CHANGES IN AIR AND BLOOD IN RESPIRATION. 683 



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 diffusipn 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 

 as that of the carbon dioxid in the alveoli of the lungs namely, 

 5 per cent, of the alveolar atmosphere (35-40 mms.). In the sys- 

 temic 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 pres- 

 sure,* 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 substance 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. CARBON DIOXID. 

 Arterial blood . . ................ 100 mms. 35 mms. 



Wall of capillary. . . _ ! _ - _ _ i _ . 



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



* This conclusion is doubted by some observers, see Barcroft, " Respiratory 

 Function of the Blood," 1914, p. 165. 



