Human Physiology, 14th edition (2016)

Respiratory Physiology 567

16.8 ACID-BASE BALANCE

OF THE BLOOD

The pH of blood plasma is maintained within a narrow

range of values through the functions of the lungs and

kidneys. The lungs regulate the carbon dioxide concentra-

tion of the blood, and the kidneys regulate the bicarbonate

concentration.

oxyhemoglobin has a weaker affinity for H^1 than does deoxy-
hemoglobin, hydrogen ions are released within the red blood
cells. This attracts HC O^3 2 from the plasma, which combines
with H^1 to form carbonic acid:

H 1 1 HC O 2 3 → H^2 C O^3
Under conditions of lower P^ CO 2 , as occurs in the pulmonary
capillaries, carbonic anhydrase catalyzes the conversion of car-
bonic acid to carbon dioxide and water:

carbonic anhydrase

H^2 C O^3 C O (^2 1) H (^2) O
Low P^ CO 2
In review, as blood goes through the systemic capillaries
the carbonic anhydrase within the red blood cells converts car-
bon dioxide into carbonic acid. Dissociation of the carbonic
acid into bicarbonate and H^1 results in the diffusion of bicar-
bonate out of the red blood cells into the plasma in exchange
for chloride. This part of the carbon dioxide transport story is
described as the chloride shift.
A reverse chloride shift operates in the pulmonary capil-
laries to convert carbonic acid to H 2 O and CO 2 gas, which is
eliminated in the expired breath ( fig. 16.39 ). The P^ CO 2 , carbonic
acid, H^1 , and bicarbonate concentrations in the systemic arter-
ies are thus maintained relatively constant by normal ventila-
tion. This is required to maintain the acid-base balance of the
blood ( fig. 16.40 ), as discussed in section 16.8.
Figure 16.40 The effect of bicarbonate on blood pH. Bicarbonate released into the plasma from red blood cells buffers
the H^1 produced by the ionization of metabolic acids (lactic acid, fatty acids, ketone bodies, and others). Binding of H^1 to hemoglobin
also promotes the unloading of O 2.
Plasma
Bicarbonate
buffer
Nonvolatile
(metabolic) acid
Red blood cells
Capillary
Tissue cells
pH = 7.40 as buffer reserve
HH++
H+
Red blob
H 2 CO 3
Cl–
HCO 3 –
Anion + H+ + HCO 3
HCO 3 –

CCl–

HCOHC 3 – H

H+

CO 2 + H 2 O

H 2 CO 3

Hemoglobin

HCO 3 –

CO 2

| CHECKPOINT

13a. List the ways in which carbon dioxide is carried by

the blood, and indicate the percentage of the total

carried for each.

13b. Where in the body does the chloride shift occur?

Describe the steps involved in the chloride shift.

13c. Where in the body does a reverse chloride shift

occur? Describe this process.

14. Using equations, show how carbonic acid and
    bicarbonate are formed. Explain how carbon dioxide
    transport influences blood pH.