroids became more rampant. Canadian Ben
Johnson was stripped of his gold medal in the 100 meter race when he tested positive
for using the anabolic steroid, Stanozolol. Since then, the drugs that are available to
improve performance have become far ranging and more sophisticated. Sharpshooters
and archers use beta blockers to slow the heart rate which can reduce nervousness and
induce calm. Endurance athletes can use drugs such as erythropoietin which boosts
red blood cell production. This allows for increased transport of oxygen to tissues.
More oxygen equates to more energy. Today, the International Olympic Committee lists
over 17 steroids and related compounds that are banned for use by athletes. Every
winning athlete is now tested for the presence of banned substances. Unfortunately, as
the tests to detect these substances improve, methods to mask their presence improves
as well.
Table 14-6 Banned Performance-Enhancing Drugs
Drug
Anabolic steroids
Stanozolol,
Androstenedoil,
Nandrolone
Peptide Hormones
Growth hormones
Erythropoietin
Beta Blockers
Atenolol, Bisoprolol,
Nandolol
Stimulants
Amphetamine
Caffeine
Pseudoephedrine
Advantage
Increases muscle
mass and strength
Side Effects
Decreases growth, kidney problems,
hair loss, oily skin, acne, shrinking
testes, infertility and cancer
Decreases fat
Diabetes, abnormalities of bones, liver,
Improves muscle mass heart and kidneys, liver disease
High blood pressure
Increases red blood
Thickens the blood increasing chances
cells that carry greater of stroke
oxygen
Heart problems
Slows heart rate
Reduces cardiac response time
Makes skin more sensitive to the sun
Increases endurance
Relief of fatigue
Improves reaction time
Increases alertness
Increases alertness
Irregular heart beat, nervousness,
difficulty sleeping
Increases blood pressure
Narrows blood vessels and increases
blood pressure
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Masking Agents
Bromantan
Probenecid
Lesson 14
Makes steroid difficult
to detect
Stops excretion of
steroids for a few
hours
unknown
Headache, tissue swelling, nausea
Because of the sophisticated science behind these drugs, it is difficult for drug testing
programs to stay as current. In the previous Olympic Games in China, the samples
provided by the athletes are being kept so as new tests are developed, old samples can
be checked for any illegal usage of performance enhancing drugs.
Key Question #14
1.
Blood sugar levels of a person with diabetes mellitus and a person without were
monitored over a period of 12 hours. Both ate an identical meal and performed 1
hour of similar exercise. Use the data provided from the diagram below to
answer the questions. (10 marks)
Source: Di Giuseppe et al. 401
a. Which hormone injection did Bill receive at the time labelled X? Provide
reasons for your answer. (2 marks)
b. What might have happened to Bill’s blood sugar level if hormone X had not
been injected? Justify your answer. (2 marks)
c. Explain what happened at time W for Bill and Farzin. (1 mark)
d. Explain why blood sugar levels begin to fall after time Y. (2 marks)
e. What hormone might Bill have received at time Z? Explain your answer.
(2 marks)
f. Why is it important that Bill and Farzin have the same body mass? (1 mark)
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Lesson 14
2.
One survey of professional athletes reported that about half would be willing to
take a drug that ensured their success, even if it shortened their lives. Create a
short essay looking at the positive and negative aspects of steroid use by
athletes in competition. (10 marks)
3.
Recent headlines have brought attention to the dangers of plastic water bottles
and other plastic products. Through use, the plastics begin to breakdown and
release a substance called bisphenol A which research suggests may be
hazardous to human health. Research the effects of bisphenol A, describing
what it is, what it does and the new guidelines for its use in the future. (10
marks)
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SBI4U
Grade 12, University Preparation
Biology
Lesson 15 – The Nervous System
SBI4U – Biology
Lesson 15
Lesson 15: The Nervous System
Introduction
Homeostatic control via the endocrine system requires time. The imbalance must first
be detected and the appropriate gland stimulated. Once the hormone is released, it
must travel through the blood stream to reach its target cells and then affect them as
required. But, there are times when the external environment changes quickly and the
organism must respond in the same fashion. The nervous system is capable of
responding to these changes within mere seconds or even fractions of seconds. This
lesson will focus on the structure of the nervous system, how it works and what it does
to affect change.
What You Will Learn
By the end of this lesson, you will be able to:
•
•
•
Use appropriate terminology related to the nervous system including synapse,
acetylcholine, threshold, reflex arc, axon and action potential
Plan an investigation into the study of a feedback system such as a stimulus
response loop
Describe the anatomy and physiology of the nervous system and explain how it
interacts to maintain homeostasis
Thought Questions
Use these questions to begin thinking about the nervous system.
A. Do nerve cells carry an electrical current?
B. Do some responses occur more quickly than others?
C. How does the nervous system know that a change in the external environment has
taken place?
D. How is a message sent to the brain?
Organization of the Nervous System
The nervous system is an elaborate and extremely complicated communication system
that contains more that 100 billion nerve cells in the brain alone. It is divided into two
main areas: the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS consists of the nerves of the brain and spinal cord and is responsible
for coordinating all incoming and outgoing information. The PNS consists of the nerves
that carry information between all areas of the body and the CNS.
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Due to its complexity, the PNS can be further sub-divided into the somatic and
autonomic nerves. The somatic nerves control the skeletal muscle, bones and skin and
relays information about the external environment back to the brain. Other nerves
control the movement of muscle in response to the brains signals. This is the portion of
the nervous system where we have some measure of awareness and voluntary control.
We can sense changes in the environment such as heat, light, touch, taste, smell and
can respond by laughing, smiling, and moving our bodies. The autonomic portion of the
PNS is not under our control. This portion consists of nerves that control the internal
organs of the body. This would include such things as the churning of the stomach
during digestion, or the movement of digested food through the intestines. We are not
generally aware of these occurrences and have no conscious control. This involuntary
portion of the PNS is maintained by the sympathetic and parasympathetic systems
which balance each other like on and off switches.
