10.1 – Overview of the Excretory System
(Taken from Biology 12, MHR, 2011)
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The excretory system (urinary system) produces urine and conducts it to outside
the body.
Functions of the Excretory System
1) Excretion of Metabolic Wastes
- The kidneys excrete metabolic waste (mainly nitrogen-containing waste).
- Nitrogenous wastes include ammonia, urea, and uric acid.
2) Maintenance of Water-Salt Balance
- By regulating salts in the blood, the kidneys are involving in regulating blood
pressure.
- The kidneys also maintain appropriate levels of potassium (K+), bicarbonate
(HCO3-), and calcium (Ca2+) in the blood.
3) Maintenance of Acid-Base Balance
The kidneys monitor and help keep the blood pH at about 7.4 by excreting
hydrogen ions (H+) and reabsorbing the bicarbonate ions (HCO3-) when needed.
4) Secretion of Hormones
The kidneys secrete two hormones: calcitriol and erythropoietin.
Calcitriol (active form of vitamin D) promotes absorption of calcium (Ca2+) from the
digestive system.
Erythropoietin stimulates the production of red blood cells and is released in
response to increase oxygen demand.
The kidneys also secrete renin which leads to the secretion of aldosterone from the
adrenal cortex.
Organs of the Excretory System

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See Figure 10.2, pg 445
Kidneys are supplied “junk” blood by renal arteries.
Renal veins return “clean” blood to the heart so it can be circulated back around
the body.
At any given time, up to 25% of the body’s blood is held by the kidneys.
The waste that filters out of the blood collects in the ducts of the kidneys and is
moved into the bladder via the ureters.
At the base of the bladder is a sphincter muscle that acts as a valve, permitting the
storage of urine.
As the bladder fills, first to 200mL, then 400mL, it activates stretch receptors and
sends a message to the brain to urinate.
If this is ignored and the bladder fills up to 600mL, the sphincter loses control, and
the urine enters the urethra.
The Kidneys
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The kidneys are bean shaped and reddish in colour.
Figure 10.3, pg 446 illustrates a cross-section of the kidney

the outer layer, the renal cortex, encircles the kidney

the inner layer, the renal medulla, is found beneath the cortex

a hollow chamber, the renal pelvis, joins the kidney with the ureter
The Nephron
Nephrons are functional units of the kidney. They are embedded within the renal
cortex and extend into the renal medulla.
The nephrons are responsible for filtering various substances from blood,
transforming it into urine.
Each nephron is organized into three main regions: a filter, a tubule, and a
collecting duct.
See Figure 10.4, pg 447
Filter
Tubule
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Each nephron is supplied with blood by the afferent arterioles, which branch into
a capillary bed, called the glomerulus  a cluster of arterioles enclosed in a
capsule is called Bowman’s capsule.
The glomerulus act as a filtration device.
They are impermeable to proteins, large molecules, and red blood cells therefore
they remain in the blood.
Water, small molecules, ions, and urea are filtered from the blood. This filtered
fluid that enters the Bowman’s capsule is referred to as filtrate and will eventually
become urine.
Blood leaves the glomerulus through other arterioles, the efferent arterioles.
The tubule has three sections: the proximal tubule, the loop of Henle, and the
distal tubule.
This tubule absorbs substances that are needed by the body (glucose and variety
of ions) from the filtrate.
The tubules also secrete substances into the tissues surrounding it.
Collecting Duct
The collecting duct is a large pipe-like channel that functions as a waterconservation device, reabsorbing water from the filtrate so that little water is lost
from the body.
The remaining filtrate is now called urine.
Summary of Nephron Structure and Description
Structure
afferent arteriole
glomerulus
efferent arteriole
peritubular capillary bed
venule
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HOMEWORK:
Description
-carries blood to the glomerulus
-a high-pressure capillary bed enclosed by the
Bowman’s capsule
-carries arteriolar blood away from the glomerulus
-capillaries that network around the nephron
-reabsorbs solute from the nephron into the blood
and secretes solute from the blood into the nephron
-carries filtered blood back to the heart
pg 448 #1-10
10.2 – Urine Formation in the Nephron
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There are 4 processes that result in the formation of urine: glomerular filtration,
tubular reabsorption, tubular secretion, and water reabsorption.
1)
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Glomerular Filtration
The movement of water and solutes from the blood into the Bowman’s capsule
(nephron) is called filtration. This fluid is called the filtrate.
