Figure 39-4 shows the effect of both alveolar ventilation and rate of oxygen absorption into the blood on the alveolar partial pressure of oxygen (Po2). One curve represents oxygen absorption at a rate of 250 ml/min, and the other curve represents a rate of 1000 ml/min. At a normal ventilatory rate of 4.2 L/min and an oxygen consumption of 250 ml/min, the normal operating point in Figure 39-4 is point A. The figure also shows that when 1000 milliliters of oxygen is being absorbed each minute, as occurs during moderate exercise, the rate of alveolar ventilation must increase fourfold to maintain the alveolar Po2 at the normal value of 104 mm Hg.
Another effect shown in Figure 39-4 is that an extremely marked increase in alveolar ventilation can never increase the alveolar Po2 above 149 mm Hg as long as the person is breathing normal atmospheric air at sea level pressure, because this is the maximum Po2 in humidified air at this pressure. If the person breathes gases that contain partial pressures of oxygen higher than 149 mm Hg, the alveolar Po2 can approach these higher pressures at high rates of ventilation.
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