Figure 15-1 The Divisions of the Nervous System
Source: Di Giuseppe et al. 412
The Nerve Cell
The neuron or nerve cell is the functional unit of the nervous system. There are three
main types of neurons, each with their own specific functions. The sensory neurons or
afferent neurons sense and relay information from the environment to the CNS for
processing. They are located in clusters or tracts called ganglia found outside the
spinal cord. The interneurons or association neurons link neurons within the body,
predominantly in the brain and spinal cord. The motor neurons or efferent neurons
relay information from the brain to the effectors such as muscles, organs and glands to
counteract the recognized change in the environment.
All neurons are composed of three basic parts. The dendrite receives information from
other nerve cells and directs it toward the cell body. The cell body contains the
standard cell organelles and carries out regular metabolic processes. The cell body
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Lesson 15
then forms an extension of the cytoplasm called the axon. This part of the nerve cell
directs the nerve impulse or information away from the cell body. Many axons are
covered with an insulating material of fatty protein called the myelin sheath. This sheath
is created by specialized glial cells called Schwann cells and helps prevent the loss of
charged ions from the nerve cell. The nodes of Ranvier are the areas between the
sections of myelin sheath which help to speed up the transmission of nerve impulses as
the signal jumps from node to node.
Neurons within the PNS contain an additional feature called the neruilemma which
surrounds the axon. This thin membrane allows axons to regenerate when they are
damaged. In the brain, this type of regenerative cell is surrounded by its myelin sheath
and is referred to as white matter. The unmyelinated fibbers are grey in colour and
named accordingly. They are not able to regenerate after injury. Thus, it is only in
certain areas of the nervous system that function can be restored as damaged nerves
repair themselves. This process occurs very slowly, with restoration rates being as slow
as 1 mm per year of regrowth. Some people with nerve damage in their limbs,
characterized by a feeling of numbness, have reported the gradual return of sensation
over a number of years. Unfortunately, the neurons of the spinal cord lack this
regenerative ability leading to paralysis if nerve damage occurs in this location.
Figure 15-2 The Nerve Cell
Source: Di Giuseppe et al. 413
Support Question
(do not send for evaluation)
1.
Briefly describe the function of the following parts of a neuron: dendrites, myelin
sheath, Schwann cells, cell body and axon.
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The Electrochemical Impulse
When the neuron is not relaying information, it is said to be at rest. In this situation,
there is a charge difference across the membrane due to the unequal concentration of
positive ions on either side of the membrane. This charge difference or resting
membrane potential is -70 mV (millivolts). Potassium ions are concentrated on the
inside of the cell and sodium ions are concentrated outside the cell. Both ions move
across the membrane but the membrane is 50 times more permeable to potassium
ions, so more potassium ions diffuse out of the cell than sodium ions diffuse in.
Because of this unequal movement, the exterior of the cell becomes positive relative to
the interior. So, at rest, the membrane is said to be polarized.
The nerve impulse is an electrochemical message generated by the movement of ions
through the membrane. When the nerve is excited, the potential charge difference
across the membrane changes to +40 mV. This is called the action potential. When
excitation of the nerve occurs, the membrane becomes more permeable to sodium ions.
It is believed that the membrane has sodium ion channels embedded within it. As the
sodium gates open, the potassium gates close. This allows for the rapid flow of sodium
ions into the cell, causing a charge reversal which is referred to as depolarization. Once
the interior of the nerve cell becomes positive, the sodium and potassium gates reverse
themselves and a sodium-potassium pump restores the ion concentrations to their
resting levels. The nerve cannot send another impulse again until the membrane has
been restored or repolarized. The refractory period can last from 1 to 10 milliseconds.
The wave of depolarization sweeps down the length of the axon in sections. A similar
analogy would be a crowd doing the wave in a stadium.
But, what causes neuron excitation? When a sensory neuron detects a change in the
environment known as a stimulus, it has to be strong enough to trigger the
depolarization of the membrane. The intensity of the stimulus must reach a set level
called the threshold level before the signal will be sent. This threshold is important for it
prevents small changes that don’t have an effect from sending a signal to the brain.
Without the threshold, the sensory neurons would send signals continuously which
would overwhelm the brain. Once the threshold level is reached the neuron will fire at
the same intensity and the same speed for any stimulus. The size of the change cannot
affect these factors of the nerve impulse. A greater stimulus does not cause a greater
depolarization of the membrane. This is known as the all-or-none response. The
neuron either sends the signal or it does not.
But, how does the brain interpret a signal of different strengths? For example, how does
the brain know when you stub your toe that it hurts only a little bit or a lot? The brain is
able to interpret intensity by the frequency of the impulses being sent. Therefore, if the
stimulus is small a few signals will be sent, one after the other. When the stimulus is
intense, the signal will be sent repeatedly as soon as the membrane has repolarizes.
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Lesson 15
Figure 15-3 The Nerve Impulse
Source: Di Giuseppe et al. 421
Please visit the following website(s)
http://highered.mcgraw-hill.com/sites/dl/free/0072495855/291136/nerveImpulse.swf
Synaptic Transmission
The distance between the peripheral nervous system and the brain is too great for a
single neuron to stretch the entire distance. Therefore, the information from the sensory
neurons must be passed to the interneurons which can then relay the information to the
brain. This means that when the signal reaches the end of the sensory neurons, there
has to be a mechanism in place to transfer the signal to the interneuron. This is
achieved through synaptic transmission. The small space between the ends of the
neurons is called the synapse. When the signal reaches the end of the neuron,
chemical neurotransmitters move across the space or synaptic cleft. They are small
molecules that are held in tiny vesicles at the end of the first neuron, referred to as the
pre-synaptic neuron. The vesicles move to the end of the neuron, fuse with the
membrane and dump their contents into the cleft. The next neuron in line, called the
post-synaptic neuron, has receptors on its end where the neurotransmitter can bind.
