The Exchange of Gases Between the Tissues and Blood Also Occurs by Diffusion
The Pao2 of blood entering the tissue capillaries from the systemic arteries is 85 to IOOO mm Hg, and the Paco2 is 40 mm Hg. As blood passes through the capillaries, it is exposed to the tissues that are consuming oxygen and producing carbon dioxide.
Tissue oxygen tension is determined by the rate of delivery of oxygen in relation to its rate of consumption, but it averages 40 mm Hg. Similarly, tissue carbon dioxide tension is determined by the rate of tissue production in relation to the rate of removal by the blood, but it averages 46 mm Hg. As a result of the partial pressure differences between the tissues and capillaries, oxygen diffuses into the tissues and carbon dioxide diffuses into the blood until the partial pressures of blood and tissue are equal. Tissues with a high oxygen demand have more capillaries per gram of tissue. This provides a larger surface for diffusion and also means that the maximal distance between the tissue and the nearest capillary is less than in the poorly vascularized tissues (Figure 47-4).During exercise, muscle blood flow increases in part as a result of recruitment of capillaries that are not perfused in the resting animal. Capillary recruitment brings blood closer to the metabolizing tissues and slows the rate of blood flow, which allows more time for diffusion equilibrium. In addition, exercise lowers the Po2 and Pco2 of the muscle, which increases the driving pressure gradients for diffusion.
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