Biology 20Excretory 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 ExcretionExcretion 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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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 Where the Waste OriginatesUrea Detoxification of ammonia in the liver.Uric Acid Breaking down nucleic acids that make up DNA.Ketones Breaking down body fat.Ammonium Breaking down protein.Urochrome Broken down blood cells – pigment that makes the

urine yellow.Creatinine 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. Glomerulus2. Bowman’s capsule3. Proximal convoluted tubule4. Loop of Henle5. Distal convoluted tubule6. 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.

Worksheet – Proximal Tubule

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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.

5. Distal Convoluted Tubule

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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 DuctThis 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.

Worksheet – Loop of Henle and Distal Tubule

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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

renal vein

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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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.

B. Effect of ADH on the Nephron

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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.

Worksheet – ADH

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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 pressureSTIMULUS in osmoticpressure of body fluids

SENSORosmoreceptors

in the hypothalamus

stimulate

CONTROLCENTREpituitary

gland

ADH

EFFECTORdistal

tubules and collecting ducts in

nephrons

RESPONSE water

reabsorption into the blood

because tubule walls

are permeable to water (urine

is concentrated

and scant)

NEGATIVEFEEDBACK 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 in bloodpressure

SENSORkidneyssecreterenin*

CONTROLCENTREadrenalcortexgland

EFFECTOR

nephrons RESPONSE Na+ and

H2Oreabsorption

increases body fluid

volume angiotensin II aldosterone

NEGATIVEFEEDBACK 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 (all)

0.9 0.9 <0.9 – 3.6 <1 – 4×

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

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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? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________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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