Once they bind, the threshold level is reached and the signal moves down the length of
the neuron. The diffusion of the transmitter across the cleft slows down the
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Lesson 15
transmission of the signal so the greater the number of synapses, the slower the signal
travels. Acetylcholine is a common neurotransmitter that is capable of opening the
sodium ion channels in the post-synaptic neuron. Once the message has been
transmitted, the enzyme cholinesterase is released into the synaptic cleft and breaks
the acetylcholine down.
Figure 15-4 The Synapse
Source: Di Giuseppe et al. 423
Please visit the following website(s)
http://highered.mcgrawhill.com/sites/dl/free/0072495855/291136/transmission_across_synapse.swf
Support Questions
(do not send for evaluation)
2.
What changes take place along a nerve cell membrane as it moves from a
resting potential to an action potential to a refractory period?
3.
Use what you have learned about threshold levels to explain why some
individuals can tolerate more pain than others.
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4.
Lesson 15
Botulism and curare are poisons that inhibit the action of acetylcholine. What
symptoms would you expect to find from someone exposed to either poison?
Provide an explanation for the symptoms.
Reflex Arcs
In most situations, the nervous system works quickly enough to respond to changes in
the environment. But, there are some situations where an almost instantaneous
response is required to keep the organism from harm. This is the function of the reflex
arc. Each reflex arc contains five necessary components: the receptor, the sensory
neuron, the interneuron in the spinal cord, the motor neuron and the effector. The
sensor is the portion of the nervous system that is able to detect the environmental
change. It activates the sensory neuron which sends the signal towards the brain.
Once the signal reaches the spinal cord, specialized interneurons that are preprogrammed with a specific response, send a message back through the motor neuron
to the effector which immediately makes the change. At the same time, the signal
travels along other interneurons to the brain for processing. This system cuts down on
the lag time required for the signal to reach the brain, have the brain process the
information and then send the appropriate response. A perfect example of this is the
heat response reflex. When the hand touches a hot object, the reflex arc causes the
hand to pull away before serious damage can be done. It is only after the hand has
pulled back that the brain actually perceives the feeling of heat. So in this case, the
reaction comes first and then the sensation.
Figure 15-5 The Reflex Arc
Copyright © 2009, Durham Continuing Education
Source: Di Giuseppe et al. 416
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Lesson 15
Please visit the following website(s)
http://www.sumanasinc.com/webcontent/animations/content/reflexarcs.html
The Central Nervous System
The CNS consists of the brain and the spinal cord and acts as the coordinating center
for all activity. This system is very well protected by both bone and tough membrane.
There are three membranes or meninges that serve as a protective barrier. The outer,
toughest membrane is the dura mater. This is followed by the middle layer, the
arachnoid mater and then the inner layer, the pia mater. They form the blood-brain
barrier which limits the substances that move from the bloodstream into the nerve cells.
Between the middle and inner layers is the cerebrospinal fluid which acts as a shock
absorber and a means of transporting nutrients to the nerve cells. Wastes are also
removed and sent to the blood for disposal.
The Spinal Cord
This is the portion of the CNS that carries information from the PNS to the brain for
processing. The spinal cord extends down from the brain through a canal in the
backbone. As mentioned earlier, both myelinated and unmyelinated fibbers travel
through the cord in specific tracts. The dorsal nerve tract brings sensory information
into the cord while the ventral nerve tract carries motor information from the brain to the
periphery.
Figure 15-6 The Spinal Cord
Source: Di Giuseppe et al. 427
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The Brain
It is the complexity of the brain that accounts for the differences between humans and
other animals but its overall structure is relatively the same. The brain can be divided
up into three distinct regions: the forebrain, the midbrain and the hindbrain. In humans,
the forebrain is the largest section. The forebrain contains the olfactory lobes which
process information concerning smell. The cerebrum is also located here. This is the
portion that coordinates the sensory information is receives and formulates the proper
response. In humans, speech, reasoning, memory and personality are located in this
area. The surface of the cerebrum is known as the cerebral cortex. It is composed of
grey matter and intricately folded to increase the surface area. Each half of this part of
the brain stores different kinds of information. The right brain is associated with visual
and spatial awareness while the left brain is linked to verbal skills. The two sides are
able to interact through a bridge called the corpus callosum. The cerebrum can be
further sub-divided into four lobes each of which has its own specific functions.
Figure 15-7 The Human Brain
Source: Di Giuseppe et al. 429
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Table 15-1 The Lobes of the Brain and Their Functions
Lobe
Frontal lobe
Function
Motor areas control movement of voluntary muscles
Association areas are linked to intellectual activities and personality
Temporal lobe Sensory areas are associated with vision and hearing
Association areas are linked to memory and interpretation of sensory
information
Parietal lobe
Sensory areas associated with touch and temperature awareness
Association areas have been linked to emotions and interpreting
speech
Occipital lobe Sensory areas are associated with vision
Association areas interpret visual information
The midbrain consists of four spheres of grey matter that acts as a relay center for
some eye and ear reflexes. The hindbrain is found posterior to the midbrain and joins
with the spinal cord. It consists of three major regions: the cerebellum, pons and
medulla oblongata. The cerebellum is the largest section and is found immediately
behind the cerebrum. It controls limb movements, balance and muscle tone. The pons
is a relay station that passes information between the two regions of the cerebellum and
between the cerebellum and the medulla oblongata. This last section acts as the
connection between the PNS and the CNS. It also controls involuntary muscle action.
It regulates such things as breathing movements, diameter of the blood vessels and the
heart rate, as well as being the coordinating center for the autonomic nervous system.
