4.2
The Kidney
E X P E C TAT I O N S
Explain the role of the kidney in maintaining water and ion balance.
Design and carry out an experiment to investigate the physiological effects
experienced by people who consume coffee.
Describe issues and present informed opinions about problems related to
kidney functions and kidney transplants.
Describe the contribution made to knowledge and technology in the field
of homeostasis by Dr. Gordon Murray’s development of the kidney dialysis
machine.
Humans have two fist-sized kidneys, which are
found in the lower back on either side of the spine.
The kidneys release their waste product (urine)
into tubes called ureters, which carry the fluid to
the urinary bladder where it is temporarily stored
(see Figure 4.8). The bladder can hold a maximum
of about 600 mL of fluid. When there is about
250 mL of urine in the bladder, we become aware
of it, and at 500 mL we become very uncomfortable.
Drainage from the bladder is controlled by two
rings of muscles called sphincters. One sphincter
is involuntarily controlled by the brain. During
childhood we learn to voluntarily control the other.
Urine exits the bladder through a tube called the
urethra. In males, the urethra is approximately
20 cm long and merges with the vas deferens from
the testes to form a single urogenital tract. In females,
renal veins
renal arteries
kidney
abdominal
aorta
vena cava
ureters
the urethra is about 4 cm long and the reproductive
and urinary tracts have separate openings.
Kidney Functions and Structure
The kidney’s principal function is to filter the
blood in order to remove cellular waste products
from the body. The essential connection between
the kidney and the blood is illustrated by the fact
that at any given time, 20 percent of the body’s
blood is in the kidneys. Although most people have
two kidneys, the human body is capable of
functioning with only one. If one kidney ceases
to work or if a single kidney is transplanted into
a patient, the functioning kidney increases in size
to handle the increased workload.
Although the kidney has other important
functions, this organ is usually associated with the
excretion of cellular waste. The main metabolic
wastes are urea, uric acid, and creatinine, all of
which have nitrogen as a major component. Figure
4.9 shows the formula by which urea is produced
in the liver from the breakdown of excess amino
acids that are the building blocks of proteins. The
amine group (NH2 ) is removed to release the rest
of the amino acid molecule, which can then be
converted to carbohydrates or fats. Although the
amine group can combine with a hydrogen ion to
form toxic ammonia, the ammonia is transformed
in the liver into the less toxic urea before being
released into the bloodstream.
O
2 NH 3
ammonia
urinary
bladder
urethra
Figure 4.8 The human urinary system
112
MHR • Unit 2 Homeostasis
+
CO2
carbon
dioxide
H 2N
C
NH2
urea
Figure 4.9 The liver combines ammonia with carbon
dioxide to form urea and water.
+
H2O
Canadians in Biology
Cellophane and Imagination —
Dr. Gordon Murray
dialysis (also known as “hemodialysis”). Dr. Murray became
renowned as a hard-working and imaginative scientist.
On December 6, 1946, Dr. Gordon Murray was called
into Toronto General Hospital. A female patient in a coma
was wheeled into the room on a stretcher. She was
“uremic,” which means her kidneys were not functioning
and the toxins in her blood were poisoning her body.
Staff from all over the hospital watched as Murray set up
an innovative apparatus. He inserted one end of 46 m of
tubing into a vein in one of the patient’s legs and the
other end into an artery in her other leg. He then turned
on a pump. The patient’s blood immediately began to
circulate through the tubing, which was strapped to a
cylinder, immersed in a bath, coiled around and around,
and then diverted back to the patient. Regular readings
were taken of both the patient’s blood and the bath
solution to see if the toxins were leaving the blood. As
time passed, the readings proved the device was working.
The patient regained consciousness in six hours.
An impediment to successful kidney dialysis was how to
get impure blood outside the body and keep it from
clotting. In 1935, Dr. Murray pioneered the use of the
drug heparin, an anticoagulant that keeps blood from
clotting and keeps sutures pliable after surgery. Heparin
brought Dr. Murray one step closer to successful dialysis.
