EXCRETION AND HOMEOSTASIS
Excretion is the removal of
waste products and harmful
substances that are formed by
metabolic activity from the
body.
Excretion
These products include water,
carbon dioxide, oxygen,
nitrogenous compunds (
ammonia, urea and uric acid)
and bile pigments.
The word metabolism is used to describe the
thousands of chemical reactions that take place
inside of cells.
Importance
of Excretion
Cell metabolism breaks down compounds and
makes new compounds.
The waste products formed during metabolism
could be harmful to the body if they allowed to
build up in the body, so they need to be
removed by excretion.
Excretion is vital so that the internal
environment is kept constant so that cells in
the body can function efficiently.
Roles of the Excretory Organs
Metabolic Waste
Source of Waste
How waste is excreted
Carbon dioxide
Aerobic respiration
Removed from the blood in the alveoli
and breathed out of the lungs during
expiration.
Mineral salts
Absorbed from food when in
excess
Removed from the blood in sweat
glands and deposited on the surfaceof
the skin.
Removed from the blood by the
kidneys and incorporated in urine.
Water
Absorbed from food when in
excess; produced in some
metabolic reactions.
Breathed out by the lungs during
expiration. Removed from the blood
in sweat glands and evaporated as
sweat. Removed from the blood by
the kidneys and incorporated in urine.
The Excretory System
Metabolic Waste
Source of waste
How waste is excreted
Urea
Breakdown of surplus
amino acids in the liver
(deamination)
Small amount removed from
the blood in sweat glands
and deposited on the surface
of the skin. Most of the urea
is removed from the blood
by the kidneys and
incorporated in urine.
Blood pigments
Breakdown of haemoglobin
from old red blood cells in
the liver
Released into the gut as prt
of bile
•
•
•
The Urinary System
In humans, the kidneys
are the main excretory
organs.
There are paired
organs which form part
of the urinary system.
The job of the kidneys
is to take unwanted
substances from the
blood and to pass
them on to the
bladder, to be
excreted.
The kidneys have four main functions:
Functions
of the
Kidneys
They are the main organs for excretion of waste
products such as urea.
They play a role in regulating the water content
of the body (osmoregulation).
They help to maintain the osmotic pressure of
body fluids by secreting excess salts and by
maintain water as needed.
They regulate the pH of the blood by controlling
the acid-base balance in the blood.
The
Structure
of The
Kidney
The kidney has three main
parts – the cortex, the
medulla and the pelvis.
Leading from the pelvis is
a tube called the ureter.
The ureter carries the
urine that the kidney has
made to the bladder.
The kidneys are made up of thousands
of tiny tubules or nephrons.
The
Structure
of the
Kidney
Each nephron begins in the cortex,
loops down into the medulla, back
into the cortex, and then down again
through the medulla to the pelvis.
The nephrons join up with the ureter
in the pelvis.
THE STRUCTURE OF A NEPHRON
THE STRUCTURE OF A NEPHRON
The Structure of A Nephron
Filtration
• Filtration is the process which separates
particles of different sizes. The term
ultra-filtration is used when the
particles being separated are very small
.
Blood is brought to the Bowman’s capsule in a
branch of the renal artery.
Filtration in
the
Bowman’s
Capsule
Blood in the glomerulus is under high pressure.
This comes about because the blood vessel
(afferent arteriole) entering the glomerulus has a
larger internal diameter that the blood vessel
(efferent arteriole) leaving the glomerulus.
This creates a build up of high pressure in the
capillaries. This pressure forces small molecules
from the blood into the Bowman’s capsule.
Filtration in
the
Bowman’s
capsule
Small molecules, including water,
amino acids, glucose, mineral salts
and urea are squeezed out of the
blood into the Bowman’s capsule.
The mixture of small molecules and
ions which forms inside the
Bowman’s capsule is called the
ultra filtrate.
Red blood cells and large
molecules such as proteins remain
in the blood.
The ultra-filtrate passes from the Bowman’s
capsule into the proximal convoluted tubule.
Selective
Reasbsorption.
The cells lining the proximal convoluted
tubule absorb substances from the ultrafiltrate and them back into the bloodstream.
As the ultrafiltrate travels through the
nephron, glucose, mineral salts, amino acids
and other useful substances are reabsorbed
into the blood by active transport ( energy is
required).
The loop of Henle’ works to
increase the salt content of
the tissues in the medulla.
The Loop
Of Henle’
This helps in the
reabsorption of water from
the ultra-filtrate in the
second coiled tubules and
collecting ducts.
The distal convoluted tubule and
the collecting duct are also
surrounded by a capillary network.