The Father of Neurosurgery
The functioning and structure of the brain has been a mystery for hundreds of years.
Canadian physician Wilder Penfield is considered to be the original pioneer in brain
mapping-figuring out what the different parts of the brain actually do. He used
electrodes to electrically stimulate different areas of the brain to determine their
function. He focused on mapping the cerebral cortex using epilepsy patients. Epilepsy
is caused by massive electrical misfiring of neurons within the brain. He was able to
determine the area where the problem originated so the tissue could be removed
leaving the rest of the cortex intact.
Support Question
(do not send for evaluation)
5.
A physician makes an incision completely through the corpus callosum. How
might this affect the patient?
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Lesson 15
Please visit the following website(s)
http://learn.genetics.utah.edu/content/addiction/drugs/mouse.html
The Autonomic Nervous System
The ANS works in concert with the endocrine system to adjust the internal environment.
This entire system works without conscious control. The two antagonistic portions of
this system work to maintain balance. The sympathetic nervous system prepares the
body for stress while the parasympathetic restores normal function. Below is a table
that shows the effects of both systems on various organs of the body.
Table 15-2 The Organs of the Autonomic Nervous System
Organ
Heart
Digestive
Liver
Eyes
Bladder
Skin
Adrenal gland
Sympathetic
Increases heart rate
Decreases peristalsis
Increases the release of
glucose
Dilates pupils
Relaxes sphincter
Increases blood flow
Causes release of
epinephrine
Parasympathetic
Decreases heart rate
Increases peristalsis
Stores glucose
Constricts pupils
Contracts sphincter
Decreases blood flow
No effect
Sensory Receptors
As humans, we perceive and interpret the stimuli from the outside world using special
sensors called receptors, the specially modified ends of sensory neurons. Each type of
receptor responds to a particular type of stimulus. The information provided allows the
brain to respond appropriately to maintain homeostasis and to prevent damage to the
organism. For some stimuli, we respond instinctively while other responses have to be
learned. Below is a chart of the different types of receptors and the information they
provide to the brain.
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Table 15-3 The Human Sensory Receptors
Receptor
Taste
Stimulus
Chemical
Smell
Pressure
Chemical
Mechanical
Proprioceptors
Balance (inner ear)
Outer ear
Eye
thermoregulators
Mechanical
Mechanical
Sound
Light
heat
Information Provided
Presence of specific chemicals identified by taste
buds
Presence of chemicals detected by olfactory cells
Movement of the skin or changes in the body
surface (touch)
Movement of the limbs
Body movement
Sound waves
Changes in light intensity, movement and colour
Flow of heat- can detect heat or the lack of
In humans, taste and smell go together to help us identify the specific taste of foods.
The majority of what we actually taste is due to the smell of foods. If you have ever had
a cold you realize that when your nose is stuffed up, you can’t taste food or at least not
very well. Proprioceptors respond to the stretching and relaxation of muscles. Even
with our eyes closed, we are aware of the position of our limbs due to the information
concerning what muscles are relaxed and which are contracted. This is something that
has to be learned. If you have ever watched a baby, they have little conscious control
over limb movement. It is only through repetition in childhood, that we learn this control.
Balance is controlled by the inner ear. The semicircular canals contain fluid that moves
around based on the position of the body. Tiny hairs in the canals sense the movement
of the fluid and send the information to the brain. The brain is then able to interpret the
information from the sensors to determine if the individual is standing upright, leaning
over, etc. If you spin in a circle for a few moments and stop, the room spins. This is
because the fluid keeps moving for a few moments after you have stopped and sends
that information to the brain. The brain interprets this to mean that the body is still
moving. The touch sensors on our bodies are extremely concentrated in some areas
and less concentrated in others. It simply depends on the sensitivity of the area and
how important it is for that area of the body to be very sensitive or slightly sensitive.
The fingertips have a high concentration of touch sensors as does the neck region.
Because we use our finger tips for investigation of the external environment, sensitivity
is important. In the neck region, because this area houses the spinal cord, increased
sensitivity is important for protection. Both the ear and the eye are special sensors that
provide the organism with important information. Hearing allows us to gather
information about the environment from far away, as a kind of early warning system.
The eye allows us to perceive objects at a distance and closely examine objects that
are near. This system reacts to light stimulus. For protection, the eye is able to adjust
the amount of light that enters the eye through dilation and contraction of the pupil.
Lastly, the body is able to sense heat and cold to prevent damage to the organism.
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Key Question #15
1.
Use what you know about the transmission of nerve impulses to formulate a
hypothesis about how local anaesthetics work (ie. The dentist freezing your gum
before filling a cavity). (4 marks)
2.
Both alcohol and caffeine affect the neurological system. Although alcohol is a
controlled substance, caffeine is not. Develop an argument to make caffeine
(coffee and other caffeinated beverages) a controlled substance. (5 marks)
3.
This question is a mini-lab concerning human response to stimulus. Follow the
procedures outlined below and answer the questions provided. (17 marks)
Mini-Lab Reflex Arcs
Reflex arcs make up the neural circuit that travels through the spinal cord,
providing a framework for reflex actions. Simple physical tests are used to check
reflexes. In this investigation, you will observe the presence and strength of a
number of reflex arcs.
Materials
Rubber reflex hammer or the edge of your hand
Penlight or small flashlight
Procedure
Part I: Knee Jerk
1. Find a partner. You will act as each other’s subjects.
2. Have your subject sit on a chair with their legs crossed. The subject’s upper
leg should remain relaxed.
3. Locate the position of the kneecap and find the large tendon below the
midline of the kneecap.
