In Chapter 37, it was pointed out that the average male functional residual capacity of the lungs (the volume of air remaining in the lungs at the end of normal expiration) measures about 2300 milliliters. Yet only 350 milliliters of new air is brought into the alveoli with each normal inspiration, and this same amount of old alveolar air is expired. Therefore, the volume of alveolar air replaced by new atmospheric air with each breath is only one seventh of the total, so that multiple breaths are required to exchange most of the alveolar air. Figure 39-2 shows this slow rate of renewal of the alveolar air. In the first alveolus of the figure, an excess amount of a gas is present in the alveoli, but note that even at the end of 16 breaths, the excess gas still has not been completely removed from the alveoli.
Figure 39-3 demonstrates graphically the rate at which excess gas in the alveoli is normally removed, showing that with normal alveolar ventilation, about one half the gas is removed in 17 seconds. When a person's rate of alveolar ventilation is only one half normal, one half the gas is removed in 34 seconds, and when the rate of ventilation is twice normal, one half is removed in about 8 seconds.
Importance of the Slow Replacement of Alveolar Air. The slow replacement of alveolar air is of particular importance in preventing sudden changes in gas concentrations in the blood. This makes the respiratory control mechanism much more stable than it would be otherwise, and it helps prevent excessive increases and decreases in tissue oxygenation, tissue carbon dioxide concentration, and tissue pH when respiration is temporarily interrupted.
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