Human Physiology, 14th edition (2016)

568 Chapter 16

Metabolic acidosis can result from excessive production of non-

volatile acids; for example, it can result from excessive production

of ketone bodies in uncontrolled diabetes mellitus (chapter 19,

section 19.4). It can also result from the loss of bicarbonate, in

which case there would not be sufficient free bicarbonate to buf-

fer the nonvolatile acids. (This occurs in diarrhea because of the

loss of bicarbonate derived from pancreatic juice—chapter 18,

section 18.5.) Metabolic alkalosis, by contrast, can be caused by

either too much bicarbonate (perhaps from an intravenous infu-

sion) or inadequate nonvolatile acids (perhaps as a result of exces-

sive vomiting). Excessive vomiting may cause metabolic alkalosis

through loss of the acid in gastric juice, which is normally absorbed

from the intestine into the blood.

Since the respiratory component of acid-base balance is

represented by the plasma carbon dioxide concentration and

the metabolic component is represented by the free bicarbon-

ate concentration, the study of acid-base balance can be sim-

plified. A normal arterial blood pH is obtained when there is

a proper ratio of bicarbonate to carbon dioxide. The pH can

be calculated given these values, and a normal pH is obtained

when the ratio of these concentrations is 20 to 1. This is given

by the Henderson-Hasselbalch equation:

pH 5 6.1 1 log

[HC O^23 ]

0.03 PC O (^2)
where P^ CO 2 is the partial pressure of CO 2 , which is proportional
to its concentration.
Respiratory acidosis or alkalosis occurs when the car-
bon dioxide concentrations are abnormal. Metabolic acidosis
and alkalosis occur when the bicarbonate concentrations are

Principles of Acid-Base Balance

The blood plasma within arteries normally has a pH between
7.35 and 7.45, with an average of 7.40. Using the definition of
pH described in chapter 2, this means that arterial blood has a
H^1 concentration of about 10^2 7.4 molar. Some of these hydro-
gen ions are derived from the ionization of carbonic acid formed
from carbon dioxide and water as indicated in these equations:

C O^2 1 H^2 O →← H^2 C O^3
H^2 C O^3 →← H 1 1 HC O^3 2
As previously described, carbon dioxide produced by tis-
sue cells through aerobic respiration is transported mostly as
bicarbonate in the blood plasma (see fig.  16.39 ). During the
reverse chloride shift that occurs in pulmonary capillaries,
bicarbonate is converted into carbonic acid and then changed
into carbon dioxide. Because CO 2 is a volatile gas released
in the expired breath, carbonic acid is referred to as a vola-
tile acid. This is significant because its blood concentration is
uniquely regulated by breathing. All other acids in the blood—
including lactic acid, fatty acids, ketone bodies, and so on—are
nonvolatile acids that cannot be eliminated through ventilation.
Under normal conditions, the H^1 released by nonvolatile met-
abolic acids does not affect the blood pH because these hydrogen
ions are bound to molecules that function as buffers. The major
buffer in the plasma is the bicarbonate (HC O^3 2 ) ion, and it buffers
H^1 as shown in figure 16.40 and described in this equation:

HC O 2 3 1 H 1 → H^2 C O^3
This buffering reaction could not go on forever because the
free HC O^3 2 would eventually disappear. If this were to occur,
the H^1 concentration would increase and the pH of the blood
would decrease. Under normal conditions, however, exces-
sive H^1 is eliminated in the urine by the kidneys. Through this
action, and through their ability to produce bicarbonate, the
kidneys are responsible for maintaining a normal concentra-
tion of free bicarbonate in the plasma. The role of the kidneys
in acid-base balance is described in chapter 17, section 17.5.
A fall in blood pH below 7.35 is called acidosis because
the pH is to the acid side of normal. Acidosis does not mean
acidic (pH less than 7); a blood pH of 7.2, for example, repre-
sents serious acidosis. Similarly, a rise in blood pH above 7.45
is called alkalosis. Both of these conditions are categorized
into respiratory and metabolic components of acid-base bal-
ance ( table 16.10 ).
Respiratory acidosis is caused by inadequate ventilation
(hypoventilation), which results in a rise in the plasma concentration
of carbon dioxide, and thus carbonic acid. Respiratory alkalosis,
by contrast, is caused by excessive ventilation (hyperventilation).

Table 16.10 | Terms Used to Describe

Acid-Base Balance

Term Definition

Acidosis,

respiratory

Increased CO 2 retention (due to hypoventilation),

which can result in the accumulation of

carbonic acid and thus a fall in blood pH to

below normal

Acidosis,

metabolic

Increased production of “nonvolatile” acids,

such as lactic acid, fatty acids, and ketone

bodies, or loss of blood bicarbonate (such as

by diarrhea), resulting in a fall in blood pH to

below normal

Alkalosis,

respiratory

A rise in blood pH due to loss of CO 2 and

carbonic acid (through hyperventilation)

Alkalosis,

metabolic

A rise in blood pH produced by loss of nonvolatile

acids (such as excessive vomiting) or by

excessive accumulation of bicarbonate base

Compensated

acidosis

or alkalosis

Metabolic acidosis or alkalosis are partially

compensated for by opposite changes in

blood carbonic acid levels (through changes in

ventilation). Respiratory acidosis or alkalosis are

partially compensated for by increased retention

or excretion of bicarbonate in the urine.

LEARNING OUTCOMES

After studying this section, you should be able to:

15. Describe the acid-base balance of the blood, and
    how it is influenced by the respiratory system.