348 LOADING UP 



absorption coefficient of the gas, but also on a factor Ic — the 

 diffusion coefficient. Ic is a constant for each gas and each 

 temperature. The appended Table LI. (from Loewy) will amplify 

 this. 



TABLE LI 



Diffusion Coefficients 



The product of a and k gives the diffusion rate in cm. per 

 24 hours through a layer of water, 1 cm. X 1 sq. cm. with a pressure 

 gradient of 1 atmos. For example, at 37° C. carbon-dioxide has 

 a diffusion rate of 1*43 x 0*57 = 0"815 cm. per 24 hours. 



It has been found that k bears a definite inverse relationship 

 to the square root of the molecular weight of the gas. The result 

 of multiplying the diffusion coefficient by the square root of the 

 molecular weight of the gas is thus a constant for all gases. This 

 diffusion factor kym has a value, for water, of 0*0649. 



The diffusion rate through lung substance, because of its large 

 content of lipoids and lipins, must be greater than that through 

 water. Experiments with soap bubbles and with frogs' lungs 

 have confirmed this deduction. It has also been found that the 

 velocity of diffusion is absolutely unaltered by slight alterations 

 in the jjH of the lung tissue. Loewy maintains that the rate of 

 diffusion is the same in dead and in living lung tissue. The 

 diffusion factor through lung has been estimated as 0-139. Ex- 

 periment has shown definitely that COg passes just as readily in 

 either direction through the lung wall. This has been amply 

 confirmed by Krogh, who found that the direction of diffusion 

 depended entirely on the direction of the gradient of pressure, and 

 the rate of diffusion was regulated by the steepness of this gradient. 



The volume of gas diffusing per minute through 1 sq. cm. 

 of alveolar wall may be calculated from this formula : 



V 



760 Vni • d 

 Dealing with carbon-dioxide we may evaluate as follows : 



a at 37° = 0-57, 



Pj^ = COg tension in the blood of the pulmonary artery 

 = about 46 mm. Hg. 



^2 = CO2 tension in alveolar air = about 40 mm. Hg, 

 Pi — P2 = -i^ — 40 = ^ mm. Hg. 