Each nephron of the kidney has an independent blood supply, and blood moves
through the afferent arteriole into the glomerulus, a high pressure filter.
The pressure in the Bowman’s capsule is about 4x as much as in normal capillary
beds.
Plasma proteins, blood cells, and platelets are too large to move through the walls
of the glomerulus.
Smaller molecules pass through the walls and enter the nephron.
According to Table 10.1, pg 451, the filtrate is identical to blood plasma – minus
proteins and blood cells.
2)
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Tubular Reabsorption
Tubule reabsorption removes useful substances (sodium) from the filtrate and
returns them into the blood for reuse by body systems.
About 65% of the filtrate that passes through the proximal tubule and loop of
Henle is reabsorbed and returned to the body.
If none of the filtrate were reabsorbed, you would form 120mL of urine each
minute, and would be requiring 1L of fluids every 10 minutes to maintain water
balance.
Both active and passive transport help reabsorb the fluids.
See Figure 10.7, pg 451
Active Transport
Nutrients (e.g., glucose, amino acids, Na+, K+) are actively reabsorbed.
Carrier molecules move Na+ ions across the cell membranes of the cells that line
the nephron.
Reabsorption occurs until the threshold level of a substance is reached, excess NaCl
remains in the nephron and is excreted with the urine.
Other molecules are actively transported from the proximal tubule, such a glucose
and amino acids, as they “hitch a ride” with specific carrier molecules that “shuttle”
them out of the proximal tubule and back into the blood.
Passive Transport
Negative ions, such as Cl-, are passively reabsorbed by electrical attraction to Na+.
Water is reabsorbed by osmosis.
Focussing on the Loop of Henle in the Proximal Tubule
3)
4)
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The function of the loop of Henle is to reabsorb water and ions from the glomerular
filtrate.
The descending limb of the loop of Henle is permeable to water and only slightly
permeable to ions.
As a result of the salty environment of the medulla and permeability of the
descending limb, water diffuses from the filtrate to the capillaries by osmosis.
See Figure 10.8A, pg 452
Near the bend in the loop of Henle (thin portion of the ascending limb), the tubule
is impermeable to water and slightly permeable to solutes.
Sodium ions diffuse from filtrate and pass into nearby blood vessels.
See Figure 10.8B, pg 452
At the thick portion of the ascending limb, sodium ions are moved out of the filtrate
by active transport.
See Figure 10.C, pg 452
By now, about two thirds of the Na+ and water from the filtrate has been
reabsorbed.
Tubular Reabsorption and Secretion in the Distal Tubule
The movement of additional wastes and excess substances from the blood back
into the nephron (filtrate) is called secretion.
Potassium (K+), excess H+ ions, and other minerals are balanced with secretion.
Like reabsorption, tubular secretion occurs by active transport – unlike
reabsorption, molecules are shuttled from the blood into the nephron.
Reabsorption and secretion in the distal tubule are controlled by hormones.
See Figure 10.9
Water Reabsorption from the Collecting Ducts
Water reabsorption removes water from the filtrate and returns it to the blood for
reuse by body systems.
Because the collecting ducts extend into the medulla, the concentration of ions
along its length increases.
This causes the passive reabsorption of water from the filtrate in the collecting
ducts by osmosis.
If the blood plasma is too concentrated, the permeability to water in the distal
tubule and the collecting duct is increased.
The body wants to conserve water so more water is reabsorbed into the
surrounding capillaries.
Hormones also control reabsorption and secretion in the collecting ducts.