The second problem to overcome was how to filter
impurities from the blood. Dr. Murray considered using
the principle of osmosis. He looked for a material that
was porous enough to let the smaller, toxic particles in
the blood pass through into another solution while
keeping in the larger particles (plasma).
In a series of experiments, Murray tried a variety of
materials, including leather and nylon, to form the tubing
for his machine. Finally, he tried cellophane designed for
use as sausage casing. It was the ideal filter; it retained the
important contents of blood while allowing the toxins to
escape into a solution in which the tubing was immersed.
In Dr. Murray’s first experiment on an animal, he attached
one end of his tubing to an artery and the other to a vein,
letting the heart pump the blood from the artery through
the tubes and back into the vein. The experiment was
successful — the toxins left the animal’s blood. However,
recalling that blood returning to the heart from the body’s
extremities (venous blood) is the most impure blood,
Murray developed a pump that would simulate the heart’s
action and pump the blood the opposite way through
the tube.
Dr. Gordon
Murray
The Steps to Success
Dr. Gordon Murray was born in 1894 in Stratford,
Ontario. He was awarded the Order of Canada in 1967
as the first North American to develop and use kidney
Another waste product found in the blood is
uric acid, which is usually produced by the
breakdown of nucleic acids such as DNA and RNA.
Creatinine is a waste product of muscle action. All
of these waste products are potentially harmful to
the body and therefore must be removed.
The kidneys are more than excretory organs;
they are one of the major homeostatic organs of the
body. In addition to filtering the blood to remove
wastes they also control the water balance, pH, and
levels of sodium, potassium, bicarbonate, and
A Partial Victory
Today, kidney dialysis (or the “artificial kidney”) enables
people with kidney disease to live relatively normal lives.
However, these patients must undergo regular dialysis
sessions — usually three six-hour sessions per week.
Researchers are striving to invent a machine to make
this labour-intensive process obsolete.
calcium ions in the blood. They also secrete a
hormone (erythropoietin) that stimulates red blood
cell production, and they activate vitamin D
production in the skin. Since the kidneys are
involved with so many of the body’s functions, the
analysis of a urine sample can tell a physician a
great deal about a patient. For example, diabetes
and pregnancy can be determined using a urine test.
Each kidney is composed of three sections — the
outer cortex, the medulla, and the hollow inner
pelvis where urine accumulates before it travels
Chapter 4 Homeostatic Mechanisms • MHR
113
down the ureters. These sections are shown to the
left of Figure 4.10. Within the cortex and medulla
of each kidney are about one million tiny filters
called nephrons. As Figure 4.10 illustrates, each
nephron consists of five parts — the Bowman’s
capsule, the proximal tubule, the loop of Henle,
the distal tubule, and the collecting duct. The
upper portions of the nephron are found in the
renal cortex, while the loop of Henle is located in
the renal medulla. The tubes of the nephron are
surrounded by cells, and a network of blood vessels
spreads throughout the tissue. Any material that
leaves the nephron enters the surrounding cells
and eventually returns to the bloodstream through
the network of blood vessels. By controlling what
leaves and what remains in the nephrons, the
kidneys keep the levels of water, ions, and other
materials nearly constant and within the limits
necessary to maintain homeostasis.
Blood enters the cavity of the ball-shaped
Bowman’s capsule through a tiny artery that
branches to form a network of porous, thin-walled
capillaries called the glomerulus. Under the
influence of blood pressure, some blood plasma
and small particles are forced out of the capillaries
and into the surrounding capsule. Larger blood
components, such as blood cells and proteins,
remain in the capillaries. The fluid in the
Bowman’s capsule is called nephric filtrate, and
it is pushed out of the capsule into the proximal
tubule. About 20 percent of the blood plasma that
enters the kidney becomes nephric filtrate.