4. Using a reflex hammer, gently strike the tendon below the kneecap.
a. Describe the movement of the leg. (1 mark)
5. Ask the subject to clench a book with both hands and then strike the tendon
of the upper leg once again.
b. Compare the movement of the leg while the subject is clenching the
book with the movement in the previous procedure. (1 mark)
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Lesson 15
Part II: Babinski Reflex
6. Ask the subject to remove a sock. Have the subject sit in a chair, then place
the heel of the bare foot on another chair for support. Quickly slide the reflex
hammer (any hard object will do-ruler, wooden spoon) along the subject’s
foot, beginning at the heel and moving toward the toes.
c. Describe the movement of the toes. (1 mark)
Part III: Pupillary Reflex
7. Have the subject close one eye for approximately 1 min. Ask them to open
the closed eye. Compare the size of the pupils.
d. Which pupil is larger? (1 mark)
8. Ask the subject to close both eyes for 1 min, and then open both eyes. Shine
a penlight in one of the eyes.
e. Describe any change in the pupils of the subject. (1mark)
Analysis
f. How does the knee-jerk reflex change when the subject is clenching
the book? Why do you think this occurs? (2 marks)
g. What is the purpose of testing different reflexes? (1 mark)
Evaluation and Synthesis
h. Explain why the knee-jerk reflex is important in walking. (1 mark)
i. A person touches a stove, withdraws their hand and then yells. Why
does the yelling occur after the hand is withdrawn? (4 marks)
j. While examining the victim of a serious car accident, a physician
lightly pokes the patient’s leg with a needle. The pokes begin near
the ankle and gradually progress toward the knee. Why is the
physician poking the patient? Why begin near the foot? (4 marks)
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SBI4U
Grade 12, University Preparation
Biology
Lesson 16 – The Immune System
SBI4U – Biology
Lesson 16
Lesson 16 – The Immune System
Introduction
Studying the systems in the last three lessons, you have learned how the body
maintains itself in proper working order to operate at peak efficiency. These systems
respond to changes in the external environment to guard against those imbalances. But
what happens when something enters in internal environment and upsets the balance
from within? The immune system is responsible for keeping disease causing organisms
from entering the organism and upsetting the balance. If keeping them out is
impossible, this system has mechanisms in place to deal with the intruder. Once this is
accomplished, the body can return to its normal state. The focus of this lesson will be
on the immune system, how it prevents intrusion and how it deals with intruders that
manage to enter.
What You Will Learn
By the end of this lesson, you will be able to:
•
•
•
Use appropriate terminology related to homeostasis including inflammatory
response, antigen, antibody, anaphylaxis and immunity
Evaluate some of the human health issues that arise from the impact of human
activity on the environment
Describe the functioning of the immune system and how it relates to homeostasis
Thought Questions
Use these questions to help you to begin thinking about the immune system.
A. What are the disease causing organisms and how do they invade the body?
B. How is the body able to fight off disease causing organisms?
C. What happens when the immune system does not work properly?
The Body’s Defences
Every moment of the day, the body is bombarded with disease causing organisms
called pathogens that could cause harm. For the most part, we are unaware that this
occurs due to the body’s defences. It is only when one of these pathogens manages to
enter our systems and begins to multiply that we show the symptoms of the attack.
Pathogens come in many forms. Bacteria are small single celled organisms that will
multiply in our tissues. Viruses lack the cellular machinery to replicate so they invade
our cells, literally hijack them and force the replication of more virus particles. Viruses
are very cell specific meaning that certain viruses will only invade certain cell types. In
the last couple of decades, a new pathogen has appeared on the horizon called the
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Lesson 16
prion. This name is derived from its meaning of proteinaceous infectious particle.
Prions are abnormal proteins that can influence normal proteins to assume an abnormal
shape. The best known example of this is mad-cow disease. At this point, there are
very few defences against them.
The First Line of Defence
The single greatest defence against pathogens is the skin. It provides a tough
protective barrier that cannot usually be penetrated as long as it is intact. But, there are
regions of the body where this tough form of skin is not present and other defences are
needed. The mucous membranes around the body openings are vulnerable to
pathogens. Some membranes release acidic secretions, maintaining an acidic
environment where pathogens cannot grow. Good examples of this would be the mouth
and the female reproductive tract. Tears, saliva, mucous secretions and sweat all
contain lysozyme, an enzyme that breaks down bacterial cell walls. The mucous
secretions and the cilia (tiny hairs) lining the respiratory tract, trap microbes and debris
while coughing expels them. The hydrochloric acid in the stomach, along with protein
digesting enzymes destroys any bacteria carried into the body with food. If a microbe
manages to bypass these defences, the second line of defence is mobilized.
The Second Line of Defence
This line of defence is called the non-specific immune response. This means that it
responds to any form of pathogen that is detected and responds in a same way. Cells
from the immune system constantly travel through the body’s tissues searching for
foreign material that should not be present. When this foreign material is detected,
white blood cells (WBC) called leukocytes begin the attack by engulfing the pathogen.
It also sends out a signal which causes the inflammatory response. Blood flow to the
site of the attack increases. This produces swelling, redness, heat and pain at the site
but, it also allows more WBC to arrive at the site quickly. The next WBC to arrive is
called the monocyte. A number of them will join together to form large macrophages
which will engulf the pathogens, releasing an enzyme that destroys them. Yet another
type of WBC, the neutrophil ensures that everything is destroyed. It engulfs the
pathogen and then releases enzymes that will destroy them both. The remains of the
attack, namely the fragments of protein, dead WBC and the dead invader are called
pus. We tend to experience this with localized infections when the skin has been
broken and bacteria have entered. If this attack has not completely eliminated the
pathogen, the third line of defence is called into action.
