Human Physiology, 14th edition (2016)

Respiratory Physiology 559

Figure 16.31 The oxygen content of blood. Plasma

and whole blood that are brought into equilibrium with the same

gas mixture have the same PO 2 , and thus the same number of

dissolved oxygen molecules (shown as blue dots). The oxygen

content of whole blood, however, is much higher than that of

plasma because of the binding of oxygen to hemoglobin, which

is normally only found in red blood cells (not shown).

Oxygen

content

0.3 ml O 2

Po 2 = 100

Plasma Whole blood

O 2 Oxyhemoglobin

Po 2 = 100

100 ml

20.0 ml O 2

100 ml

Gas tank

PO 2 = 100 mmHg

normal, this blood carries approximately 20 ml of O 2 per 100 ml

of blood ( fig. 16.31 ).

Hemoglobin

Most of the oxygen in the blood is contained within the red

blood cells, where it is chemically bonded to hemoglobin.

Each hemoglobin molecule consists of four polypeptide chains

called globins and four iron-containing, disc-shaped organic

pigment molecules called hemes ( fig. 16.32 ).

The protein part of hemoglobin is composed of two identical

alpha chains, each 141 amino acids long, and two identical beta

chains, each 146 amino acids long. Each of the four polypeptide

chains is combined with one heme group. In the center of each

heme group is one atom of iron, which can combine with one

molecule of oxygen. One hemoglobin molecule can thus com-

bine with four molecules of oxygen—and since there are about

280 million hemoglobin molecules per red blood cell, each red

blood cell can carry over a billion molecules of oxygen.

Normal heme contains iron in the reduced form (Fe^2 1 ,

or ferrous iron). In this form, the iron can share electrons and

bond with oxygen to form oxyhemoglobin. When oxyhemo-

globin dissociates to release oxygen to the tissues, the heme

iron is still in the reduced (Fe^2 1 ) form and the hemoglobin is

called deoxyhemoglobin, or reduced hemoglobin. The term

oxy-hemoglobin is thus not equivalent to oxidized hemoglobin;

hemoglobin does not lose an electron (and become oxidized)

when it combines with oxygen. Oxidized hemoglobin, or

methemoglobin, has iron in the oxidized (Fe^3 1 , or ferric) state.

Methemoglobin thus lacks the electron it needs to form a bond

the newborn. Pulmonary stretch receptors in adults, however,
are probably not active at normal resting tidal volumes (500 ml
per breath) but may contribute to respiratory control at high
tidal volumes, as during exercise.

| CHECKPOINT

9a. Describe the roles of centers in the brain stem and

cervical spinal cord in the regulation of breathing.

9b. Describe the effects of voluntary hyperventilation

and breath holding on arterial PCO 2 , pH, and oxygen

content. Indicate the relative degree of changes in

these values.

10a. Using a flowchart to show a negative feedback loop,

explain the relationship between ventilation and

arterial PCO 2.

10b. Explain the effect of increased arterial PCO 2 on

(a) chemoreceptors in the medulla oblongata and

(b) chemoreceptors in the aortic and carotid bodies.

10c. Explain the role of arterial PO 2 in the regulation of

breathing. Why does ventilation increase when a

person goes to a high altitude?

16.6 HEMOGLOBIN AND OXYGEN

TRANSPORT

Deoxyhemoglobin loads with oxygen to form oxyhemo-

globin in the pulmonary capillaries, and a portion of the

oxyhemoglobin unloads its oxygen in the capillaries of the

systemic circulation. The bond strength between hemo-

globin and oxygen, and thus the extent of unloading, is

changed under different conditions.

LEARNING OUTCOMES

After studying this section, you should be able to:

11. Describe the changes in percent oxyhemoglobin as
    a function of arterial PO 2 and explain how this relates
    to oxygen transport.


12. Describe the various conditions that influence the
    oxyhemoglobin dissociation curve and oxygen
    transport.

If the lungs are functioning properly, blood leaving in the pul-
monary veins and traveling in the systemic arteries has a P O 2 of
about 100 mmHg, indicating a plasma oxygen concentration
of about 0.3 ml O 2 per 100 ml blood. The total oxygen content of
the blood, however, cannot be derived if only the P O 2 of plasma
is known. The total oxygen content depends not only on the P O 2
but also on the hemoglobin concentration. Arterial blood can
carry 1.34 ml of oxygen per gram of hemoglobin. Therefore, if
the P O 2 and hemoglobin concentration of the arterial blood are