The Oxyhemoglobin Dissociation Curve

Blood in the systemic arteries, at a PO2 of 100 mmHg, has apercent oxyhemoglobin saturation of 97% (which means that 97% of the hemoglobin is in the form of oxyhemoglobin). This blood is delivered to the systemic capillaries, where oxygen diffuses into the cells and is consumed in aerobic respiration. Blood leaving in the systemic veins is thus reduced in oxygen; it has a PO2 of about 40 mmHg and a percent oxyhemoglobin saturation of about 75% when a person is at rest (table 16.7). Expressed another way, blood entering the tissues contains 20 ml O2 per 100 ml blood, and blood leaving the tissues contains 15.5 ml O2

per 100 ml blood (fig. 16.34). Thus, 22%, or 4.5 ml of O2 out of the 20 ml of O2 per 100 ml blood, is unloaded to the tissues.

A graphic illustration of the percent oxyhemoglobin saturation at different values of PO2 is called an oxyhemoglobin dissociation curve (fig. 16.34). The values in this graph are obtained by subjecting samples of blood in vitro to different partial oxygen pressures. The percent oxyhemoglobin saturations obtained, however, can be used to predict what the unloading percentages would be in vivo with a given difference in arterial and venous PO2 values.

Figure 16.34 shows the difference between the arterial and venous PO2 and the percent oxyhemoglobin saturation at rest. The relatively large amount of oxyhemoglobin remaining in the venous blood at rest serves as an oxygen reserve. If a person stops breathing, a sufficient reserve of oxygen in the blood will keep the brain and heart alive for about 4 to 5 minutes without using cardiopulmonary resuscitation (CPR) techniques. This reserve supply of oxygen can also be tapped when a tissue's requirements for oxygen are raised.

The oxyhemoglobin dissociation curve is S-shaped, or sig-moidal. The fact that it is relatively flat at high PO2 values indicates that changes in PO2 within this range have little effect on the loading reaction. One would have to ascend as high as 10,000 feet, for example, before the oxyhemoglobin saturation of arterial blood would decrease from 97% to 93%. At more common elevations, the percent oxyhemoglobin saturation would not be significantly different from the 97% value at sea level.

Table 16.8 Effect of pH on

Hemoglobin Affinity for Oxygen and Unloading

of Oxygen to the Tissues

Arterial O2 Content

Venous O2 Content

O2 Unloaded to

pH Affinity

per 100 ml

per 100 ml

Tissues per 100 ml

7.40 Normal

19.8 ml O2

14.8 ml O2

5.0 ml O2

7.60 Increased

20.0 ml O2

17.0 ml O2

3.0 ml O2

7.20 Decreased

19.2 ml O2

12.6 ml O2

6.6 ml O2

10 20 30 40 50 60 70 80 90 100 110 120 130 140 Po2 (mmHg)

■ Figure 16.35 The effect of pH on the oxyhemoglobin dissociation curve. A decrease in blood pH (an increase in H+ concentration) decreases the affinity of hemoglobin for oxygen at each PO2 value, resulting in a "shift to the right" of the oxyhemoglobin dissociation curve. This is called the Bohr effect. A curve that is shifted to the right has a lower percent oxyhemoglobin saturation at each POr

At the steep part of the sigmoidal curve, however, small changes in PO2 values produce large differences in percent saturation. A decrease in venous PO2 from 40 mmHg to 30 mmHg, as might occur during mild exercise, corresponds to a change in percent saturation from 75% to 58%. Since the arterial percent saturation is usually still 97% during exercise, the lowered venous percent saturation indicates that more oxygen has been unloaded to the tissues. The difference between the arterial and venous percent saturations indicates the percent unloading. In the preceding example, 97% - 75% = 22% unloading at rest, and 97% - 58% = 39% unloading during mild exercise. During heavier exercise, the venous PO2 can drop to 20 mmHg or lower, indicating a percent unloading of about 80%.

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Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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