When the arterial blood reaches the peripheral tissues, its Po2 in the capillaries is still 95 mm Hg.Yet, as shown in Figure 40-3, the Po2 in the interstitial fluid that surrounds the tissue cells averages only 40 mm Hg. Thus, there is a tremendous initial pressure difference that causes oxygen to diffuse rapidly from the capillary
Effect of blood flow and rate of oxygen consumption on tissue Po2
blood into the tissues—so rapidly that the capillary Po2 falls almost to equal the 40 mmHg pressure in the interstitium. Therefore, the Po2 of the blood leaving the tissue capillaries and entering the systemic veins is also about 40 mm Hg.
Effect of Rate of Blood Flow on Interstitial Fluid Po2. If the blood flow through a particular tissue is increased, greater quantities of oxygen are transported into the tissue, and the tissue Po2 becomes correspondingly higher. This is shown in Figure 40-4. Note that an increase in flow to 400 per cent of normal increases the Po2 from 40 mm Hg (at point A in the figure) to 66 mmHg (at point B). However, the upper limit to which the Po2 can rise, even with maximal blood flow, is 95 mm Hg, because this is the oxygen pressure in the arterial blood. Conversely, if blood flow through the tissue decreases, the tissue Po2 also decreases, as shown at point C.
Effect of Rate of Tissue Metabolism on Interstitial Fluid Po2. If the cells use more oxygen for metabolism than normally, this reduces the interstitial fluid Po2. Figure 40-4 also demonstrates this effect, showing reduced interstitial fluid Po2 when the cellular oxygen consumption is increased, and increased Po2 when consumption is decreased.
In summary, tissue Po2 is determined by a balance between (1) the rate of oxygen transport to the tissues in the blood and (2) the rate at which the oxygen is used by the tissues.
Diffusion of Oxygen from the Peripheral Capillaries to the Tissue Cells
Oxygen is always being used by the cells. Therefore, the intracellular Po2 in the peripheral tissue cells remains lower than the Po2 in the peripheral capillaries. Also, in many instances, there is considerable physical distance between the capillaries and the cells. Therefore, the normal intracellular Po2 ranges from as low as 5 mm Hg to as high as 40 mm Hg, averaging (by direct measurement in lower animals) 23 mm Hg. Because only 1 to 3 mm Hg of oxygen pressure is normally required for full support of the chemical processes that use oxygen in the cell, one can see that even this low intracellular Po2 of 23 mm Hg is more than adequate and provides a large safety factor.
Diffusion of Carbon Dioxide from the Peripheral Tissue Cells into the Capillaries and from the Pulmonary Capillaries into the Alveoli
When oxygen is used by the cells, virtually all of it becomes carbon dioxide, and this increases the intra-cellular Pco2; because of this high tissue cell Pco2, carbon dioxide diffuses from the cells into the tissue capillaries and is then carried by the blood to the lungs. In the lungs, it diffuses from the pulmonary capillaries into the alveoli and is expired.
Thus, at each point in the gas transport chain, carbon dioxide diffuses in the direction exactly opposite to the diffusion of oxygen. Yet there is one major difference between diffusion of carbon dioxide and of oxygen: carbon dioxide can diffuse about 20 times as rapidly as oxygen. Therefore, the pressure differences required to cause carbon dioxide diffusion are, in each instance, far less than the pressure differences required to cause oxygen diffusion. The CO2 pressures are approximately the following:
1. Intracellular Pco2, 46 mm Hg; interstitial Pco2,
45 mm Hg. Thus, there is only a 1 mm Hg pressure differential, as shown in Figure 40-5.
2. Pco2 of the arterial blood entering the tissues, 40 mm Hg; Pco2 of the venous blood leaving the tissues, 45 mm Hg. Thus, as shown in Figure 40-5, the tissue capillary blood comes almost exactly to equilibrium with the interstitial Pco2 of
3. Pco2 of the blood entering the pulmonary capillaries at the arterial end, 45 mm Hg; Pco2 of the alveolar air, 40 mm Hg. Thus, only a 5 mm Hg pressure difference causes all the required carbon dioxide diffusion out of the pulmonary capillaries
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