GASEOUS DIFFUSION 349 



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

 experiment. The blood loses carbon-dioxide, i.e. p^ decreases ; COg passes 

 into the alveolar air, i.e. p^ increases, and p^ — ^g-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, 

 \/m = V44 = 6-63, 



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



_ 0-57 X 3 X 0-139 

 ~ 760 X 6-63 X 0-004 

 = 0-01 c.c. 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 X 0-01 = 10,000 c.c. 



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 X 760 -y/m X d 



Pi -P2 



a X c 



Evaluating this, 

 Pi ~Pz 



_ 250 X 760 X 6-63 X 0004 

 "~ 0-57 X 0-139 X 100 X 10,000 

 = 0-063 mm. Hg. 



That is, a difference of COg tension between blood and alveolar 

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

 cause 250 c.c. of COg 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. 



(1) The lungs may be considered as secretory glands. Fish 

 have a swim-bladder which is, like the lungs, an outgrowth from 

 the alimentary canal. Oxygen is secreted by it so as to equalise 

 the specific gravities of fish and water. The fish may secrete 

 oxygen against the pressure produced in the bladder by immersion 

 to a great depth, e.g. against the pressure of hundreds of atmo- 

 spheres. Against this view may be opposed the histological fact 

 that cells composing the walls of the swim-bladder structuralh' do 