BIO FACT
People who are trying to increase muscle mass sometimes
use a diet high in proteins or amino acids. The problem with
a diet like this is that it creates an excess of amino acids,
which are broken down in the liver to form the carbohydrates
necessary for metabolism. The excess amine groups in turn
produce high levels of urea, which is released into the
bloodstream to be removed by the kidneys. However, high
urea levels can damage the kidneys, so it is necessary to
find ways to bring these levels down. The simplest way to
accomplish this is to drink plenty of fluids.
When the nephric filtrate enters the proximal
tubule, re-absorption begins. Re-absorption is the
proximal tubule
Bowman’s capsule
distal tubule
glomerulus
renal artery
Renal Cortex
renal vein
outer layer
(cortex)
inner layer
(medulla)
capillary
network
loop of
Henle
inner
collecting
area (pelvis)
descending
loop
ascending
loop
Renal Medulla
collecting duct
ureter
Figure 4.10 A nephron is composed of the Bowman’s
capsule, the proximal tubule, the loop of the nephron
114
MHR • Unit 2 Homeostasis
(called the loop of Henle), the distal tubule, and the
collecting duct.
MINI
inner
medulla
descending loop
outer
medulla
H2O
ascending loop
cortex
increasing Na+ concentration in renal medulla
process by which materials required by the body
are removed from the filtrate and returned to the
bloodstream. Osmosis, diffusion, and active
transport draw water, glucose, amino acids, and ions
from the filtrate into the surrounding cells. From
here the materials return to the bloodstream. This
process is aided by active transport of glucose and
amino acids out of the filtrate. The lining of the
proximal tubule is covered with tiny projections
(like the villi of the small intestine) to increase
the surface area and speed up the process of reabsorption. When the filtrate reaches the end of
the proximal tubule, the fluid is isotonic with the
surrounding cells, and the glucose and amino acids
have been removed from the filtrate. We say a fluid
is isotonic when it has the same concentration of
water and solutes as that in the cells surrounding it.
From the proximal tubule, the filtrate moves
to the loop of Henle. The primary function of the
loop of Henle, which first descends into the inner
renal medulla and then turns to ascend back
towards the cortex, is to remove water from the
filtrate by the process of osmosis (see Figure 4.11).
The cells of the medulla have an increased
concentration of sodium ions (Na+ ). These ions
increase in a gradient starting from the area closest
to the cortex and moving toward the inner pelvis of
the kidney. This increasing gradient acts to draw
water from the filtrate in the loop of Henle. This
process continues down the length of the
descending loop due to the increasing level of Na+
in the surrounding tissue. You will observe a
similar process in the MiniLab below.
Na+ Cl −
H2O
Na+ Cl −
H2O
+
Na Cl
H2O
−
H2O
H2O
loop of the Henle
urea
collecting
duct
Figure 4.11 As the filtrate travels down the descending
loop of Henle, water moves out by osmosis. What prevents
the water from being re-absorbed into the ascending loop?
The high levels of Na+ in the surrounding medulla
tissue are the result of active transport of Na+ out of
the ascending loop of Henle. The amount of water
removed from the filtrate by the time it reaches the
bottom of the loop of Henle results in an increased
concentration of all of the materials dissolved in
the remaining filtrate, including Na+ . Thus, as the
filtrate moves up the ascending loop of Henle, Na+
LAB
The Effect of Salt Concentration
Common table salt (NaCl) is an important component of
cells and the fluids that surround them in the body. Life
evolved in the salt water environment of the ocean and
salt plays an important role in cellular function. The
concentration of salt affects osmosis or the movement
of water in the body’s cells. This movement can be
demonstrated in other cells, for example, in those of an
onion. In this MiniLab, you will use a piece of coloured
onion to observe the effect of increased salt concentration.
Make a wet mount slide of a thin layer of red onion skin.
Draw a diagram of two of the cells that you can see at 100x
or 400x. Label the cell wall, cell membrane, cytoplasm, and
nucleus (if visible). This usually works better if you use the
microscope’s diaphragm to decrease the amount of light.
