Biology 12

Chapter 4 Homeostatic Mechanisms • MHR 117

Urine Output

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.

Figure 4.13When 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.

Blood pH and the Kidney

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 buffersto

Continued on page 120 ➥

http://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.

WEB LINK

For more information about how the kidney functions, refer

to your Electronic Learning Partner.

ELECTRONIC LEARNING PARTNER

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.

BIO FACT

Kidney returns less water

to the blood, resulting

in increased urine output.

high fluid intake

hypothalamus

reduced ADH

production