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Figure 16-1 The Non-Specific Immune Response
Source: Di Giuseppe et al. 463
The Third Line of Defence – The Immune Response
This line of defence is usually engaged when a bacterial infection runs out of control
and becomes system wide, or a virus runs out of control destroying its target cells in an
effort to replicate more virus particles. In this case, a response more specific to the
pathogen is question is required. Lymphocytes are the specialized WBC that are
responsible for the body’s more specific immune response. To help slow down the
invading pathogen, the neutrophils and macrophages release a chemical that travels
through the bloodstream to the hypothalamus. These chemical reset the body’s
thermostat at a higher temperature of approximately 40°C. This increased temperature
makes it difficult for bacteria to survive and gives the body’s defences time to work. We
experience this as a fever. In reality, reducing a fever can actually prolong the infection.
If body temperature rises above 40°C it can be detrimental with cell death occurring
above 43°C.
Every type of bacteria and virus particle has characteristic proteins attached to its
surface that can be used to identify it from other cells. These protein patterns are called
antigens. T-cells, produced in the bone marrow and stored in the thymus, are able to
seek out and identify foreign intruders by these antigens and signal the attack. As the
macrophages arrive and engulf the invader, the antigens on the surface of the invader
are pushed to the cell surface of the macrophage. Helper T-cells copy the shape of the
foreign antigens and release a chemical messenger called lymphokine which causes
the B-cells to divide rapidly. The helper T-cells then take the information about the
foreign antigens to the B-cells so that specific antibodies can be mass produced.
Antibodies are molecules that are designed to fit over the antigens on the surface of the
invader. Each type of antibody will specifically fit the antigens on that particular invader.
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Lesson 16
Figure 16-2 The Difference Between Self and Foreign Antigens
Source: Di Giuseppe et al. 467
Meanwhile, the attack on the pathogen continues. Cytotoxic or killer T-cells are also
activated to help destroy the pathogen. These cells are very generalized in their attack.
They release enzymes that destroy not only the pathogens, but the body’s own cells as
well. When you are sick with a cold or the flu, you experience this part of the attack
when your throat feels sore and raw. The killer T-cells are in the process of destroying
the virus but, the epithelial cells lining your throat, which the virus likes to invade, are
destroyed as well. This defence by the killer T-cells and the macrophages will continue
until the antibodies arrive at the scene.
This war that occurs in the body between the pathogen and white blood cells, boils
down to a numbers game. This form of immune response takes time which gives the
pathogen the opportunity to multiply is vast numbers. By the time the immune system
initiates this form of attack, there are millions upon millions of pathogens present. The
macrophages and killer T-cells are doing their best, but they are vastly outnumbered.
Even as they are destroying the pathogens, more are being created. It is the antibody
that helps to even the odds.
Once the antibody arrives, its specifically designed Y-shaped form fits over the antigens
on the surface of the pathogens. This neutralizes the pathogen, preventing it from
multiplying further or invading more cells. This stabilizes the number of pathogen
particles and allows the macrophages and other T-cells the time needed to destroy the
pathogens that are present. When the battle has been won, suppressor T-cells signal
the immune system to terminate the response. Most of the antibodies and T-cells will
die off but, some will remain to guard the site to ensure that the battle is truly over. The
macrophages will continue to clean up the site until all the pathogens are eliminated.
Memory B-cells are created that carry an imprint of the foreign antigens. They will
survive and continue to circulate through the body. If the pathogen ever enters the
system again, the memory B-cells will quickly identify it, allowing it to be destroyed
before it can establish itself in the body. The individual is immune to that invader.
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Figure 16-3 The Specific Immune Response
Source: Di Giuseppe et al. 469
Please visit the following website(s)
http://highered.mcgraw-hill.com/sites/dl/free/0072507470/291136/immResponse.swf
Support Questions
(do not send for evaluation)
1.
Describe three ways in which a pathogen can enter the body.
2.
Describe three body defences that prevent pathogens from entering the body.
3.
Explain in your own words the function of each of the following: killer T-cells,
helper T-cells, suppressor T-cells and memory T-cells.
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Lesson 16
Active and Passive Immunity
When immunity is created because of an immune response, it is called active immunity.
The body is now on guard against that particular invader and will not be given the
chance to cause any further problems. If the pathogen ever enters the system again,
we are unaware of it because it is destroyed before is causes any damage. Passive
immunity is acquired when antibodies are passed directly into the body. This transfer
occurs between mother and baby through the placenta. Some antibodies are also
passed on during breastfeeding. This type of immunity will only last for a period of time.
Vaccination
Vaccination or immunization is a way to create active immunity without having to suffer
the full effects of the pathogen. This process began in ancient China where bits of
smallpox diseased skin were inhaled by healthy people. In Western culture, Edward
Jenner developed the first vaccine against smallpox. He made use of a similar disease
called cowpox which activated the immune system to produce antibodies against both.
Since then, many other vaccines have been produced. Louis Pasteur, who invented the
technique of pasteurization which is used to reduce the amount of microbes in milk, also
invented the vaccine for rabies. There is no virus closely related to rabies so Jenner’s
method could not be used. Instead, by trial and error, Pasteur managed to weaken the
virus so that it could be injected without causing the disease but, antibodies could still
be created. In the mid-twentieth century, Jonas Salk used formaldehyde to inactivate
the polio virus and create a usable vaccine.
Today, vaccines are developed in the same manner as those earlier vaccines. Some
form of the virus, a dead form or a weakened form or even part of the virus is used to
activate the immune response to create antibodies which will prevent the disease from
invading the system. Immunizations are now routine and required by law. The more
traditional vaccines immunize against diphtheria, tetanus, polio, pertussus (whooping
cough), measles, mumps and rubella (German measles). More recently, vaccines that
immunize against chicken pox, hepatitis, and certain forms of pneumonia and meningitis
have been added to the list.
This use of the immune system has saved countless lives. In fact, smallpox, the original
virus that first led to the creation of a vaccine, has been eradicated from the Earth. Only
samples of the virus are left and they reside in the freezers of the Centers for Disease
Control in Atlanta and a government research facility in Russia.