Reabsorption
in the Distal
Convoluted
Tubule
Cells lining this tubule absorb
mineral salts from the ultra filtrate
and pass them back into the
capillary.
The major function of the distal
convoluted tubule is to absorb
water.
After reabsorption, the
remaining fluid continue on its
way along the nephron.
URINE
By the time it gets to the
collecting duct, it is mostly water
, with urea and salts dissolved in
it. It is called urine.
The urine from all of the
nephrons in the kidneys flows
into the ureters. The ureters take
it to the bladder.
The urethra is a long tube which leads
out of the bladder.
URINATION
The urethra has a sphincter muscle at
the top which is usually tightly closed.
When the bladder is full, the sphincter
muscle opens, so that urine flows
along the urethra and out of the body.
How the kidneys are adapted for
reabsorption
Feature
Explanation of how this helps absorption
Tubules are long
Increases surface area for reabsorption
Tubules are coiled
Allows tubules to be long but contained
within a small space
Each kidney has over a million tubules
Increases surface area for reabsorption
Cells lining the tubule have microvilli on
their inner surface membrane
Increases the surface area of each cell
lining the tubule without increasing the
overall size.
Cells lining the tubule have many
mitochondria in their cytoplasm
Mitochondria produce ATP which is a
source of energy for the reabsorption
process.
Kidney Dialysis
Kidney Dialysis
For our bodies to work properly
the conditions surrounding the
cells which make up the body must
be kept as constant as possible.
Homeostasis
It is vital that our bodies maintain
a constant internal environment,
which is not influenced by external
conditions.
The maintenance of a constant
internal environment is known as
HOMEOSTASIS.
The nervous and hormonal
systems play an important role in
maintaining this important
balance in our bodies.
FEEDBACK
MECHANISMS
Feedback mechanisms play an
important role in Homeostasis.
However, most of the control
systems in the body are examples
of negative feedback.
NEGATIVE
FEEDBACK
When the levels of a
substance in the body
rise, changes are made
which lower the levels
again.
When levels of a
substance fall, changes
are made so it rises again
to the original levels.
OSMOREGULATION
The amount of water lost from the
kidney is controlled by the sensitive
feedback mechanism involving the
Anti-Diuretic Hormone (ADH).
If the water content of the blood is too low
(so the salt concentration of the blood
increases) special sense organs known as
osmoreceptors in our brain detect this.
Low water
content in
the Blood.
They stimulate the pituitary gland in the
brain to release ADH into the blood.
This hormone affects the distal convoluted
tubules of the kidneys, making them more
permeable to water so more water is
reabsorbed back into the blood.
This means that less water is
left in the kidney tubules so a
more concentrated urine is
formed.
As a result, the amount of
water in the blood increases
and so the concentration of
salts in the blood return to
normal.
What happens
when there is
high water
content in the
blood(dilute
blood
concentration)?
If the water content of the blood is too
high, the pituitary gland secretes
much less ADH (or none at all) into the
blood.
The kidney then reabsorbs less water
back into the blood, producing a large
volume of dilute urine.
Water is lost from the blood and the
concentration of salts return to
normal.
This system of
osmoregulation is an example
of negative feedback.
As the water concentration in
the blood falls, the level of
ADH produced rises.
As the concentration of water
in the blood rises, the level of
ADH released falls.
The sugar carried in the blood is
glucose.
Regulation
of Blood
Sugar
The concentration of glucose in the
blood is monitored closely by the
pancreas and a normal
concentration of 80mg per 100 cm3
The pancreas and the liver are the
organs responsible for controlling
blood sugar.
There are a special group of cells in
the pancreas known as the Islets of
Langerhans. These cells secrete two
hormones, insulin and glucagon.
Regulation
of Blood
Sugar
If the blood glucose concentration
increases e.g after a heavy meal, the
cells detect this and release more
insulin and less glucagon.
The insulin travels in the blood to the
liver and tells it to do a number of
things.
The Liver under the influence
of the hormone insulin:
Regulation
of Blood
Sugar
It converts glucose to
glycogen and stores it
It converts glucose to fat
As a result the blood glucose
concentration decreases.
If the blood glucose decreases, the reverse
happens.
Control
of Blood
Sugar
The islets of Langerhans detects a decrease
in glucose concentration in the blood and
secretes more glucagon and no insulin.
The glucagon causes the liver to convert
glycogen in to glucose.
This increases the blood glucose
concentration.
The Control of Blood Sugar
Control of
CO2
The breathing centre located
near the rear of the brain
(medulla oblongata) regulates
our breathing movements.
