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Biology 20
Excretory System
General Outcome 3 – Students will explain the role of the excretory system in
maintaining an internal equilibrium in humans through the exchange of energy and matter
with the environment.
A. The Systems of Excretion
Excretion involves the removal of metabolic wastes that are ingested or produced as the
body works. If these wastes build up in the blood, the body would be quickly poisoned.
The entire body, therefore, works together for the removal of wastes. The major organs
involved in filtering the blood and excreting metabolic wastes include the skin, lungs,
digestive tract, and kidneys.
1. The skin is designed to remove waste body heat, urea and other wastes found in
sweat.
2. The lungs remove a very dangerous waste gas, CO2.
3. The kidneys remove urea, uric acid and extra water and salts.
4. The large intestine removes solid wastes and undigested materials.
5. The liver transforms ingested toxins such as alcohol and heavy metals into soluble
compounds that can be eliminated by the kidneys.
 The liver also transforms the hazardous products of protein metabolism
into metabolites, which are then eliminated by the kidneys. Proteins
contain a nitrogen molecule. The nitrogen and 2 hydrogen ions must be
removed in a process called deamination. Deamination occurs in the
liver, and the process produces ammonia. Ammonia is extremely toxic
and can easily kill you. Ammonia is converted into urea.
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B. Structures of the Urinary System
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3
1. Kidneys
The body contains two kidneys on the
back wall of the abdominal cavity. The
functions of the kidneys are as follows:
a) Kidneys filter the blood by
removing liquid wastes (urine).
b) Kidneys are made up of three
layers: 1) the cortex – outer layer
of connective tissue. 2) The
medulla – inner layer containing
nephrons. 3) The pelvis – hollow
chamber joining the kidney to the
ureter.
c) The kidneys control potassium
and salt levels in the blood.
Aldosterone promotes the
increased absorption of sodium
by the kidneys.
d) The kidneys control water balance. Antidiuretic hormone (ADH) – promotes the
reabsorption of water by the kidneys.
e) Blood acidity is controlled by the kidneys.

Blood enters the kidney through the renal artery. It is filtered and then clean blood
exits the kidney through the renal vein.