A Summary of Nephron Functions (Figure 10.3, pg 454)
Part of the Nephron
Glomerulus
Function
Filtration
 Glomerular blood pressure forces some of the water
and dissolved substances from the blood plasma
through the pores of the glomerular walls
Receives filtrate from glomerulus
Reabsorption
 Active reabsorption of all nutrients, including glucose
and amino acids
 Active reabsorption of positively charged ions such as
sodium (Na+), potassium (K+), and calcium (Ca2+)
 Passive reabsorption of water by osmosis
 Passive reabsorption of negatively charged ions such
as chloride (Cl-) and bicarbonate (HCO3-)by electrical
attraction to positively charged ions
Bowman’s capsule
Proximal tubule
Descending loop of Henle
Ascending loop of Henle
Distal tubule
Secretion
 Active secretion of hydrogen ions (H+)
Reabsorption
 Passive reabsorption of water by osmosis
Reabsorption
 Active reabsorption of sodium ions (Na+)
 Passive reabsorption of chloride and potassium ions
Reabsorption
 Active reabsorption of sodium ions (Na+)
 Passive reabsorption of water by osmosis
 Passive reabsorption of negatively charged ions such
as chloride (Cl-) and bicarbonate (HCO3-)
Secretion
 Active secretion of hydrogen ions (H+)
 Passive secretion of potassium ions (K+) by electrical
attraction to chloride ions (Cl-)
Reabsorption
 Passive reabsorption of water by osmosis
Collecting duct
Animation of the Kidney
http://www.biologymad.com/resources/kidney.swf
HOMEWORK:
pg 455 #1 - 12
Urine Formation
(Table 2, pg 351)
Site
Description of Process
Substances Transported
1.glomerulus and
Bowman’s capsule
-Filtration of water and dissolved solutes
occurs as blood is forced through walls of
glomerulus into Bowman’s capsule by fluid
pressure in capillaries.
2.proximal tubule
-Selective reabsorption of nutrients from
filtrate back into blood by active and passive
transport.
-Within proximal tubule, pH is controlled by
secretion of hydrogen ions (H+) and
reabsorption of bicarbonate ions (HCO3-).
-Descending limb of loop of Henle is
permeable to water, resulting in loss of water
from filtrate by osmosis.
-Salt (NaCl) becomes concentrated in filtrate
as descending limb penetrates inner medulla
of kidney.
-Thin segment of ascending limb of loop of
Henle is permeable to salt, resulting in
diffusion of salt out of ascending limb.
-Salt continues to pass from filtrate to
interstitial fluid in thick segment of ascending
limb.
-Selective reabsorption of nutrients from
blood into nephron by active transport. Distal
tubule helps regulate potassium (K+) and salt
(NaCl) concentration of body fluids.
-As in proximal tubule, pH is controlled by
tubular secretion of hydrogen ions (H+) and
reabsorption of bicarbonate ions (HCO3-).
-Urine formation.
-sodium ions (Na+), chloride
ions (Cl-), water (H2O),
hydrogen ions (H+), glucose,
amino acids, vitamins, minerals,
urea, uric acid
-bicarbonate ions (HCO3-), salt
(NaCl), water (H2O), potassium
ions (K+), hydrogen ions (H+),
ammonia (NH3), glucose, amino
acids, vitamins, urea
3.descending limb
of loop of Henle
4.ascending limb
of loop of Henle
5.distal tubule
6.collecting duct
-water (H2O)
-salt (NaCl)
-salt (NaCl), potassium ion (K+),
water (H2O), hydrogen ions
(H+), bicarbonate ions (HCO3-),
uric acid, ammonia (NH3)
-water (H2O), salt (NaCl), urea,
uric acid, minerals
10.3 – Other Functions and Disorders of the Excretory System
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Basically, the body adjusts for increased water intake by increasing urine output, and it
adjusts for low water levels in the blood by reducing urine output (more concentrated).
In order for these adjustments to take place, 2 body systems must interact: nervous
system and endocrine system.
Regulating Water-Salt Balance
Reabsorption of Water (Blood Volume)
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The body produces a hormone called antidiuretic hormone (ADH) which helps
regulate the osmotic pressure of body fluids by causing the kidneys to increase water
reabsorption.
When ADH is released, a more concentrated urine is produced  conserving water.
ADH is produced by specialized nerve cells in the hypothalamus. The pituitary gland
stores and releases ADH into the blood.
Changes in osmotic pressure in the blood are picked up by specialized nerve receptors
in the brain called osmoreceptors.
If your blood water level drops because of sweating, blood solutes become more
concentrated, which increases the blood’s osmotic pressure – water moves out of cells
and into the bloodstream.
When the cells of the hypothalamus shrink, a nerve message is sent to the pituitary,
signaling the release of ADH.
The ADH is carried by blood to the kidneys, making the tubules more permeable to
water, which allows more water to be reabsorbed into the blood  more
concentrated urine is produced.
A greater amount of water in the blood causes its osmotic pressure to stop increasing
and prevents body cells from losing water to the blood and become dehydrated.
The shrinking of the hypothalamus cells also initiates a behavioural response – the
sensation of thirst.
