1122 



PHYSIOLOGY 



The tensions in the alveolar air of man may be taken as follows : 



Oxygen 

 Carbon dioxide 



107 mm. Hg. 

 40 



As the venous blood enters the lungs there is thus a difference of pressure 

 of 107 37 = 70 mm. Hg., which will tend to cause a flow of oxygen from 



so 3 



FIG. 516. Tensions of O 2 and C0 2 in alveoli compared with those in arterial 



blood of rabbit. 



The dotted lines represent the tensions in the alveolar air, the uninterrupted 

 lines the tensions of the gases in the arterial blood. (KROGH.) 



FIG. 517. Tensions of gases in alveolar air and in arterial blood. 



A; during artificial increase 'of oxygen tension in alveoli; B, during artificial 



increase of C0 2 tension in alveoli. 



alveolar air to blood and a difference of 46 40 = 6 mm. Hg., tending 

 to cause a flow of carbon dioxide from blood to alveolar air. Is this differ- 

 ence sufficient to account for the amount of gas given off or taken up by 

 the blood in its passage through the lungs ? In a state of medium distension 

 the 3000 c.c. of air contained by the lungs have been estimated to occupy 

 seven hundred million alveoli, each of which has a diameter of 0'2 mm., 

 so that the total surface over which the blood is exposed to the alveolar 



