DIFFUSION 263 



Dealing with carbon dioxide we may evaluate as follows : 



a at 37 =0-57, 



p x =CO 2 tension in the blood of the pulmonary artery 



=about 46 mm. Hg, 



p 2 =CO 2 tension in alveolar air=about 40 mm. Hg, 

 p 1 p 2 =,46 40=^6 mm. Hg. 



This difference of pressure, of course, only exists at the beginning of 

 the experiment. The blood loses carbon dioxide, i.e. p t decreases ; C0 2 

 passes into the alveolar air, i.e. p z increases, and p 1 p 2 tends towards 

 zero. It is, therefore, necessary to take a mean value between 6 and 0, 

 i.e. 3 mm. Hg. 



C=diffusion factor=0-139, 



d=thickness of alveolar wall=0-004 mm., 



0-57X3XO-139 

 ~760X6-63XO-004 

 =0-01 c.cm. per minute. 



As the effective absorptive surface of the lung is about 100 sq. 

 metres, there can pass through it each minute 



100 x 10,000 xO -01 =10,000 c.cm. 



of carbon dioxide by simple diffusion. 



One may consider the problem from another aspect and deter- 

 mine the gradient of pressure necessary to furnish the 250 c.c. of 

 carbon dioxide normally expired per minute. Transposing the 

 formula one gets 



v X760 Jm xd 

 P '- p2= axe 



Evaluating this, 



250x760x6-63x0-004 

 Pi ~P*Q. 57 xO-139 XlOO X 10,000 



=0 -063 mm. Hg. 



That is, a difference of CO 2 tension between blood and alveolar 

 air of only 2 xO -063=0 -12 mm. Hg. would be quite sufficient to 

 cause 250 c.c. of CO 2 to pass through the lung wall per minute. 

 During work the amount of carbon dioxide eliminated by the 

 lungs may be increased tenfold. The above figures show that there 

 is ample wharf-space for this exportation. 



The transference of oxygen from alveolar air to blood has been 

 the cause of much controversy. Two conflicting views both 

 backed by experimental facts are held. 