The functional unit of the kidney is the nephron.
This is the site for filtration, reabsorption, and secretion. Each kidney contains over
a million nephrons. All of the blood in the body is filtered through the nephrons
every half hour. The fluids are being filtered out of the blood as this occurs. This
means that 180-200L of fluid is being filtered out of your blood each day. You do
not have that much fluid in your body; therefore, 99% of the water that has been
filtered out must be recycled back into the blood. The nephron will be explained in
more detail later.
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
The materials filtered out of the blood collect in the medulla of the kidney as urine.
The many metabolic waste products that make up urine include ammonium, uric
acid, urea, ketones, urochrome, creatinine and excess water, potassium, sodium,
chlorine, hydrogen ions and other trace elements. Glucose, amino acids and other
needed salts are reabsorbed into the blood and saved from the urine.
Waste
Urea
Uric Acid
Ketones
Ammonium
Urochrome
Creatinine
Where the Waste Originates
Detoxification of ammonia in the liver.
Breaking down nucleic acids that make up DNA.
Breaking down body fat.
Breaking down protein.
Broken down blood cells – pigment that makes the
urine yellow.
Working muscles as they use up phosphocreatine.
Kidney Worksheet
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Study the diagrams above and answer the questions below.
1. The volume of blood entering the kidney through the renal artery in one day is more
than the volume leaving through the renal vein. What does this tell you about where
urine comes from?
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If you study the overall structure of the kidney (Diagrams A and B), you will learn
very little about how it cleanses the blood. This is because the cleansing occurs in
millions of microscopic nephrons. The renal cortex of the kidney contains the curly
parts of the nephrons. The renal medulla contains the long loops and collecting ducts.
These are shown in Diagram C.
2. Study the arrangement of the collecting ducts in relation to the renal pelvis. What
does this indicate about the function of the renal pelvis?
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2. Ureters
The ureters are tiny tubes that attach to the renal pelvis of the kidney and lead to the
bladder. They have the unpleasant job of transporting freshly produced urine from the
kidneys to the bladder. Ureters are about 25cm long and are made of three layers.
a) The inner layer is made of mucous membrane.
b) The middle layer is composed of muscular tissue.
c) The outer layer is made of a protective fibrous tissue.
 The muscles within the ureters are capable of producing peristaltic waves
or contractions to move the urine into the bladder.
3. Urinary Bladder
The bladder is a hollow stretchable bag. The bladder has the honorable function of
storing urine. It is made of three muscular tissue layers. The bladder is controlled by two
valves like structures called urinary sphincters.
4. Urethra
The urethra is a tube that leads from the bladder to the outside of the body. The urethra
acts as a passageway for urine to the outside. It carries only urine in females, but is able
to carry urine and semen in males – however, not at the same time.
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C. The Nephron
The nephron is the microscopic filtering worker of the kidney. It does not work alone, as
there are over one million of these workers in each kidney. The major individual parts
that make up the nephron include:
1. Glomerulus
2. Bowman’s capsule
3. Proximal convoluted tubule
4. Loop of Henle
5. Distal convoluted tubule
6. Collecting duct
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1. Glomerulus
 The glomerulus is a microscopic glob of capillaries that has blood circulating
throughout it at an increased pressure. The increased blood pressure causes fluids
to leak out of the capillaries. This process is called filtration. Fluids and materials
are forced out of the glomerulus and into the Bowman’s capsule.
 The fluids and material of the blood or filtrate contains both waste materials and
good, required materials. Almost everything other than blood cells and large
proteins are filtered out of the glomerulus. The filtrate or exiting material contains
water, salts, sugars, amino acids, uric acid, urea and other materials.
 Two arterioles are involved in increasing the pressure of blood in the glomerulus.
One is the afferent arteriole and the other is the efferent arteriole.
a) Afferent arteriole – controls the blood flow into the glomerulus. It dilates to
allow for increased blood flow or volume into the glomerulus.
b) Efferent arteriole – controls the blood flow out of the glomerulus. It constricts to
increase the pressure as it backs up all the blood that the afferent arteriole has
taken the liberty to allow in.
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Worksheet – Glomeruluar Function
Urine formation occurs as blood pressure forces filtrate from the glomerulus into the
capsule. This bulk flow of fluids into the capsules of the nephrons in both kidneys creates
about 180 L of filtrate per day. All but about 1 L will be actively reabsorbed back into the
blood, with great expenditure of ATP.
1. How does bulk flow of filtrate into the capsule differ from diffusion?
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2. In what ways is nephric filtration similar to formation of interstitial fluid (lymph) in
other tissues in the body?
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3. One of the effects of a drug overdose is a serious decrease in blood pressure. How
might this affect kidney function?
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4. Explain why blood cells and proteins are not usually found in the filtrate.
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5. Why are useful molecules like glucose and other nutrients found in the filtrate along
with urea and other wastes?