Lift the cover slip and put two drops of saturated salt (NaCl)
solution on the onion cells. After two minutes, examine the
onion cells again.
CAUTION: Handle the microscope slides and cover
slips carefully. Wash your hands after completing
the MiniLab.
Analyze
1. Draw two of the cells that have changed. Label the cell
wall, cell membrane, cytoplasm, and nucleus.
2. Explain why the cytoplasm changed in the presence of
the salt solution.
Chapter 4 Homeostatic Mechanisms • MHR
115
is actively pulled from the filtrate into the
surrounding tissue. At the same time, the water
that left the descending loop cannot re-enter the
ascending loop because this loop is impermeable
to water.
Chloride ions tend to follow the sodium ions
because of the electrical attraction between the
negative chloride ions and the positive sodium
ions. In addition, as the water concentration in the
filtrate decreases, the chloride ion concentration in
the filtrate increases, resulting in still more
chloride diffusion out of the ascending loop.
A Glomerular Filtration
Water, salts, nutrient molecules,
and waste molecules move
from the glomerulus to the
inside of the Bowman’s capsule.
These small molecules are
called the glomerular filtrate.
As shown in Figure 4.12, the distal tubule is
responsible for a process called tubular secretion.
Tubular secretion involves active transport to pull
substances such as hydrogen ions, creatinine, and
drugs such as penicillin out of the blood and into
the filtrate. The fluid from a number of nephrons
moves from the distal tubules into a common
collecting duct, which carries what can now be
called urine into the renal pelvis. At that point,
99 percent of the water that entered the proximal
tubule as nephric filtrate has been returned to the
body. In addition, nutrients such as glucose and
amino acids have been reclaimed.
B Active Recovery
ATP is used to actively transport amino acids
and glucose out of the filtrate back into the
body. This makes the filtrate more dilute, so
water leaves passively by osmosis as the fluid
flows through the descending loop of Henle.
C Water Recovery
Removing more water
concentrates the urine. Active
transport pumps sodium ions
outside the descending loop
of Henle to create a
hypertonic environment.
proximal tubule
Bowman’s capsule
H2O
glucose
amino
acids
drugs
creatinine
H+
distal
tubule
artery
vein
collecting duct
loop of
Henle
Figure 4.12 Active and passive transport are both used to
maintain a balance of solutes and water. At A, the pressure
of the blood flowing into the glomerulus pushes solutes and
water into the Bowman’s capsule. At B, active transport is
used to recover amino acids and glucose. This makes the
filtrate relatively dilute, so water also moves out of the
116
MHR • Unit 2 Homeostasis
capillary
network
H2O
solutes
nephron. At the bottom of the loop of Henle, the filtrate is
almost isotonic. Diffusion of urea and the active transport of
sodium ions out of the ascending loop of Henle creates a
relatively hypertonic environment. At C, water can leave the
distal tubule, resulting in more concentrated urine.
Urine Output
BIO FACT
The permeability of the distal tubule and collecting
duct is controlled by a hormone called anti-diuretic
hormone (ADH). ADH is secreted by a gland
attached to the hypothalamus called the pituitary
gland. ADH increases the permeability of the distal
tubule and collecting duct, thus allowing more
water to be removed from the nephric filtrate when
the body has a need to conserve water.
The pituitary, as you will see in Chapter 6, is
controlled by the hypothalamus. As shown in
Figure 4.13, the hypothalamus acts to regulate the
body’s feedback systems. When the body needs to
eliminate excess water, ADH is inhibited and more
water is excreted in the urine. Drugs such as
alcohol and caffeine block the release of ADH
and increase the volume of urine. Increased urine
output can also be a symptom of conditions such
as diabetes. In people who have diabetes, the
increased level of blood sugar can overload the
active transport system of the proximal tubule,
which causes glucose to remain in the nephric
filtrate as it moves through the loop of Henle, distal
tubule, and collecting duct. The glucose retains
water in the filtrate, offsetting the system that is
designed to remove it. The result is that large
volumes of sugary urine are produced, which is
one of the major symptoms of diabetes.
high fluid intake
hypothalamus
Penicillin is an acid that the body actively secretes into
urine. About four hours after penicillin is ingested,
50 percent of the penicillin in the blood is secreted and
removed from circulation. In the early days of penicillin
use the drug was difficult to obtain, so hospitals recycled
penicillin by collecting patients’ urine and separating the
penicillin for re-use.
