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ESSENTIALS OF PHYSIOLOGY. 



of opinion that it is true only for the resting individual, and that during 

 exercise, when the need for oxygen is greater, or, at high altitudes, when 

 the supply of oxygen is deficient, the tension of oxygen in the blood 

 is higher than that of alveolar air, and that this difference is due to 

 the secretion of oxygen into the blood by the pulmonary epithelium. 



Similarly, it has been found that the tension of carbonic acid in 

 venous blood corresponds with a partial pressure of about 46 mm. Hg, 

 and is very slightly higher than that in alveolar air (fig. 102). If the 



% Co 2 in, h spired i 



FIG. 102. The tension of gases in the alveolar air and in blood. 

 From Barcroft, Respiratory Function of the Blood. 



The dotted lines represent the tension in the alveolar air, and the continuous lines 

 show the tension in the blood. During three periods the amount of C0 2 in 

 inspired air was increased. 



tension of carbonic acid in alveolar air is raised, the tension in the 

 blood shows a corresponding rise and still remains slightly higher than 

 that i^ the alveolar air. 



In man the difference between the tension of carbonic acid in venous 

 blood and in alveolar air is probably about 6 mm. Hg. This difference 

 is much smaller than that between the tension of oxygen in alveolar 

 air and venous blood respectively, but carbonic acid diffuses through a 

 membrane such as that separating the air in the lungs from the blood, 

 twenty-five times as quickly as oxygen ; and the difference between the 