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6. Sometimes bacterial infection causes nephritis—an inflammation of membranes in
the glomerulus and capsule. Large pores are created through which blood cells enter
the nephron. What symptom would indicate this problem?
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2. Bowman’s Capsule
 The Bowman’s capsule is a cup-like structure that collects the leaking fluids
(filtrate) from the glomerulus and sends it into the winding and connecting tube
called the proximal convoluted tubule.
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3. Proximal Convoluted Tubule
 The proximal convoluted tubule is responsible for the movement of the filtrate
back into the blood. This is called tubular reabsorption. The “good” materials
are reabsorbed and the “bad” materials remain in the tubules and are destined to
be excreted. The much need materials such as water, glucose, amino acids,
vitamins, minerals and some salts are removed from the tubes back into the
blood.
 It is important to reabsorb some of these materials actively instead of leaving it
up to chance with the passive form of diffusion. Here, glucose, amino acids,
and some salts are actively removed from the filtrate. This is accomplished
through the use of ATP, which is generated in the high quantities of
mitochondria located in the walls of the tubule.
 Once the good materials are transported out of the filtrate, they are left to
diffuse back into the capillaries that are circulating in the cortex of the kidneys.
The blood becomes enriched with good materials while the bad materials
remain in the tubules. About 70% of the reabsorption of needed materials occurs
in the proximal convoluted tubules.
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Worksheet – Proximal Tubule
1. In terms of energy costs to the cells in the proximal tubule, the reabsorption of salt
(Na+ and Cl–) and water has been called a deal where we get “3 for the price of 1.”
Explain.
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2. What other useful substances, in addition to Na+, Cl–, and water, are reabsorbed in the
proximal tubule?
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4. Loop of Henle
 The loop of Henle is a long microscopic tube that loops into the medulla area of
the kidney and creates an environment that encourages water reabsorption.
 The loop creates a salty medulla, which allows the kidney to draw out a little extra
water from the other tubes that are running through this area. The extra water
moves into the area by osmosis and is then returned to the blood.
 Animals that need to conserve water tend to have long loops of Henle.
 Descending loop – water reabsoption.
 Ascending loop – sodium ion reabsorption.
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5. Distal Convoluted Tubule
 The distal convoluted tubule is a winding tube that comes after the loop of Henle.
Other items that still need to be reabsorbed into the blood are caught here.
Therefore, these tubules, like the proximal tubules, are involved in reabsorption.
 The distal convoluted tubule is able to aid in the removal of any material directly
out of the blood supply at any time. Therefore, any “bad” materials still remaining
in the blood can be removed in a process called tubular secretion.
 Tubular secretion is exactly opposite to reabsorption as it is the process that
removed unneeded materials from the blood stream into the tubules of the
nephron in order to eliminate them from the body.
 The capillaries near the
convoluted tubule aid in
secretion as the carrier
proteins in the walls of the
capillaries are transporting
waste materials by active
transport.
6. Collecting Duct
This is the final tube that takes the
waste materials that have not been
reabsorbed towards the center of the
kidney where they will be dumped.
Tubular secretion also occurs
between the collecting duct and the
surrounding blood supply. Many
collecting ducts coming from various
regions of the kidney’s cortex come
together to form the “renal
pyramids” of the kidney’s medulla.
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Worksheet – Loop of Henle and Distal Tubule
Use the following diagram, which shows a nephron lying within the cortex and the
medulla, and your textbook or other resource to answer the questions below.
1. Why are the two parts of the nephron loop called “descending” and “ascending”?
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2. Explain the process of reabsorption from the descending loop.
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3. Where is the highest concentration of Na+ found?
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4. Explain the process of reabsorption from the ascending loop.
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5. Explain how reabsorption of ions and water occurs from the distal tubule.
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Use this additional information to answer the next question.
Excess ions and other substances are added to the filtrate from the surrounding
capillaries in a process called tubular secretion. This has been called “reabsorption in
reverse.”
6. List three examples of substances that are actively secreted into the filtrate. How is
blood pH maintained by tubular secretion?
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The structures of the nephron are labelled here to help outline the processes by which
blood becomes urine in the nephron. The word “proximal” (in proximal tubule) means
nearby and refers to the fact that this part of the tubule is located near the Bowman’s
capsule. The word “distal” (in distal tubule) means distant and refers to the fact that this
part of the tubule is located more distantly from the Bowman’s capsule.
The arrangement of blood vessels associated with nephrons is unique. Unlike those found
in most parts of the body, this system of vessels contains two capillary beds in sequence,
with two kinds of arterioles. Blood flows around each nephron as follows:
renal
artery