ELECTRONIC LEARNING PARTNER
For more information about how the kidney functions, refer
to your Electronic Learning Partner.
WEB LINK
www.mcgrawhill.ca/links/biology12
The kidney is a vital organ. Unfortunately, various diseases of
the kidney and other medical conditions (such as diabetes)
can seriously impair normal kidney function. Patients who
experience loss of kidney function must maintain a continual
program of regular hemodialysis.
You learned about hemodialysis in Canadians in Biology
on page 113. To access articles describing the process of
hemodialysis, go to the web site above, and click on Web
Links. Describe the major causes of kidney failure. How does
hemodialysis compensate for normal kidney function? Explain
how an artificial kidney removes waste products from a
patient’s blood. Make a sketch to illustrate how substances
are filtered out of the blood in an artificial kidney. Describe
the health risks associated with hemodialysis treatment.
Blood pH and the Kidney
reduced ADH
production
Kidney returns less water
to the blood, resulting
in increased urine output.
Figure 4.13 When you drink a large amount of water, the
fluid level in your blood vessels increases. This increase
triggers the hypothalamus to slow down production of ADH.
As a result, you eliminate more water. When the water level
in the blood drops too low, the hypothalamus produces
more ADH.
The kidneys regulate the acid-base balance of the
blood. To remain healthy, our blood pH should stay
around 7.4, which is slightly basic. One way in
which blood pH is controlled at this level is by
regulation of the active transport of hydrogen ions
(H+ ) into the nephric filtrate. If blood pH fluctuates,
the secretion of H+ either slows or increases until
the pH returns to normal. As a result of this
fluctuation, urine can have a pH as low as 4.5 or as
high as 8.0. Normally, urine has a pH of about 6.0.
While the kidneys are ultimately responsible
for the removal of excess hydrogen ions from the
blood, the respiratory system works with the kidneys
to help maintain the pH of the blood at 7.4. The
two systems depend on chemicals called buffers to
Continued on page 120
Chapter 4 Homeostatic Mechanisms • MHR
➥
117
Biology Magazine
TECHNOLOGY • SOCIETY • ENVIRONMENT
Kidney Transplants
When the first kidney transplant occurred 40 years ago,
it was a major medical breakthrough. In the year 2000
there were 1112 kidney transplants performed in Canada.
Of these, 724 used cadaveric (deceased) donors and 388
used living donors.
relatives must give permission for transplantation of
organs. In contrast, many European countries have an
“opting out” policy, in which permission is assumed
unless the potential donor has specifically requested
not to be an organ donor.
What Are the Ethical Considerations?
The major problem with transplant surgery is not in the
operating room — it is in finding suitable donors. In
Canada, most donors are victims of stroke or head
trauma (often related to motor vehicle accidents) who are
being maintained on a ventilator. Only two to three
percent of all deaths in Canada are the result of brain
death, and that pool of potential donors is further limited
by the fact that hospitals require permission from the
relatives of the donor to perform a transplant. Ninety-six
percent of relatives agree to organ donation if they know
the wishes of the potential donor, while only 58 percent
agree if the issue has not been discussed in advance.
The use of living donors is a growing trend. People can
survive with only one kidney, but the donation of a kidney
by a living donor creates a number of ethical problems for
the medical community. There are risks associated with
any surgery — should a doctor risk the life of a healthy
person in an attempt to aid someone who is ill? In
addition, there is the problem of “informed consent.” Can
a doctor be sure that a living donor is a willing participant
and is not the victim of pressure from relatives? Can a
doctor proceed with the operation if he or she suspects
that the donor is receiving some form of benefit in
exchange for donating a kidney?
