608 PHYSIOLOGY OF RESPIRATION. 



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. (tifWfif)- 



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 (28.1 to 41.8 mms. Hg). 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 

 C0 2 is present under a tension (Strassburg) of 7 to 9 per cent. (53.2 

 to 68.4 mms.). The high tension of the C0 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 substance is combined in some way 

 in a chemical compound too firm to dissociate. The physical con- 

 ditions are therefore such as would cause a stream of C0 2 from 

 tissue to blood and a stream of oxygen in the reverse direction. 



Oxygen. Carbon Dioxid. 



Arterial blood 75 mms. 28.1 to 41.8 mms. 



Wall of capillary J- 



Tissues mm. 53.2 to 68.4 mms. 



It is to be remembered that in this exchange the blood and 

 the lymph act as intermediaries. The C0 2 diffuses from lymph 

 to plasma and from tissues to lymph. The oxygen diffuses from 

 lymph to tissues, from plasma to lymph, and from oxyhemo- 

 globin to plasma. Bohr* has found experimentally that in blood, 

 when the oxygen tension is low, an increase in the C0 2 pressure tends 

 to dissociate the oxyhemoglobin. Since these conditions prevail in 

 the capillaries of the body it may be that the mere presence of the 

 C0 2 in increased amounts facilitates the liberation of the oxygen. 



Suggested Secretory Activity in the Respiratory Exchange. The 

 view that the exchange of gases in the lungs and tissues is entirely explained 

 by the diffusion of the gases from points of high tension to points of low ten- 

 sion, and that the membranes interposed are entirely passive in the process 

 has not passed unchallenged. Certain observers (Bohr, Haldane and Smith) f 



* " Skandinavisches Archiv f. Physiologie," 16, 402, 1904. 



t See Haldane and Smith, " Journal of Physiology," 20, 497, 1896. 