incoming
arteriole 
glomerulus
(tuft of

capillaries)
outgoing
arteriole 
capillary
network
renal
 venule 
This is an excellent example of the biological principle “Form follows function.” That is,
the structure (form) of an organ matches the way it works. Once you have learned how
the nephron cleanses blood, the significance of its structure and its double capillary beds
will become clear.
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renal
vein
Worksheet – Functions of the Nephron
How the nephron cleanses the blood
Cleansing the blood of wastes, excess salts, and water can be compared to house
cleaning. Cleaning out a desk drawer involves separating useful items from garbage, then
keeping the useful items and discarding the garbage. You can choose one of two ways to
do this. In one method, you would dump everything out of the drawer and then reclaim
the useful items, leaving a pile of garbage. An alternative would be to sort through the
materials in the drawer, removing the garbage and leaving the useful items in place.
The cleansing of blood in nephrons involves mostly the first method. Useful molecules as
well as wastes and excess substances are filtered out of the blood. Then useful molecules
are reclaimed back into the blood, leaving wastes (urine) to be excreted. A review of
blood composition will help you to understand how it is cleansed.
1. List all the components of whole blood, beginning with the largest particles. Note that
some components are waste molecules and some are useful. Some useful components
occur in excess amounts.
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Study the diagram of a nephron with its surrounding blood vessels that follows.
Remember that blood passes through two capillary beds as it flows around the nephron,
and that exchange of components between blood and surrounding tissues can only occur
through thin capillary walls.
2. Identify where useful molecules and wastes are filtered out of the blood and into the
nephron.
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3. Where are useful molecules reabsorbed from the nephron back into the blood?
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4. Which structure transports the waste molecules (urine) left behind after reabsorption?
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5. Which parts of the nephron are in the cortex of the kidney? Which parts penetrate
deep into the medulla?
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6. A common misconception is that “kidneys filter wastes out of the blood.” Use the
words “filtration” and “reabsorption” to explain why this statement is false.
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D. Water Balance and ADH
A. Regulation of ADH
 The function of ADH (Antidiuretic hormone) is to increase water reabsorption by
the kidney. Nerve cells in the brain (hypothalamus) produce ADH, and it is
stored in the pituitary gland in the brain until it is needed.
 Osmoreceptors in the hypothalamus detect changes in the osmotic pressure in the
bloodstream. As water loss occurs, water moves from the tissues of the body into
the bloodstream, which increases the blood’s osmotic pressure.
 The cells in the hypothalamus shrink, causing a nerve message to be sent to the
pituitary gland stimulating the release of ADH and triggering the thirst response.
 The kidneys will then reabsorb more water and produce more concentrated urine.
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B. Effect of ADH on the Nephron
 The proximal tubule and descending loop of Henle are permeable to water. The
ascending loop of Henle, distal convoluted tubule, and the collecting duct are
impermeable to water. If there is no ADH, the 15% of water that didn’t get
reabsorbed in the proximal tubule is lost in the urine. ADH causes the distal
tubule and collecting duct to become permeable to water, therefore, decreasing
the amount of water lost in the urine.
 Cold weather, caffeine and alcohol all inhibit the release of ADH, decreasing
water reabsorption, thereby increasing the volume of dilute urine being produced.
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Worksheet – ADH
After eating a salty meal or neglecting to drink water regularly, the osmotic pressure
(“saltiness”) of body fluids increases. This is the stimulus that initiates a series of events
in which urine becomes scant and concentrated because more water is reabsorbed from
the urine as it passes through the salty medullary tissues. A hormone called ADH
increases the permeability of the distal tubules and collecting ducts, allowing osmosis to
occur. This response tends to return osmotic pressure of body fluids to normal, especially
when an accompanying thirst causes increased water intake.
How ADH lowers osmotic pressure
STIMULUS
 in osmotic
pressure of
body fluids