Canada has one of the lowest organ donation rates
among industrialized countries. There are fewer than
14 donors per million in Canada, compared with more
than 30 per million in Spain. Why? This disparity is partly
due to Canada’s “opting in” policy — donors and/or their
An additional ethical problem is looming on the
biotechnological horizon. Pig tissue (not organs) has been
used in clinical trials to replace damaged human tissue.
Pig organs are similar in size and shape to their human
counterparts, which makes pigs good candidates for
DESIGN YOUR OWN
Investigation
SKILL FOCUS
4 • A
Hypothesizing
The Physiological Effects of Coffee
This investigation gives you an opportunity to explore one of the body’s feedback
systems. You will discover how coffee, which is consumed by millions of
Canadians each day, affects the homeostatic processes of the human body.
Coffee contains caffeine, a stimulant and diuretic that affects the body in a variety
of ways. Begin your investigation by using the Internet or your library to research
the positive and negative effects of coffee.
Problem
Materials
How does coffee affect the physiology of the body?
Select your own materials.
Hypothesis
Experimental Plan
Create a hypothesis related to one physiological effect
of consuming coffee.
CAUTION: Due to health concerns, it may not be
appropriate for some students to participate as
subjects in this investigation. Be sure that
students do not exceed their normal coffee intake.
Initiating and planning
Performing and recording
Analyzing and interpreting
Communicating results
1. After deciding which physiological reaction you want
to measure, design an experiment that allows you to
measure the effect of coffee.
2. Be sure to establish proper controls so you can
compare your results before and after the ingestion
of coffee.
3. Establish the amount of time required, and ask your
teacher to approve your experimental design and to
arrange for any equipment you may need.
118
MHR • Unit 2 Homeostasis
organ donation. One of the major concerns with the use
of animal organs has been the possibility of transmitting
animal viruses into humans. These fears were somewhat
diminished by a report that showed that none of the
160 people who had received heart valves or other
tissue from pigs had become infected. Concern persists,
however, about potential transfer of viruses.
donation. The genetic manipulation of animals is ongoing,
and the creation of transgenic animals (animals that have
genes from more than one species) is producing animals
with new characteristics. Some experts predict that
clinical trials using pig organs as donors for humans
could begin in less than two years.
Follow-up
1. Debate with classmates whether Canada should
adopt an “opting out” policy to increase the number
of cadaveric donors. What problems might this create?
Pigs are good potential candidates for organ donation.
As you will learn in Chapter 9, the next step in using pigs
as organ donors will be to genetically modify the pig
genome to decrease the risk of rejection after organ
Checking the Plan
2. Consider the following cost comparison of kidney
transplant versus dialysis. The operation costs
$20 000 and requires $6000 per year in follow-up
treatments. Compare this with the $50 000 per year
required to maintain a patient on dialysis (an artificial
kidney). Over a five-year period, the costs are
$50 000 for the transplant and $250 000 for dialysis.
Kidney transplant operations have a 98 percent
success rate using living donors and a 95 percent
success rate with cadaveric donors. Should a
destitute person be allowed to sell one of his or her
kidneys to avoid starvation? Is this different from a
family member donating a kidney?
Conclude and Apply
1. What are your dependent and independent
variables? What are your controlled variables?
4. What conclusions can you make about one
physiological response to the intake of coffee?
2. What will be your control?
5. Based on your results, predict what other
measurable effects coffee would have on the
human body.
3. What will you measure and how?
4. How will you record and graph your data?
Exploring Further
Data and Observations
Conduct your investigation and make your
measurements. Graph your results and then enter the
data in a summary table.
Analyze
1. (a) Was the variable you investigated affected by
coffee?
(b) If so, how was it affected?
2. Were your results consistent?
6. Using the Internet,
find the results of
various research
studies that
have explored
the positive and
negative effects
of coffee. How
might you account
for the conflicting
conclusions?
