The top part of Figure 1 shows a pulmonary alveolus adjacent to a pulmonary capillary, demonstrating diffusion of O2 between the alveolar air and the pulmonary blood. The PO2 of the gaseous O2 in the alveolus averages 104 mm Hg, whereas the PO2 of the venous blood entering the pulmonary capillary at its arterial end averages only 40 mm Hg because a large amount of O2 was removed from this blood as it passed through the peripheral tissues. Therefore, the initial pressure difference that causes O2 to diffuse into the pulmonary capillary is 104 − 40, or 64 mm Hg. In the graph at the bottom of the figure, the curve shows the rapid rise in blood PO2 as the blood passes through the capillary; the blood PO2 rises almost to that of the alveolar air by the time the blood has moved a third of the distance through the capillary, becoming almost 104 mm Hg.
Fig1. Uptake of oxygen by the pulmonary capillary blood. (Data from Milhorn HT Jr, Pulley PE Jr: A theoretical study of pulmonary capillary gas exchange and venous admixture. Biophys J 8:337, 1968.)
Uptake of Oxygen by the Pulmonary Blood during Exercise. During strenuous exercise, a person’s body may require as much as 20 times the normal amount of oxygen. Also, because of increased cardiac output during exercise, the time that the blood remains in the pulmonary capillary may be reduced to less than one- half normal. Yet because of the great safety factor for diffusion of O2 through the pulmonary membrane, the blood still becomes almost saturated with O2 by the time it leaves the pulmonary capillaries. This can be explained as follows.
First, it was pointed out in Chapter 40 that the diffusing capacity for O2 increases almost threefold during exercise; this results mainly from increased surface area of capillaries participating in the diffusion and also from a more nearly ideal ventilation-perfusion ratio in the upper part of the lungs.
Second, note in the curve of Figure 1 that under non-exercising conditions, the blood becomes almost saturated with O2 by the time it has passed through one third of the pulmonary capillary, and little additional O2 normally enters the blood during the latter two thirds of its transit. That is, the blood normally stays in the lung capillaries about three times as long as needed to cause full oxygenation. Therefore, during exercise, even with a shortened time of exposure in the capillaries, the blood can still become almost fully oxygenated.