SENSOR
osmoreceptors
in the
hypothalamus
stimulate

CONTROL
CENTRE
pituitary
gland
ADH

EFFECTOR
distal
tubules and
collecting
ducts in
nephrons

RESPONSE
 water
reabsorption
into the blood
because
tubule walls
are
permeable to
water (urine
is
concentrated
and scant)
NEGATIVE
FEEDBACK
 in osmotic
pressure of
body fluids
1. Redraw the chart to show the response to drinking several glasses of water, which
decreases the osmotic pressure of body fluids. Choose an appropriate title for your
chart.
2. How do diuretics such as alcohol and caffeine affect this homeostatic mechanism?
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E. Blood Pressure and Aldosterone



The function of Aldosterone is to increase blood pressure by increasing the
reabsorption of water. Blood volumes can regulate blood pressure – as volume
decreases, blood pressure also decreases.
Aldosterone is produced in the adrenal gland which is located above the kidney.
Blood pressure is monitored by the osmoreceptors in the juxtaglomerular
apparatus (cells located in the afferent arteriole near the glomerulus). When
blood pressure drops, the cells in the juxtaglomerular apparatus release rennin
which converts angiotensinogen (a plasma protein secreted by the liver) to
angiotensin (active form). Angiotensin constricts blood vessels and causes
Aldosterone to be released – both of which increase blood pressure.
Aldosterone is released and causes an increase in sodium ion reabsorption from
the distal tubule and collecting duct in the nephron. As more NaCl leaves the
nephron (ascending loop), the osmotic gradient increases. With a higher
concentration of water in the nephron, water moves into the bloodstream by
osmosis, therefore, increasing blood volume and blood pressure.
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Worksheet – Aldosterone
When someone suffers a serious extensive blood loss, body fluid volume decreases. The
decrease in fluid volume tends to decrease blood pressure, but the homeostatic
mechanism presented below helps to maintain blood pressure until blood loss becomes
critical and death is imminent.
How aldosterone raises blood pressure
STIMULUS
SENSOR
 in blood  kidneys 
secrete
pressure
renin*
CONTROL
CENTRE
adrenal
cortex
gland
EFFECTOR
nephrons


RESPONSE
 Na+ and
H2O
reabsorption
increases
body fluid
volume
angiotensin II
aldosterone
NEGATIVE
FEEDBACK
 in blood
pressure
* renin from the kidneys causes the plasma protein angiotensinogen to change to
angiotensin I. An enzyme in the lungs changes angiotensin I into angiotensin II.
1. Aldosterone stimulates the reabsorption of sodium ions in the nephrons. How does
this lead to an increase in water reabsorption?
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2. Drinking salty water tends to increase body fluid volume. What effect might this
increased volume have on secretion of aldosterone? How might the nephrons
respond?
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Worksheet – Composition of Urine
The following table compares the composition of blood plasma, nephric filtrate, and
urine. Study it carefully and answer the questions that follow.
Comparison of concentrations of substances in Plasma, Filtrate and Urine
(mg/100mL)
Substance
Plasma
Filtrate
Urine
Concentration
Change
Inorganic ions 0.9
0.9
<0.9 – 3.6
<1 – 4×
(all)
K+
0.02
0.02
0.15
7.5×
Amino acids
0.05
0.05
none
–
Proteins
8.0
none
none
–
Glucose
0.01
0.01
none
–
Urea
0.03
0.03
1.8
60×
Note: The pH of blood plasma and filtrate is 7.4. The pH of urine is 4.8–7.5.
1. Study the concentrations of the ions, the amino acids, glucose and urea. Why are their
concentrations in the filtrate identical to those in the plasma?
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2. Neither glucose nor proteins are present in urine, but for different reasons. Explain.
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3. Although urea (a waste molecule) undergoes less reabsorption than glucose, its
concentration in the urine has increased about 60-fold. Account for the increase.
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4. Give two reasons to explain why K+ is more concentrated in the urine than in the
filtrate.
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5. Which ion accounts for the low pH of the urine? How and where is this ion
transported into the urine? Why is the elimination of this ion important to survival?
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F. Disorders
1. Diabetes Mellitus