3. What factors of your population may have affected
your results?
Chapter 4 Homeostatic Mechanisms • MHR
119
control pH. You first encountered the action of
buffers in Chapter 1. You learned that buffers resist
changes in pH by taking up or releasing H+ or
hydroxide ions (OH− ). The main buffer in the blood
is carbonic acid (H2CO3 ), a weak acid that reacts to
release H+ and the bicarbonate ion (HCO3 − ). If the
blood contains excess H+ , it is too acidic. In this
situation, excess H+ combines with the HCO3 − to
form H2CO3 .
H+
HCO3 −
+
hydrogen ion
H2CO3
bicarbonate ion
carbonic acid
In basic conditions, when there is a low
concentration of H+ , carbonic acid dissolves to form
H+ and HCO3 .
H2CO3
H+
+
HCO3 −
carbonic acid
hydrogen ion
bicarbonate ion
These reactions prevent major change in blood
pH. The reactions are linked to the respiratory
system, because H2CO3 reacts in solution to form
carbon dioxide (CO2 ) and water. Therefore, levels
of H2CO3 are linked to levels of CO2 , which are
regulated by breathing. When the reactions are
combined, the result is a series of reversible
reactions that act to control pH.
H+
+
hydrogen
ion
HCO3 −
bicarbonate
ion
SECTION
1.
2.
H2CO3
carbonic
acid
H2O + CO2
water carbon
dioxide
When carbon dioxide (CO2 ) is exhaled, this
reaction shifts toward the right, and the hydrogen
ions that were free (and that were lowering the pH)
are now in the neutral form of water. Thus,
breathing rate increases if the body’s receptors
detect a drop in pH. Conversely, if pH increases,
breathing rate slows down, moving the reaction
toward the left. (The negative feedback loops are
monitored by sensory receptors in the carotid
arteries and the aorta. If these receptors detect a
fluctuation in the hydrogen ion concentration in
the blood, they communicate with the respiratory
centre of the brain. If the pH decreases, breathing
rate increases. As more CO2 is exhaled, the pH
will increase.)
Another example of buffering is the combining
of hydrogen ions in the blood with ammonia from
the cells that line the nephron. The ammonia is
formed from the breakdown of amino acids and is
changed to the less toxic ion ammonium (NH4 +),
while raising pH.
NH3
ammonia
+
H+
HN4 +
hydrogen ion
ammonium ion
Like the kidney, the pancreas uses receptors,
integrators, and effectors in a feedback system to
keep the body working at peak efficiency. The next
section introduces you to this key organ in the
homeostatic process.
REVIEW
C Draw the shape of a nephron found in human
kidney tissue. Label the four major sections of the
nephron.
On the diagram you drew for question 1, show
where the following processes occur:
6.
K/U
Identify the factors that contribute to:
(a) an increase in urine production over a 24-h period
(b) a decrease in concentration of urine over a
24-h period
K/U
7.
MC People who have diabetes experience increased
risk of kidney failure. Investigate possible links
between diabetes and impaired kidney function. What
recommendations would you provide to people with
diabetes that might help them minimize the possibility
of developing kidney problems?
8.
I Alcohol, like caffeine, acts as a diuretic. How
does alcohol affect the feedback loop that controls
the concentration of urine produced? Propose a
testable question that could answer this problem.
What factors would you need to control to test
your question?
(a) glomerular filtration
(b) glucose leaves the filtrate
(c) water leaves the nephron
(d) salt ions are removed from the nephron
(e) urea diffuses out of the nephron
120
3.
Describe the factors that determine the final
concentration of sodium ions (Na+ ) and potassium
ions (K+ ) in urine.
4.
C Draw a negative feedback loop that shows how
the kidney keeps the pH level of blood plasma and
other body fluids at about 7.4.
5.
C What is the role of the hypothalamus in the
production of urine?
K/U
MHR • Unit 2 Homeostasis