Human Physiology, 14th edition (2016)

Respiratory Physiology 565

16.7 CARBON DIOXIDE

TRANSPORT

Carbon dioxide is transported in the blood primarily in the

form of bicarbonate (HC O^3  2 ), which is released when car-

bonic acid dissociates. Carbonic acid is produced mostly

in the red blood cells as blood passes through systemic

capillaries.

but it has one heme rather than four; therefore, it can combine
with only one molecule of oxygen.
Myoglobin has a higher affinity for oxygen than does hemo-
globin, and its dissociation curve is therefore to the left of the
oxyhemoglobin dissociation curve ( fig.  16.37 ). The shape of the
myoglobin curve is also different from the oxyhemoglobin disso-
ciation curve. The myoglobin curve is rectangular, indicating that
oxygen will be released only when the P^ O 2 becomes very low.
Since the P^ O 2 in mitochondria is very low (because oxy-
gen is incorporated into water here), myoglobin may act as
a “go-between” in the transfer of oxygen from blood to the
mitochondria within muscle cells. Myoglobin may also have
an oxygen-storage function, which is of particular importance
in the heart. During diastole, when the coronary blood flow
is greatest, myoglobin can load up with oxygen. This stored
oxygen can then be released during systole, when the coronary
arteries are squeezed closed by the contracting myocardium.

Figure 16.37 A comparison of the dissociation curves for hemoglobin and myoglobin. Myoglobin is an oxygen-
binding pigment in skeletal muscles. At the PO 2 of venous blood, the myoglobin retains almost all of its oxygen, indicating that it has a
higher affinity than hemoglobin for oxygen. The myoglobin, however, does release its oxygen at the very low PO 2 values found inside the
mitochondria.

Myoglobin

Hemoglobin

Venous blood Arterial blood

100

80

60

40

20

0

0 20 40 60 80 100 120

Percent oxygen saturation

PO 2 (mmHg)

| CHECKPOINT

11a. Use a graph to illustrate the effects of P^ O 2 on the

loading and unloading reactions.

11b. Draw an oxyhemoglobin dissociation curve and

label the P^ O 2 values for arterial and venous blood

under resting conditions. Use this graph to show the

changes in unloading that occur during exercise.

12a. Explain how changes in pH and temperature affect

oxygen transport, and state when such changes occur.

12b. Explain how a person who is anemic or a person at

high altitude could have an increase in the percent

unloading of oxygen by hemoglobin.

LEARNING OUTCOMES

After studying this section, you should be able to:

13. Explain how carbon dioxide is transported by the
    blood.


14. Explain the relationship between blood levels of
    carbon dioxide and the blood pH.

Carbon dioxide is carried by the blood in three forms: (1) as

dissolved CO 2 in the plasma—carbon dioxide is about 21 times

more soluble than oxygen in water, and about one-tenth of

the total blood CO 2 is dissolved in plasma; (2) as carbamino-

hemoglobin —about one-fifth of the total blood CO 2 is carried

attached to an amino acid in hemoglobin (carbaminohemoglo-

bin should not be confused with carboxyhemoglobin, formed

when carbon monoxide binds to the heme groups of hemoglo-

bin); and (3) as bicarbonate ion, which accounts for most of

the CO 2 carried by the blood ( fig. 16.38 ).

Carbon dioxide is able to combine with water to form car-

bonic acid. This reaction occurs spontaneously in the plasma

at a slow rate, but it occurs much more rapidly within the red