As soon as the amount of water in the blood increases and the hypothalamus cells
swell, the nerve messages signal the pituitary to stop, ADH is not released, and less
water is reabsorbed from the nephrons.
ADH and the Nephron
- ADH makes the upper part of the distal tubule and collecting duct permeable to water.
The water leaves the upper part of the distal tubule and enters the blood. This makes
the blood more dilute while making the urine more concentrated.
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See Figure 10.11, pg 457
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In a condition called diabetes insipidus, ADH activity is insufficient so the reabsorption
of water cannot occur.
A person with diabetes insipidus urinates excessively – as much as 4 L to 8 L per day.
They may need to take synthetic ADH to restore the balance of water reabsorption.
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Alcoholic beverages and Caffeine
- Both alcohol beverages and caffeine are diuretics  increase volume of urine.
- Diuretics inhibit the release of ADH which decreases the permeability of the tubules
and collecting ducts for water. As a result, less water is reabsorbed into the blood and
more water stays in the urine.
Reabsorption of Salts (Blood Pressure)
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Kidneys play a role in regulating blood pressure by adjusting for blood volumes.
A hormone called aldosterone (produced in the cortex of the adrenal gland) acts on
the nephrons to increase Na+ reabsorption in the distal tubules and collecting ducts.
As NaCl reabsorption increases, the osmotic gradient increases and more water moves
out of the nephron by osmosis.
Blood pressure receptors in the juxtaglomerular apparatus (found near the glomerulus)
detect low blood pressure.
Specialized cells within the structure release renin, an enzyme that converts
angiotensinogen – a plasma protein – into angiotensin.
Angiotensinogen has two important functions:
1) Causes constriction of blood vessels.
Blood pressure increases when the diameter of blood vessels is reduced.
2) Stimulates the release of aldosterone from the adrenal gland.
Aldosterone is then carried in the blood to the kidneys, where it acts on the cells of
the distal tubule and collecting duct to increase Na+ transport. This causes the
fluid level of the body to increase thus increasing blood pressure.
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Since Na+ reabsorption is followed passively by chloride ions (Cl-) and water,
aldosterone has the net effect of retaining both salt and water. As a result, blood
volume and blood pressure increase.
Juxtaglomerular Apparatus
http://www.wisc-online.com/objects/ap2204/ap2204.swf
Regulating Blood pH
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The pH of the body remains relatively constant, between 7.3 and 7.5.
In addition to regulating body fluid volumes and maintaining the composition of salts
in the blood, the kidneys also maintain pH balance.
There are three mechanisms that keep blood pH at about 7.4: acid-base buffer
system, respiration, and the function of the kidneys.
Acid-Base Buffer System
- An acid-base balance is maintained by buffer systems that absorb excess H+ ions or
ions that act as bases.
- Excess H+ ions from metabolic processes are buffered by bicarbonate ions (HCO3-) in
the blood. Carbonic acid (H2CO3) is produced in the process.
- In turn, the carbonic acid breaks down to form carbon dioxide and water.
Respiration
- If H+ ion concentration in the blood increases, the medulla oblongata increases the
breathing rate.
- The carbon dioxide is then transported to the lungs where much of it is exhaled.
- When carbon dioxide is exhaled, the number of hydrogen ions is reduced.
The Kidneys
- The kidneys excrete H+ and reabsorb HCO3- as needed to maintain normal blood pH.
- If the blood is too acidic, H+ is excreted and HCO3- is reabsorbed.
- If the blood is too basic, H+ is not excreted and HCO3- is not reabsorbed.
- Ammonia (NH3) provides another means of buffering and removing the hydrogen ions
in the urine. Remember, ammonia is produced by the breakdown of amino acids.
- See Figure 10.12, pg 459
Releasing Hormones
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The kidneys secrete renin which leads to the secretion of aldosterone from the adrenal
cortex.
The kidneys also secrete two hormones: calcitriol and erythropoietin.
Calcitriol
- If the level of Ca2+ in the blood falls below normal, parathyroid hormone (PTH) is
released by the parathyroid gland.
- PTH stimulates the release of calcitriol by the kidneys.
- Calcitriol (active form of vitamin D) promotes absorption of calcium (Ca2+) from the
digestive system.
Erythropoietin
Erythropoietin stimulates the production of red blood cells and is released in
response to increase oxygen demand or when the oxygen-carrying capacity of
blood decreases.
