Effect of blood Po2 on the quantity of oxygen bound with hemoglobin in each 100 milliliters of blood.
systemic arterial blood, which is 97 per cent saturated, is about 19.4 milliliters per 100 milliliters of blood.This is shown in Figure 40-9. On passing through the tissue capillaries, this amount is reduced, on average, to 14.4 milliliters (Po2 of 40 mm Hg, 75 per cent saturated hemoglobin). Thus, under normal conditions, about 5 milliliters of oxygen are transported from the lungs to the tissues by each 100 milliliters of blood flow.
Transport of Oxygen During Strenuous Exercise. During heavy exercise, the muscle cells use oxygen 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 oxygen remain bound with the hemoglobin in each 100 milliliters of blood, as shown in Figure 40-9. Thus, 19.4 - 4.4, or 15 milliliters, is the quantity of oxygen actually delivered to the tissues by each 100 milliliters of blood flow. Thus, three times as much oxygen as normal is delivered in each volume of blood that passes through the tissues. And 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 (six- to sevenfold) by the increase in oxygen transport in each volume of blood (threefold) gives a 20-fold increase in oxygen transport to the tissues. We see later in the chapter that several other factors facilitate delivery of oxygen into muscles during exercise, so that muscle tissue Po2 often falls very little below normal even during very strenuous exercise.
Utilization Coefficient. The percentage of the blood that gives up its oxygen as it passes through the tissue capillaries is called the utilization coefficient. The normal value for this is about 25 per cent, as is evident from the preceding discussion—that is, 25 per cent of the oxygenated hemoglobin gives its oxygen to the tissues. During strenuous exercise, the utilization coefficient in the entire body can increase to 75 to 85 per cent. And in local tissue areas where blood flow is extremely slow or the metabolic rate is very high, utilization coefficients approaching 100 per cent have been recorded—that is, essentially all the oxygen is given to the tissues.
Effect of Hemoglobin to "Buffer" the Tissue Po2
Although hemoglobin is necessary for the transport of oxygen to the tissues, it performs another function essential to life. This is its function as a "tissue oxygen buffer" system. That is, the hemoglobin in the blood is mainly responsible for stabilizing the oxygen pressure in the tissues. This can be explained as follows.
Role of Hemoglobin in Maintaining Nearly Constant Po2 in the Tissues. Under basal conditions, the tissues require about 5 milliliters of oxygen from each 100 milliliters of blood passing through the tissue capillaries. Referring back to the oxygen-hemoglobin dissociation curve in Figure 40-9, one can see that for the normal 5 milliliters of oxygen to be released per 100 milliliters of blood flow, the Po2 must fall to about 40 mm Hg. Therefore, the tissue Po2 normally cannot rise above this 40 mm Hg level, because if it did, the amount of oxygen needed by the tissues would not be released from the hemoglobin. In this way, the hemoglobin normally sets an upper limit on the oxygen pressure in the tissues at about 40 mm Hg.
Conversely, during heavy exercise, extra amounts of oxygen (as much as 20 times normal) must be delivered from the hemoglobin to the tissues. But this can be achieved with little further decrease in tissue Po2
because of (1) the steep slope of the dissociation curve and (2) the increase in tissue blood flow caused by the decreased Po2; that is, a very small fall in Po2 causes large amounts of extra oxygen to be released from the hemoglobin. It can be seen, then, that the hemoglobin in the blood automatically delivers oxygen to the tissues at a pressure that is held rather tightly between about 15 and 40 mm Hg.
When Atmospheric Oxygen Concentration Changes Markedly, the Buffer Effect of Hemoglobin Still Maintains Almost Constant Tissue Po2. The normal Po2 in the alveoli is about 104 mm Hg, but as one ascends a mountain or ascends in an airplane, the Po2 can easily fall to less than half this amount. Alternatively, when one enters areas of compressed air, such as deep in the sea or in pressurized chambers, the Po2 may rise to 10 times this level. Even so, the tissue Po2 changes little.
It can be seen from the oxygen-hemoglobin dissociation curve in Figure 40-8 that when the alveolar Po2 is decreased to as low as 60 mm Hg, the arterial hemoglobin is still 89 per cent saturated with oxygen—only 8 per cent below the normal saturation of 97 per cent. Further, the tissues still remove about 5 milliliters of oxygen from each 100 milliliters of blood passing through the tissues; to remove this oxygen, the Po2 of the venous blood falls to 35 mm Hg—only 5 mm Hg below the normal value of 40 mm Hg. Thus, the tissue Po2 hardly changes, despite the marked fall in alveolar Po2 from 104 to 60 mm Hg.
Conversely, when the alveolar Po2 rises as high as 500 mm Hg, the maximum oxygen saturation of hemoglobin can never rise above 100 per cent, which is only 3 per cent above the normal level of 97 per cent. Only a small amount of additional oxygen dissolves in the fluid of the blood, as will be discussed subsequently. Then, when the blood passes through the tissue capillaries and loses several milliliters of oxygen to the tissues, this reduces the Po2 of the capillary blood to a value only a few millimeters greater than the normal 40 mm Hg. Consequently, the level of alveolar oxygen may vary greatly—from 60 to more than 500 mm Hg Po2—and still the Po2 in the peripheral tissues does not vary more than a few millimeters from normal, demonstrating beautifully the tissue "oxygen buffer" function of the blood hemoglobin system.
Factors That Shift the Oxygen-Hemoglobin Dissociation Curve— Their Importance for Oxygen Transport
The oxygen-hemoglobin dissociation curves of Figures 40-8 and 40-9 are for normal, average blood. However, a number of factors can displace the dissociation curve in one direction or the other in the manner shown in Figure 40-10. This figure shows that when the blood becomes slightly acidic, with the pH decreasing from the normal value of 7.4 to 7.2, the oxygen-hemoglobin dissociation curve shifts, on
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.