Essentials of Anatomy and Physiology

know, the respiratory system may be the cause of a pH
imbalance or may help correct a pH imbalance from
some other cause.

Respiratory Acidosis and Alkalosis

Respiratory acidosis is caused by anything that
decreases the rate or efficiency of respiration. Severe
pulmonary diseases are possible causes of respiratory
acidosis. When CO 2 cannot be exhaled as fast as it
is formed during cell respiration, excess CO 2 results
in the formation of excess Hions, as shown in this
reaction:

CO 2 H 2 O →H 2 CO 3 →HHCO 3 

The excess Hions lower the pH of body fluids.
Respiratory alkalosisis far less common but is the
result of breathing more rapidly, which increases the
amount of CO 2 exhaled. Because there are fewer CO 2

molecules in the body fluids, fewer H ions are

formed, and pH tends to rise.

Respiratory Compensation

for Metabolic pH Changes

Changes in pH caused by other than a respiratory dis-

order are called metabolic acidosis or alkalosis. In

either case, the respiratory system may help prevent a

drastic change in pH.

Metabolic acidosismay be caused by kidney dis-

ease, uncontrolled diabetes mellitus, excessive diar-

rhea or vomiting, or the use of some diuretics. When

excess Hions are present in body fluids, pH begins

to decrease, and this stimulates the respiratory centers

in the medulla. The response is to increase the rate of

breathing to exhale more CO 2 to decrease Hion for-

mation. This helps raise the pH back toward the nor-

mal range.

Fluid–Electrolyte and Acid–Base Balance 449

Peritubular

capillary Filtrate

Renal tubule

Na+ Na+ Na+

Na 2

HCO 3 – HCO 3 – H+ H+

CO 2 H 2 O

HPO 4

 H 2 CO 3

+

Blood Urine

NaH 2 PO 4

Na+HPO 4 -2

Figure 19–4. The phosphate buffer system.
The reactions are shown in a kidney tubule. See
text for description.
QUESTION:From where does the kidney tubule
cell get a hydrogen ion to excrete?