HUMAN BIOLOGY

232 Chapter 12

acid–base balance The
relative amounts of acidic
and basic substances in
extracellular fluid, including
the blood.

metabolic acidosis Higher
than normal acidity in blood
and other body fluids.

metabolic alkalosis Higher
than normal alkalinity in
blood and other body fluids.

hoW do the kidneys help keep the

eXtracellular fluid from becoming

too acidic or too basic?

• The urinary system eliminates excess hydrogen ions and also
  replenishes bicarbonate used in buffering reactions.


• Serious acid–base imbalances include metabolic acidosis (body
  fluids that are too acidic) and metabolic alkalosis (body fluids
  that are too basic).

taKe-Home message

removing excess acids and other

substances in urine

of nephron tubule walls. For example, when the blood

is too acidic (a too high concentration of H^1 ), water and

carbon dioxide combine with the help of an enzyme. They

form carbonic acid that then can be broken into bicarbonate

and H^1. Figure 12.11 summarizes these steps.

As you can see, bicarbonate produced in the reactions

moves into peritubular capillaries. It ends up circulating

in the blood, where it buffers excess H^1. When the blood

is too basic (alkaline), chemical adjustments in the kidneys

normally ensure that less bicarbonate is reabsorbed into

the bloodstream.

The H^1 that is formed in the tubule cells is secreted into

the filtrate in the tubule. There the excess H^1 may combine

with phosphate ions, ammonia (NH 3 ), or even bicarbonate.

In this way the excess H^1 is excreted.

Various factors may cause serious

acid–base imbalances

Maintaining proper blood pH is crucial to homeostasis. If

the pH of blood falls outside the normal range for long, the

most serious impact occurs in the central nervous system

(brain and spinal cord). When severe diarrhea, kidney dis-

ease, or some other problem prevents kidneys from excret-

ing enough acid, the result is metabolic acidosis. Then

nerve cells cannot communicate properly and an affected

person may fall into a fatal coma.

Severe vomiting or dehydration, hormonal disorders,

and overuse of antacids are common causes of metabolic

alkalosis, or blood that is too basic. Then nerve cells are

overstimulated, so a person may suffer muscle spasms, ner-

vousness, or convulsions. In the next two sections you will

find information about other major disorders that prevent

the urinary system from functioning normally.

n    As urine forms, nephrons make adjustments that help keep

the extracellular fluid from becoming too acidic or too basic.

n    Links to pH scale 2.7, Breathing controls 10.6

the kidneys play a key role in maintaining

the balance of acids and bases in the blood

You may recall from Chapter 2 that normal pH in the blood

and other body fluids is between 7.3 and 7.5. Because acids

lower pH and bases raise it, pH reflects the body’s acid–

base balance—the relative amounts of acidic and basic

substances in extracellular fluid. Remember also that a

buffer system involves substances that reversibly bind and

release H^1 and OH^2 ions. Buffers minimize pH changes as

acidic or basic molecules enter or leave body fluids.

Chapter 10 described how bicarbonate (HCO 32 ) serves

as a buffer in the lungs. It forms when carbon dioxide com-

bines with water. The bicarbonate then reacts with H^1 to

form carbonic acid, and enzyme action converts carbonic

acid into water and carbon dioxide. The CO 2 is exhaled,

while the hydrogen ions are now a

part of water molecules. H^1 is not

eliminated permanently, however.

Only the kidneys can do that. They

also restore the buffer bicarbonate.

Depending on changes in the

acid–base balance of the blood that

enters nephrons, the kidneys can

either excrete bicarbonate or form

new bicarbonate and add it to the

blood (Table 12.3). The necessary

chemical reactions go on in the cells

(carbonic

acid)

bicarbonate

enters

bloodstream

tissue

fluid

peritubular

capillary

cells of

tubule wall

tubule

interior

H 2 O 1 CO 2

enzyme action

H 2 CO 3

H^1 1 ammonia H^1 HCO^32

H^1 1 phosphate

excreted

in

urine

F i g u r e 12 .11 Animated! The kidneys remove H^1 from the body,

preventing the blood from becoming too acidic. (© Cengage Learning)

12.5

Physiological change Response in Nephron Tubules

Blood too acidic (pH too low) More bicarbonate (HCO 32 )

enters the bloodstream,

buffers excess H^1

Blood too basic (pH too high) Bicarbonate excreted in urine

Table 12.3 Snapshot of How the Kidneys Help

Maintain Blood pH

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