Caused by inadequate insulin production, which decreases blood sugar
levels. Because of this, the amount of glucose in the blood increases, as
well as the amount of glucose in the filtrate.
There is a limit to the energy available for active transport of glucose back
into the bloodstream and the excess glucose stays in the nephron and
decreases the concentration gradient, therefore, less water is reabsorbed.
This causes increased urine volume and generally, glucose is detected in
the urine.
Type 1 – body destroys the insulin-producing cells in the pancreas.
Type 2 – insulin producing cells in the pancreas wear out or the receptors
no longer function.
2. Diabetes Insipidus


Caused by the destruction of the hypothalamus cells or the nerves leading to
the pituitary. This prohibits the release of ADH.
Individuals with this form of diabetes have increased amount of urine as no
water is being reabsorbed due to ADH.
3. Bright’s Disease

Blood vessels in the glomerulus become inflamed or destroyed. The
glomerulus becomes permeable to plasma proteins and possibly blood cells.
The body cannot reabsorb large solutes as there is no mechanism to do so.
The concentration gradient decreases, therefore, less water is reabsorbed.
Urine output increases.
4. Kidney Stones

Caused by precipitation of mineral solutes from the blood. They can then
lodge in the renal pelvis or urethra. Tissues can be torn which is extremely
painful!
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G. Dialysis

The proper functioning of our kidney is necessary for life. People who have
malfunctioning kidneys undergo dialysis. The person’s blood is transferred from
their body through a dialysis machine that behaves like a mechanical kidney. The
dialysis tubing is semipermeable…large solutes won’t pass through, but small
solutes (glucose, urea, hydrogen ions, uric acid and other electrolytes) can. The
solution surrounding the tubing has a lower concentration of the small dissolved
solutes. The dissolved solutes diffuse from the tubing into the dialysis machine,
then the “filtered” blood is returned to the body. This can go on for an extended
time, but the best option would be a kidney transplant.
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Worksheet – Kidney Transplant
The first kidney transplant was done in 1952. The donor and the recipient were identical
twins. Since then, great advances have been made in transplant technology and
immunosuppressant drugs that prevent rejection of the transplanted organ. Modern
success rates for the procedure are very high.
Viable kidneys for transplant can be harvested from nonliving (cadaveric) donors or from
living donors who are frequently close relatives of the patient. In Canada, more
transplants involving nonliving donors occur, but this procedure occurs only after a
compatible tissue match is found, and sometimes the wait for a suitable kidney is many
years. Transplants from living donors are advantageous in that tissue matches are easier
to find.
A third source of organs for transplant is animals. For example, research is underway to
perfect techniques for harvesting heart valves from pigs for transplantation into humans.
Genes for certain human proteins are transferred into pig embryos, and the embryos are
allowed to develop normally in healthy sows. Tissues from the resulting transgenic
offspring are less likely to be rejected in the human body because of the presence of some
human proteins. Transplanting animal organs or tissues into human donors is called
xenotransplantation.
Organ transplantation has always presented ethical and legal issues, but transplantation
from living donors and xenotransplantation are especially interesting.
1. Research the issue of kidney transplantation from living donors in Canada. What is
the proportion of living donor transplants compared to cadaveric transplants? What
are the relationships between donors and recipients? What are the medical risks to
donors and recipients? What are the ethical and legal issues? How do medical teams
assess the feasibility of a living donor transplant?
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2. What is the current state of research on xenotransplantation of kidneys? Is it being
done anywhere in the world? What organs or tissues are currently being researched or
transplanted? How are donor animals produced, and what is their fate after
transplantation? What are the medical risks to the recipients? What are the ethical
issues?
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