688 PHYSIOLOGY OF RESPIRATION. 



in the venous blood the oxygen exists under a tension of 5.3 per 

 cent, of the alveolar atmosphere (710 X .053 = 37.6 mms. Hg.), 

 and the CO 2 under a tension of 6 per cent. (42.6 mms. Hg.). The 

 physical relations of pressure between the alveolar air and the 

 gases in the venous blood may be represented as follows : 



OXYGEN. CARBON Dioxin. 



Alveolar air ........ 100 mms. 35 to 40 mms. 



Membrane ...... - 1 - - + - 



f I 



Venous blood . . . 37.6 mms. 42.6 mms. 



Diffusion must take place, therefore, in the direction indicated 

 by the arrows. As the oxygen passes through into the blood it is 

 combined with the hemoglobin and it is estimated that the arterial 

 blood as it flows away from the lungs is nearly saturated with 

 oxygen, lacking perhaps only 1 volume per cent, of being completely 

 saturated (Pfliiger). That is, if the normal arterial blood contains 

 19 c.c. of oxygen for each 100 c.c. of blood, it is probable that one 

 more cubic centimeter might be combined by the hemoglobin if 

 exposed fully to the air or oxygen. The difference in tension 

 between the carbon dioxid on the two sides of the membrane is not 

 so great as in the case of the oxygen, but owing to the more rapid 

 diffusion of this gas it is probable that this difference suffices to 

 explain the exchange. In this matter one must bear in mind also 

 the very large expanse of surface offered by the lungs and the very 

 complete subdivision of the mass of blood in the capillaries. Thus, 

 following a calculation made by Zuntz, the surface of the human 

 lungs may be estimated at 90 sq.ms. or 900,000 sq.cms. If we 

 assume that 300 c.c. of carbon dioxid (500 X 0.04 X 15) are given 

 off from the blood in a minute this would indicate a diffusion 

 through each square centimeter of only 0.0003 c.c. 



This same idea is expanded by Loewy as follows: The surface of the 

 lungs exposed to the air may be reckoned at 90 square meters, and the thick- 

 ness of membrane intervening between this air and the blood in the capillaries 

 may be estimated at 0.004 of a millimeter. Under these conditions as much 

 as 6083 c.c. of oxygen might diffuse into the blood in a minute. As a matter 

 of fact only about 250 to 300 c.c. of oxygen are really absorbed per minute in 

 quiet breathing, and not more than ten times this amount in the violent 

 respiration following excessive muscular exercise. It would seem, therefore, 

 that diffusion should suffice to supply the oxygen actually needed. This 

 reasoning applies a fortiori to the carbon dioxid, since the velocity of diffusion 

 of this gas through a moist membrane is much (25 times) greater. If the 

 tension of the CO 2 in the blood were only 0.03 mm. higher than that in the 

 alveoli, the known exchange might be explained by diffusion. 



Exchange of Gases in the Tissues. The arterial blood passes 

 to the tissues nearly saturated with oxygen so far as the hemo- 

 globin is concerned, and this oxygen is held under a tension 

 equivalent probably to at least 100 mms. Hg. The carbon 

 dioxid is less in quantity than on entering the lungs and exists 

 under a smaller pressure, which may be assumed to be the same 



