Human Physiology, 14th edition (2016)

Respiratory Physiology 569

A change in blood pH, produced by alterations in either

the respiratory or metabolic component of acid-base balance,

can be partially compensated for by a change in the other

component. For example, a person with metabolic acidosis

will hyperventilate. This is because the aortic and carotid

bodies are stimulated by an increased blood H^1 concentra-

tion (fall in pH). As a result of the hyperventilation, a sec-

ondary respiratory alkalosis is produced. The person is still

acidotic, but not as much so as would be the case without the

compensation. People with partially compensated metabolic

acidosis would thus have a low pH, which would be accom-

panied by a low blood P CO 2 as a result of the hyperventila-

tion. Metabolic alkalosis, similarly, is partially compensated

for by the retention of carbonic acid due to hypoventilation

( table 16.12 ).

abnormal ( table 16.11 ). Often, however, a primary disturbance
in one area (for example, metabolic acidosis) will be accom-
panied by secondary changes in another area (for example,
respiratory alkalosis). It is important for hospital personnel
to identify and treat the area of primary disturbance, but such
analysis lies outside the scope of this discussion.

Ventilation and Acid-Base Balance

In terms of acid-base regulation, the acid-base balance of the
blood is divided into the respiratory component and the meta-
bolic component. The respiratory component refers to the carbon
dioxide concentration of the blood, as measured by its P CO 2. As
implied by its name, the respiratory component is regulated by
the respiratory system. The metabolic component is controlled
by the kidneys, and is discussed in chapter 17, section 17.5.
Ventilation is normally adjusted to keep pace with the meta-
bolic rate, so that the arterial P CO 2 remains in the normal range.
In hypoventilation, the ventilation is insufficient to “blow off ”
carbon dioxide and maintain a normal P CO 2. Indeed, hypoventila-
tion can be operationally defined as an abnormally high arterial
P CO 2. Under these conditions, the concentration of carbonic acid
is excessively high and respiratory acidosis occurs.
In hyperventilation, conversely, the rate of ventilation is
greater than the rate of CO 2 production. Arterial P CO 2 therefore
decreases so that less carbonic acid is formed than under nor-
mal conditions. The depletion of carbonic acid raises the pH
of the blood and respiratory alkalosis occurs. Hyperventilation
can cause dizziness because it also raises the pH of the CSF
and brain interstitial fluid, which induces cerebral vasocon-
striction. The resulting reduction in brain blood flow produces
the dizziness.

Table 16.11 | Classification of Metabolic and Respiratory Components of Acidosis
and Alkalosis

Plasma CO 2 Plasma HCO 3 ̄ Condition Causes

Normal Low Metabolic acidosis Increased production of “nonvolatile” acids (lactic acids, ketone

bodies, and others), or loss of HCO 3 – in diarrhea

Normal High Metabolic alkalosis Vomiting of gastric acid; hypokalemia; excessive steroid administration

Low Low Respiratory alkalosis Hyperventilation

High High Respiratory acidosis Hypoventilation

Table 16.12 | Effect of Lung Function on Blood Acid-Base Balance

Condition pH PCO 2 Ventilation Cause or Compensation

Normal 7.35 –7.45 39 – 41 mmHg Normal Not applicable

Respiratory acidosis Low High Hypoventilation Cause of the acidosis

Respiratory alkalosis High Low Hyperventilation Cause of the alkalosis

Metabolic acidosis Low Low Hyperventilation Compensation for acidosis

Metabolic alkalosis High High Hypoventilation Compensation for alkalosis

| CHECKPOINT

15a. Define the terms acidosis and alkalosis. Identify the

two components of blood acid-base balance.

15b. Explain the roles of the lungs and kidneys in maintaining

the acid-base balance of the blood.

15c. Describe the functions of bicarbonate and carbonic acid

in blood.

15d. Describe the effects of hyperventilation and

hypoventilation on the blood pH, and explain the

mechanisms involved.

15e. Explain why a person with ketoacidosis

hyperventilates. What are the potential benefits of

hyperventilation under these conditions?