CHANGES IN AIR AND BLOOD IN RESPIRATION. 629 



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. (Timnnr)- 



Exchange of Gases in the Tissues. The arterial blood passes 

 to the tissues nearly saturated with oxygen so far as the hemoglobin 

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

 probably to at least 75 to 80 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 as that of the carbon 

 dioxid in the alveoli of the lungs, namely, 3.7 to 5.5 per cent, of an 

 atmosphere. In the systemic capillaries the blood comes into 

 diffusion relations with the tissues, and direct examination of the 

 latter shows that the oxygen in them exists under a very small 

 pressure, practically zero pressure, while the CO 2 is present under 

 a tension (Strassburg) of 7 to 9 per cent. The high tension of the 

 CO 2 is explained by the fact that it is being formed in the tissues 

 constantly as a result of their metabolism, while the low tension of 

 the oxygen is due to the fact that on entering the tissue this sub- 

 stance is combined in some way in a chemical compound too firm 

 to dissociate. The physical conditions are, therefore, such as would 

 cause a stream of CO 2 from tissue to blood and a stream of oxygen 

 in the reverse direction. 



OXYGEN. CAHBON DIOXID. 

 Arterial blood .................. 75 mms. 35 mms. 



Wall of capillary ................ I _ "t* 



Tissues ........................ mm. 50 to 70 mms. 



