Biology 12

120 MHR • Unit 2 Homeostasis

SECTION REVIEW

1. Draw the shape of a nephron found in human
   kidney tissue. Label the four major sections of the
   nephron.


2. On the diagram you drew for question 1, show
   where the following processes occur:
   (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


3. Describe the factors that determine the final
   concentration of sodium ions (Na+) and potassium
   ions (K+) in urine.


4. 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. What is the role of the hypothalamus in the
   production of urine?
   6. 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
   7. 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. 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?

I

MC

K/U

C

C

K/U

K/U

C

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 (H 2 CO 3 ), a weak acid that reacts to

release H+and the bicarbonate ion (HCO 3 −). If the

blood contains excess H+, it is too acidic. In this

situation, excess H+combines with the HCO 3 −to

form H 2 CO 3.

In basic conditions, when there is a low

concentration of H+, carbonic acid dissolves to form

H+andHCO 3.

These reactions prevent major change in blood

pH. The reactions are linked to the respiratory

system, because H 2 CO 3 reacts in solution to form

carbon dioxide (CO 2 ) and water. Therefore, levels

of H 2 CO 3 are linked to levels of CO 2 , which are

regulated by breathing. When the reactions are

combined, the result is a series of reversible

reactions that act to control pH.

When carbon dioxide (CO 2 ) 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 CO 2 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 (NH 4 +),

while raising pH.

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.

NH 3 HN 4 +

ammonia hydrogen ion ammonium ion

+ H+

H 2 O +CO 2

water carbon

dioxide

H+ + HCO 3 − H 2 CO 3

hydrogen

ion

bicarbonate

ion

carbonic

acid

↼⇁ ↼⇁

H 2 CO 3

carbonic acid

↼⇁ H+ + HCO 3 −

hydrogen ion bicarbonate ion

H+ + HCO 3 − ⇁↼ H 2 CO 3

hydrogen ion bicarbonate ion carbonic acid