Gas Exchange in the Lungs

The properties of oxygen, CO2 and hemoglobin as previously described show us what happens when oxygen is already in circulation but we also need to understand how gas exchange occurs in the lungs. Gas exchange occurs very rapidly in a healthy lung. It takes about 0.25 seconds for the gas-blood interface to reach equilibrium, which is approximately one-third of the blood’s transit time through the lungs. However during exercise the heart beats faster and blood moves much more quickly throughout the body. If not for acute changes that occur in the lung with exercise, there would not be sufficient time for blood to become oxygenated in the lungs. During high intensity exercise the pulmonary capillaries increase their size by about three times the resting value. This means that blood flow through the lungs as well as aeration state of the blood leaving the lungs stays almost exactly the same in a resting person when compared to that same person during rigorous exercise. This basic idea of oxygenation is illustrated by Figure 3.

Figure 3. Time required for gas exchange. At rest, blood remains in the pulmonary and tissue capillaries for about 0.75 seconds. Abnormal responses mapped with the dashed line due to pulmonary disease impairs the rate of gas transfer across the alveolar-capillary membrane, thus prolonging the time for equilibration of gases. Bloods transit time through the pulmonary capillaries during maximal exercise decreases to about 0.4 seconds, but this still remains adequate for completion of aeration in healthy lungs.

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