Malfunctions of the Immune System
The immune system is vital to the body to defend against pathogens so the body can
carry of its normal functions. Sometimes though, the immune system malfunctions
preventing the body from functioning normally.
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SBI4U – Biology
Lesson 16
Allergies
An allergy is an exaggerated response of the immune system to a harmless material.
For some reason, the system responds as if a foreign pathogen has entered the body.
There are two main types of allergic reactions: immediate and delayed. The immediate
reaction is the most common and occurs within seconds of exposure to the allergen.
When this occurs, specialized antibodies trigger the release of histamines which
increase the permeability of blood vessels. This causes the area to become red and
swollen and can also cause the release of cellular fluid to the area, resulting in watery
eyes and a runny nose. Food allergies can trigger vomiting, cramps and diarrhea.
These symptoms can be alleviated with antihistamines.
Asthma, which is the most common chronic disease in North American children, can
occur when allergens are inhaled. The subsequent release of histamines causes the
passages of the lungs to spasm and constrict making it difficult to breathe.
The delayed allergic reaction is initiated by T cells which have become sensitized to an
allergen due to previous exposure. This type of reaction occurs more slowly but also
lasts longer.
Food allergies are becoming an ever increasing concern. It is uncertain as to whether
the cause is due to more food additives or if environmental factors play a role in
increasing sensitivity. There has been a marked increased in the number of children
who suffer from peanut allergies and it continues to rise. In a severe food allergy
reaction, known as anaphylaxis, people can die if they do not get treatment. Once
present in the circulatory system, the antigen triggers a massive release of histamine.
This causes the dilation of the blood vessels throughout the cardiovascular system
resulting in the leakage of fluid and proteins out of the capillaries. This leakage causes
a rapid drop in blood pressure and reduced blood flow to the organs and tissues of the
body. Emergency treatment involves using epinephrine which increases heart rate and
strengthens the heart’s contractions. Today, many people with a severe allergy carry a
dose of epinephrine in the form of an Epi-pen to administer as soon as the attack
begins.
Health Canada has identified the following as the nine top food allergens in Canada.
1)
2)
3)
4)
5)
peanuts
tree nuts
sesame seeds
milk
eggs
6)
7)
8)
9)
seafood (fish, crustaceans and shellfish)
soy
wheat
sulphites (a food additive)
Copyright © 2009, Durham Continuing Education
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SBI4U – Biology
Lesson 16
Autoimmune Diseases
In this situation, the immune system cannot recognize self. By this, it means that the
system, for unknown reasons, sees the body’s own cells as being foreign and attacks
them. It is the destruction of the cells that triggers the symptoms of the disease. Some
theories suggest that all systems carry renegade T and B cells that are capable of
attacking the body but the suppressor T cells manage to keep them in check. The
problem arises when the suppressor T cells are weakened or their numbers decline.
Rheumatoid arthritis surfaces when the immune response is mounted against the bones
and connective tissues of the joints. Type I diabetes is caused by an attack on the
insulin-producing cells of the pancreas. Lupus is caused when the antigen-antibody
complexes build up in the walls of blood vessels, joints, kidneys and skin. Multiple
sclerosis is a disease caused when the myelin sheath around nerve cells is destroyed.
This reduces and prevents nerve signals from being sent, eventually leading to
paralysis. So far, there are no direct cures but immune suppressor drugs have helped
to reduce the severity of the attacks.
Support Questions
(do not send for evaluation)
4.
Describe how vaccinations provide protection against pathogens.
5.
Explain how the immune system causes autoimmune disease.
Key Question #16
1.
As you are hurrying around, you get a splinter in your hand. You pull it out, but
later that night your finger becomes swollen and red. A few days later, the entire
hand is swollen, the pain is intense and you develop a fever. Explain what has
been happening in each step of this situation. (6 marks)
2.
You are an immunologist and have a patient who just received a kidney
transplant. How do you explain to him that he must be given
immunosuppressant drugs? (4 marks)
3.
Allergies are the most common type of immune system disorder. Describe an
allergic reaction and explain why it may be harmful. (5 marks)
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SBI4U
Grade 12, University Preparation
Biology
Support Question Answers
SBI4U – Biology
Support Question Answers
Answers to Support Questions
Lesson 13
1.
In response to extreme heat, blood vessels in the skin dilate. This increases
blood flow from arteries into the capillaries of the skin, lowering blood pressure.
In addition, the loss of body fluids from extensive sweating will reduce the volume
of body fluids and lead to a reduction of fluids in the blood. A decrease in blood
volume will cause a decrease in blood pressure.
2.
a. This is a positive feedback system.
b. This will occur when the baby is full and stops sucking.
3.
Ecstasy interferes with the hypothalamus’s ability to regulate body temperature.
A rapid increase in core body temperature inactivates many enzymes, slowing
down metabolic activity. Essential biochemical pathways that inactivate poisons
and produce needed chemicals are impaired. This can lead to tissue damage
and in extreme cases, death. Ecstasy also increases the production of ADH
which prevents the body from excreting urine and makes the mouth and throat
feel dry. While the ecstasy is making a person thirsty, the high level of ADH is
preventing urination. Water is not eliminated from the body and this can cause
the brain to swell, leading to coma or even death.
4.
The three main processes are: filtration which involves the movement of fluids
from the glomerulus into Bowman’s capsule under high pressure. Reabsorption
involves the movement of fluids from the nephron back into the bloodstream.
Secretion involves the transport of solutes into the distal tubule of the nephron.
5.
The correct order should be: B, C, E, F, A, and D.
6.
The table should be completed as follows.
