Tissue Gas Exchange

Although O2 delivery to muscle increases during exercise, it does not increase as much as O2 demand so O2 extraction increases. In healthy subjects, hemoglobin oxygen saturation is already nearly 100% in arterial blood, so the only alternative to increase O2 extraction is to decrease venous oxygen levels. The magnitude of decrease in venous O2 levels can be calculated from the Fick principle (see Chapter 21):

The limits for decreasing venous oxygen levels are discussed below.

Several factors facilitate tissue gas exchange when O2 extraction is increased during exercise. Increased blood flow and muscular contraction result in continuous flow in all of the capillaries in a muscle bed. Compared to rest, when some capillaries are perfused only intermittently, this decreases the diffusion distances for O2. Hematocrit and hemoglobin concentration increase during acute exercise, although contraction of the spleen to release a reservoir of red blood cells is not as important in humans as it is in dogs and horses. Increased temperature, CO2, and H+ ion in exercising muscle will also decrease the affinity of hemoglobin for O2, facilitating the unloading of O2 in exercising muscle (Bohr effect; see Chapter 20). Finally, myoglobin may act as a shuttle to facilitate O2 transport within myocytes, thereby improving oxygenation near the venous ends of capillaries in muscles. Many of these features are illustrated in Fig. 5 in Chapter 21.

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