Figure 409

almost all oxygen transport from the lungs to the tissues.

Oxygen-Hemoglobin Dissociation Curve. Figure 40-8 shows the oxygen-hemoglobin dissociation curve, which demonstrates a progressive increase in the percentage of hemoglobin bound with oxygen as blood Po2 increases, which is called the per cent saturation of hemoglobin. Because the blood leaving the lungs and entering the systemic arteries usually has a Po2 of about 95 mm Hg, one can see from the dissociation curve that the usual oxygen saturation of systemic arterial blood averages 97 per cent. Conversely, in normal venous blood returning from the peripheral tissues, the Po2 is about 40 mm Hg, and the saturation of hemoglobin averages 75 per cent.

Maximum Amount of Oxygen That Can Combine with the Hemoglobin of the Blood. The blood of a normal person contains about 15 grams of hemoglobin in each 100 milliliters of blood, and each gram of hemoglobin can bind with a maximum of 1.34 milliliters of oxygen (1.39 milliliters when the hemoglobin is chemically pure, but impurities such as methemoglobin reduce this). Therefore, 15 times 1.34 equals 20.1, which means that, on average, the 15 grams of hemoglobin in 100 milliliters of blood can combine with a total of almost exactly 20 milliliters of oxygen if the hemoglobin is 100 per cent saturated. This is usually expressed as 20 volumes per cent. The oxygen-hemoglobin dissociation curve for the normal person can also be expressed in terms of volume per cent of oxygen, as shown by the far right scale in Figure 40-8, instead of per cent saturation of hemoglobin.

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