This breathing centre (also
called the master centre of the
body) uses special
chemoreceptors to measure
CO2 concentrations in the brain
and blood in the arteries.
When the blood carbon dioxide
concentration increases, this is
detected by these special
chemoreceptors in the blood
vessels e.g aorta.
Impulses are sent to the
respiratory centre in the brain.
The brain sends impulses to the
intercostal muscles and
diaphragm, causing their rate of
contraction to increase. This
causes the depth and rate of
breathing to increase, which
removes the carbon dioxide
from the body.
The skin is the largest
organ in the body.
The Skin
It helps us to resist
infection, excrete and
avoid dehydration.
It also has a major role to
play in controlling our
body temperature.
The skin cntains two
distinctive layers - the
epidermis and the dermis:
The EPIDERMIS protects the
deeper layers of the skin,
this is the layer that is mostly
affected by sun radiation.
The epidermis is split again
into two layers:
• The Malpighian layer produces all of the cells that
make up the epidermis. These cells are constantly
dividing by mitosis (a process that produces two
identical cells from one original cell). The newly
produced cells gradually move towards the surface of
the skin. As they do this, they slowly die and fill up with
keratin, a protein. These cells contain melanin which
protects the tissues underneath against ultra-violet
light.
• The Cornified layer is made up of these dead cells. The
Cornified layer protects the softer living cells
underneath from damage by friction, as the dying cells
become harder and are waterproof. There are various
parts of the body where the skin experiences much
more wear, for example the bottom of the feet. Here,
the Cornified layer grows thicker and is much more
durable.
Melanin
• This pigment absorbs the sun’s harmful ultraviolet rays. These rays can, in large enough
doses, damage the genetic information (DNA)
stored in the nucleus of each cell. This will in
turn cause the cell to die.
• Slightly smaller doses may alter the DNA
which can lead to various forms of skin cancer.
This can be more threatening than the cells
dying.
The DERMIS
• The dermis is made up of connective tissue
which contains elastic fibres. These fibres give
the skin its stretching abilities. As a person
ages, these fibres lose elasticity causing the
skin to become loose and wrinkly.
• It contains:
• Sweat glands which secrete sweat (mostly
made up of water with dissolved amounts of
urea and salt), to aid in the body’s
temperature control.
The Dermis
• Blood vessels (arteries and capillaries) which
supply the oxygen crucial to the process of
mitosis in the epidermis layer of the skin. They
also carry blood containing heat to the skin to
help in temperature regulation.
• Nerve endings (receptors) are sensitive to touch,
pain, pressure and temperature, keeping you
aware of your surrounding environment.
• Adipose (fat) layer lies beneath the epidermis.
This is a layer of triglycerides. This fat helps
insulate your body (slows down loss to heat gain)
and acts as a reserve energy store, if needed.
The Dermis
• Hair and Hair erector muscles aid in
temperature regulation as they raised and
lowered by the muscles.
• Sebaceous glands secrete sebum which
prevents the skin from cracking; it helps to
waterproof the skin and also inhibits the
growth of bacteria.
Temperature Regulation
o Heat and temperature
Heat is a form of energy usually measure in
joules.
Temperature is a measure of the degree of
hotness usually measured in degrees Celsius
or degree Farenheit
Temperature Regulation
• The body’s internal temperature needs to be kept
constant as any significant variation in the body
temperature could have damaging effects on the
body’s enzymes.
• Body enzymes work best at temperatures around
37oC.
• The skin works together with receptors in the
hypothalamus to control body temperature.
• Any changes to the external temperature are
sensed by the skin, while changes in the
temperature of the body are sensed by the
hypothalamus.
When the Body temperature falls
• When body temperature drops below the
normal 37oC, the hypothalamus is activated.
• This sends a message (via the nerves) to the
blood vessels in the skin and they constrict
(close up) to prevent heat loss.
• At the same time, the hair on the skin stand
uo so that they can act as insulation and trap a
layer of warm air next to the skin. This is why
you get goose bumps when you get cold.
• If the body temperature falls further,
messages are sent to the skeletal muscles and
they start to contract and relax very quickly,
causing you to shiver.
• Shivering generates heat and your body
temperature rises as a result.
When the Body temperature rises
• When your body temperature rises above normal
, the hypothalamus sends messages to the blood
vessels to make them dilate ( open wider)
• This allows more blood to flow close to the
surface of the skin and more heat is lost to the
environment.
• The hypothalamus also activates sweat glands, so
you sweat more, so you lose heat through the
sweat.
• The evaporation of sweat also helps to lower the
body temperature because it cools the body.
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