680 PHYSIOLOGY OF RESPIRATION. 



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

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

 following a calculation made b}^ 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 cUffusion 

 through each square centimeter of only 0.0003 c.c. (-eoWoo)- 



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

 lungs exposed to tlie 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 tlirough a moist membrane is much (25 times) greater. If the 

 tension of the CO, 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 in the arterial blood is less in quantity than when the blood 

 entered the lungs, and it 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, 5.5 per cent. (40 mms.) of the alve- 

 olar 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 CO2 is present under 

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

 CO2 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, from tissue to blood and a stream 

 of oxygen in the reverse direction. 



• Oxygen. Cakbon Dioxid. 



Arterial blood 100 mms. 35 mms. 



Wall of capillary • "^ 



Tissues mm. 50 to 70 mms. 



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



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

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