Disorders of the Excretory System
Urinary Tract Infection
- If the bladder has a bacterial or viral infection, the disorder is called cystitis; if only
the urethra is involved, the condition is called urethritis.
- UTI’s are more common in women.
- Symptoms of UTI’s include painful burning sensation during urination, need to urinate
frequently, and bloody or brown urine.
- Treatment is usually with antibiotics but in more serious cases surgery may be needed.
Kidney Stones
Most stones consist of calcium.
Most kidney stones form due to excess calcium in the urine.
Very painful.
Many stones pass through the urinary tract on their own.
Traditional treatment for kidney stones has been surgical removal followed by a
period of rest.
A new technique called extracorporeal shock-wave lithotripsy (ESWL) uses
high-energy shock waves to break the kidney stones into small fragments. After a
few days, tiny granules from the stone can be voided through the excretory
system.
Figure 10.13, pg 461
Diabetes Mellitus
- Diabetes is caused by the inadequate secretion of insulin from the pancreas.
- Insulin allows glucose to enter a cell (insulin allows the cell membrane to become
permeable).
- Because of the higher concentration of glucose in the kidneys, it opposes the osmotic
pressure of other solutes that have left the nephron.
- This results in water not being able to be reabsorbed therefore, water is lost in urine.
- Individuals with Diabetes Mellitus are always thirsty.
Diabetes Insipidus
- This is the destruction of ADH producing cells in the hypothalamus or the destruction
of the nerve tracts leading to the pituitary gland therefore, urine output increases.
- In extreme cases, an individual can lose 20 L of dilute urine in one day.
Bright’s Disease (Nephritis)
- Invading microbes destroy the tiny blood vessels in the glomerulus, thereby altering
the permeability of the nephron.
- Proteins and other large molecules are able to pass into the nephron.
- Proteins cannot be reabsorbed therefore, drawing water into the nephrons  results in
increased urine output.
Treatment of Kidney Disease
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Dialysis and transplants are currently the most common treatments for kidney disease.
Dialysis:
the exchange of substances across a semi-permeable membrane.
Like a normally functioning kidney, a dialysis machine operates on the principles of
diffusion and blood pressure. Unlike a kidney, a dialysis machine cannot perform
active transport.
There are two types of dialysis: hemodialysis and peritoneal dialysis.
Hemodialysis
- The machine is connected to the patient’s circulatory system by a vein.
- Blood is pumped through a series of dialysis tubes that are submerged in a bath of
various solutes. Glucose and a mixture of salts set up concentration gradients.
- Urea will move from the blood into the dialysis fluid until equal concentrations are
established.
- By continually flushing expended dialysis solution and replacing it, urea and other
waste solutes are continuously removed.
- During hemodialysis, the body also receives the hormones the kidneys are unable to
produce.
- See Figure 10.14A, pg 462
Peritoneal Dialysis
- 2 L of dialysis fluids is pumped into the abdominal cavity, and the membranes of the
cavity selectively filter wastes from the blood.
- Urea and other wastes diffuse from the plasma into the peritoneum and into the
dialysis fluid.
- Wastes accumulate in the dialysis fluids, which can be drained off and replaced several
times a day.
- Patient may continue with less strenuous activities. Peritoneal dialysis allows for
greater independence because patients can perform the procedure on their own at
home.
- See Figure 10.14B, pg 462
Kidney Transplants
- Individuals with 10% or less kidney function will eventually need new kidneys.
- Organ donation rate in Canada is very low compared to other developed countries.
- The success rate of organ transplantation is around 95-98%.
- Surgical techniques, such as laparoscopic surgery are constantly being improved.
- The main disadvantage with any transplant is the immune response of the recipient.
- The donor kidney is often identified as a foreign invader and the recipient’s immune
system acts in an attempt to destroy it.
- New medicines to prevent rejection of the new organ are constantly being developed
and improved.
Evaluating Kidney Function
- Physicians use urinalysis and blood tests to make reasoned inferences and
hypotheses about a person’s health and kidney function.
- See Table 10.4, pg 464 for normal values from common urine tests.
- Blood tests can also reveal information about kidney function.
- Blood can be tested to measure the amount of urea and creatinine it contains. High
levels of urea or creatinine indicate that the kidneys are not working properly.
HOMEWORK:
pg 465 #1 - 10