The concentration of oxygen in arterial blood, by volume, is about 20 mL/dL. About 98.5% of this is bound to hemoglobin and 1.5% is dissolved in the blood plasma. Hemoglobin consists of four protein (globin) chains, each with one heme group (see fig. 18.10, p. 690). Each heme group can bind 1 O2 to the ferrous ion at its center; thus, one hemoglobin molecule can carry up to 4 O2. If even one molecule of O2 is bound to hemoglobin, the compound is called oxyhemoglobin (HbO2), whereas hemoglobin with no oxygen bound to it is deoxyhemoglobin (HHb). When hemoglobin is 100% saturated, every molecule of it carries 4 O 2; if it is 75% saturated, there is an average of 3 O2 per hemoglobin molecule; if it is 50% saturated, there is an average of 2 O2 per hemoglobin; and so forth. The poisonous effect of carbon monoxide stems from its competition for the O2 binding site (see insight 22.3).

The relationship between hemoglobin saturation and Po2 is shown by an oxyhemoglobin dissociation curve (fig. 22.21). As you can see, it is not a simple linear relationship. At low Po2, the curve rises slowly; then there is a

Oxyhemoglobin Dissociation Curve

Systemic tissues Alveoli t t

Systemic tissues Alveoli

Partial pressure of O2 (Po2) in mmHg

Figure 22.21 The Oxyhemoglobin Dissociation Curve. This curve shows the relative amount of hemoglobin that is saturated with oxygen (/-axis) as a function of ambient (surrounding) oxygen concentration (x-axis). As it passes through the alveolar capillaries where the Po2 is high, hemoglobin becomes saturated with oxygen. As it passes through the systemic capillaries where the Po2 is low, it typically gives up about 22% of its oxygen (color bar at top of graph). What would be the approximate utilization coefficient if the systemic tissues had a Po2 of 20 mmHg?

rapid increase in oxygen loading as Po2 rises further; finally, at high Po2, the curve levels off as the hemoglobin approaches 100% saturation. This reflects the way hemoglobin loads oxygen. When the first heme group binds a molecule of o2, hemoglobin changes shape in a way that

Saladin: Anatomy & I 22. The Respiratory System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

864 Part Four Regulation and Maintenance facilitates uptake of the second O2 by another heme group. This, in turn, promotes the uptake of the third and then the fourth O2—hence the rapidly rising midportion of the

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