Kidney Disease
Cause of Problem
Diabetes mellitus
Lack on insulin
production
Low ADH
Diabetes insipidus
Bright’s disease
Multiple conditions
where the nephrons are
damaged
Kidney stones
Mineral solutes in blood
precipitate
Problem created by
disease
Glucose in urine will cause
dehydration
Less water reabsorbed,
urine output increases
dramatically
Proteins and other large
molecules are able to pass
into the nephron drawing
water into the nephron
increasing urine output
The sharp-sided stones may
become lodged in the ureter
or urethra tearing tissues
Copyright © 2009, Durham Continuing Education
Recommended
treatment
Insulin injection
ADH provided by
injection
None
High energy sound
waves to smash the
stones or surgery
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SBI4U – Biology
Support Question Answers
Lesson 14
1.
a. One possible hypothesis is that the thyroxine supplements will cause the
metabolic rate to go up and lead to weight loss.
b. The control group will be the rats on diet A since there are no supplements
and no nutrients are missing.
c. Group II was likely fed diet B because the thyroxine extract would cause them
to lose weight while the increased metabolic rate would lead to a higher
consumption of oxygen for cellular respiration.
d. Group IV was likely fed diet D because of the effect of the absence of iodine.
There is weight gain since the metabolic rate would drop shown by decreased
oxygen consumption.
2.
Doctors are hesitant to prescribe cortisone for long term treatment because it is a
form of cortisol. Over the long term it would cause the following symptoms.
Mood changes
Increase in bone deterioration
Weight gain due to fluid retention
Increase in blood sugar
Increase in blood pressure
Decrease in skin thickness
3.
Stress hormones provide more blood glucose to cope with the elevated energy
requirements brought on by stress. An elevation in blood pressure ensures that
the body’s tissues are receiving an enhanced supply of oxygen and nutrients
during a stress situation.
4.
a. The adrenal cortex has been removed. The cortisol exerts a negative
feedback response that normally decreases the ACTH in the blood. If cortisol
is not produced by the adrenal cortex, ACTH cannot be used, so ACTH levels
rise.
b. Insulin has been injected. Insulin increases the permeability of cells to
glucose. Glucose diffuses from the blood into the body’s cells, decreasing
blood glucose levels.
c. ADH was affected. ADH is produced in the posterior pituitary and is normally
carried in the blood. If blood cannot move from the posterior pituitary to other
regions of the body, ADH also does not move. ADH signals the body to
reabsorb water from the kidneys. If no ADH reaches the kidney, water
reabsorption does not occur and urine production increases.
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SBI4U – Biology
Support Question Answers
d. Glucagon was injected. Glucagon converts glycogen to glucose in the liver.
Glucose diffuses into the blood.
Lesson 15
1.
The dendrites are projections of cytoplasm that carry nerve impulses toward the
cell body. The myelin sheath is a fatty covering over the axon of a nerve cell that
speeds the rate of an impulse along a nerve. Schwann cells are the cells that
compose the myelin sheath. The cell body is the area of the nerve that contains
the nucleus, and the axon is the extension of the cytoplasm that carries the nerve
impulse to other nerves or effectors.
2.
The resting cell membrane is more permeable to potassium than to sodium.
Thus, the outside of the nerve is positive relative to the inside. An impulse
creates an action potential that causes the sodium gates to open, allowing
sodium ions to diffuse into the nerve, depolarizing the membrane. This
depolarization causes the sodium gates to shut and opens the potassium gates.
A sodium-potassium pump restores the resting potential of the membrane by
transporting sodium ions out of and moving potassium ions into the neurons.
This is referred to as repolarization and it occurs during the refractory period.
The sodium gates in adjacent areas open and the action potential moves along
the membrane.
3.
Individuals with nerves that have a higher threshold level are not as easily
activated. This means that it takes an impulse of greater intensity to activate
these sensory neurons so the person tends not to feel pain as easily.
4.
Without acetylcholine being produced, postsynaptic membranes would not be
depolarized. Nerve transmission would be severely inhibited because although
sensory nerves would respond to stimuli, motor neurons could not be excited.
Walking, sitting and other movements that require motor nerves could not occur.
Even breathing movements would stop causing death.
5.
If the corpus callosum is severed, then the right and left hemispheres of the brain
cannot communicate. The speech centre of the brain is in the left hemisphere. If
an object is placed in front of a person so that only his or her left eye can see it,
then the right side of the brain receives the signal that there is an object present.
If the person is asked what the object is, he or she cannot respond because the
speech centre is on the left side of the brain. The right and left hemispheres do
not communicate. The person is aware that the object is there and will act as if
the object is present but cannot call it by name.
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SBI4U – Biology
Support Question Answers
Lesson 16
1.
A pathogen can enter the body in the following ways: through a break in the
skin, through the mucous membranes, through the respiratory tract, the eyes and
through the digestive system.
2.
The body’s defences against pathogens include: the tough outer skin, acidic
environments for the mucous membranes, the production of lysozyme, using
mucous and cilia to trap particles and coughing to expel them, the non-specific
immune response and the specific immune response.
3.
The killer T-cells are able to kill the pathogen and any cells in the area that are
infected. The helper T-cells make a copy of the antigens to pass on to the Bcells so antibodies can be made. Suppressor T-cells shut off the immune
response once the pathogen is destroyed. The memory T-cells retain a copy of
the antigen so that the body will recognize and destroy the invader if it ever
enters the system again.
4.
Heat-killed or weakened pathogens are injected into the body. These pathogens
are not strong enough to produce the actual disease but they will stimulate
antibody production. The antibodies will then already be present in the blood
should the person be exposed to the actual pathogen. The antibodies will then
destroy the pathogen before it can take hold and multiply.
5.
Renegade B cells produce antibodies against the body’s own tissues and the T
cells attack these tissues. Normally suppressor T cells prevent these B and T
cells from destroying tissues but in some cases the suppressor T cells fail and
the attack takes place.
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