The chemistry of hemoglobin is presented in Chapter 33, where we pointed out that the O2 molecule combines loosely and reversibly with the heme portion of hemoglobin. When PO2 is high, as in the pulmonary capillaries, O2 binds with the hemoglobin, but when PO2 is low, as in the tissue capillaries, O2 is released from the hemoglobin. This is the basis for almost all O2 transport from the lungs to the tissues.
Oxygen-Hemoglobin Dissociation Curve. Figure 1 shows the O2-hemoglobin dissociation curve, which demonstrates a progressive increase in the percentage of hemoglobin bound with O2 as blood PO2 increases, which is called the percent 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 O2 saturation of systemic arterial blood averages 97 percent. Conversely, in normal venous blood returning from the peripheral tissues, the PO2 is about 40 mm Hg, and the saturation of hemoglobin averages 75 percent.
Fig1. Oxygen-hemoglobin dissociation curve.
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 O2 (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 milliliter of blood can combine with a total of about 20 milliliters of O2 if the hemoglobin is 100 percent saturated. This is usually expressed as 20 volumes percent. The O2 hemoglobin dissociation curve for the normal person can also be expressed in terms of volume percent of O2, as shown by the far-right scale in Figure 1, instead of percent saturation of hemoglobin.
Amount of Oxygen Released From the Hemoglobin When Systemic Arterial Blood Flows Through the Tissues. The total quantity of O2 bound with hemoglobin in normal systemic arterial blood, which is 97 percent saturated, is about 19.4 milliliters per 100 milliliters of blood, as shown in Figure 2. Upon passing through the tissue capillaries, this amount is reduced, on average, to 14.4 milliliters (PO2 of 40 mm Hg, 75 percent saturated hemoglobin). Thus, under normal conditions, about 5 milliliters of O2 are transported from the lungs to the tissues by each 100 milliliters of blood flow.
Fig2. Effect of blood PO2 on the quantity of oxygen bound with hemoglobin in each 100 milliliters of blood.
Transport of Oxygen Is Markedly Increased during Strenuous Exercise. During heavy exercise, the muscle cells use O2 at a rapid rate, which, in extreme cases, can cause the muscle interstitial fluid PO2 to fall from the normal 40 mm Hg to as low as 15 mm Hg. At this low pressure, only 4.4 milliliters of O2 remain bound with the hemoglobin in each 100 milliliters of blood, as shown in Figure2. Thus, 19.4 − 4.4, or 15 milliliters, is the quantity of O2 actually delivered to the tissues by each 100 milliliters of blood flow, meaning that three times as much O2 as normal is delivered in each volume of blood that passes through the tissues. Keep in mind that the cardiac output can increase to six to seven times normal in well-trained marathon runners. Thus, multiplying the increase in cardiac output (6- to 7-fold) by the increase in O2 transport in each volume of blood (3-fold) gives a 20-fold increase in O2 transport to the tissues. We see later in the chapter that several other factors facilitate delivery of O2 into muscles during exercise, so muscle tissue PO2 often falls just slightly below normal even during very strenuous exercise.
Utilization Coefficient. The percentage of the blood that gives up its O2 as it passes through the tissue capillaries is called the utilization coefficient. The normal value for this is about 25 percent, as is evident from the preceding discussion—that is, 25 percent of the oxygenated hemoglobin gives its O2 to the tissues. During strenuous exercise, the utilization coefficient in the entire body can increase to 75 to 85 percent. In local tissue areas where blood flow is extremely slow or the metabolic rate is very high, utilization coefficients approaching 100 percent have been recorded—that is, essentially all the O2 is given to the tissues.
