Most of the oxygen in the blood is contained within the red blood cells, where it is chemically bonded to hemoglobin. As described in chapter 13, each hemoglobin molecule consists of four polypeptide chains called globins and four nitrogen-containing, disc-shaped organic pigment molecules called hemes (fig. 16.33).

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 combine 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 (Fe2+, or ferrous iron). In this form, the iron can share electrons and bond with oxygen to form oxyhemoglobin. When oxyhemoglobin dissociates to release oxygen to the tissues, the heme iron is still in the reduced (Fe2+) form and the hemoglobin is called deoxy-hemoglobin, or reduced hemoglobin. The term oxyhemoglobin 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 (Fe3+, or ferric) state. Methemoglobin thus lacks the electron it needs to form a bond with oxygen and cannot

Test Yourself Before You Continue

1. Describe the effects of voluntary hyperventilation and breath holding on arterial PCOj, pH, and oxygen content. Indicate the relative degree of changes in these values.

2. Using a flowchart to show a negative feedback loop, explain the relationship between ventilation and arterial Pco2.

3. Explain the effect of increased arterial PCOj on (a) chemoreceptors in the medulla oblongata and (b) chemoreceptors in the aortic and carotid bodies.

4. Explain the role of arterial Po in the regulation of breathing. Why does ventilation increase when a person goes to a high altitude?


Whole blood

Po2= 100 •

Fox: Human Physiology, Eighth Edition

16. Respiratory Physiology I Text

© The McGraw-H Companies, 2003

Respiratory Physiology


Beta chain

Alpha chain

Beta chain

Alpha chain

Beta chain

Heme group

Alpha chain

Beta chain


Heme group

Alpha chain


ch2 I 2 COOH

ch2 ch2


Figure 16.33 The structure of hemoglobin. (a) An illustration of the three-dimensional structure of hemoglobin in which the two alpha and two beta polypeptide chains are shown. The four heme groups are represented as flat structures with atoms of iron (spheres) in the centers. (b) The structural formula for heme.

participate in oxygen transport. Blood normally contains only a small amount of methemoglobin, but certain drugs can increase this amount.

In carboxyhemoglobin, another abnormal form of hemoglobin, the reduced heme is combined with carbon monoxide instead of oxygen. Since the bond with carbon monoxide is about 210 times stronger than the bond with oxygen, carbon monoxide tends to displace oxygen in hemoglobin and remains attached to hemoglobin as the blood passes through systemic capillaries. In carbon monoxide poisoning—which, in severe form results primarily from smoke inhalation and suicide attempts, and in milder forms from breathing smoggy air and smoking cigarettes—the transport of oxygen to the tissues is reduced.

According to federal standards, the percentage of car-boxyhemoglobin in the blood of active nonsmokers should be no higher than 1.5%. However, concentrations of 3% in nonsmokers and 10% in smokers have been reported in some cities. Although these high levels may not cause immediate problems in healthy people, long-term adverse effects on health are possible. People with respiratory or cardiovascular diseases would be particularly vulnerable to the negative effects of carboxyhemoglobin on oxygen transport.

Clinical Investigation Clues

Remember that Harry smoked cigarettes, drove a taxi, and had a carboxyhemoglobin saturation of 18% How are these observations related?

What danger does this pose to Harry (note—be sure to take his other problems into account)?

What can he do to lower his carboxyhemoglobin saturation?

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