28 



PHYSIOLOGY 



continue independently always in excess in the same 

 direction for the same gas. If the pressure of X were 

 very much greater in A than in B the passage from the 

 side of less pressure might be so small that it could 



• ••••• ...... 



• ••••• 



• ••••• ...... 



•••••• 



• ••••• • 



V.V.V. ! .-.•.•.•.•.• 



...... •••••• 



• ••••• i *••••• 



Fig. 15.— Case 2: Two gases in equilibrium. 



be ignored in comparison with the passage from the 

 other side; we would then speak of the passage as if it 

 were in one direction only. 



Application to Respiration. — Case 2 represents the 

 condition in respiration (Fig. 14). Oxygen or X is 

 on one side of the membrane and carbon dioxide or Y 

 on the other side. Each gas passes through and estab- 

 lishes its own equilibrium quite independently of the 

 other (Fig. 15). The pressure of oxygen is so much 

 greater on the outside of the membrane than on the 

 inside that we ignore the small amount that passes 

 out, and speak only of the taking in of oxygen ; and the 

 pressure of carbon dioxide is so much greater on the 

 inside than on the outside that we ignore the small 

 amount that passes in and speak only of the giving 

 off of carbon dioxide. No equilibrium can be estab- 

 lished in either case, for in the tissues oxygen is con- 

 tinually being used and carbon dioxide is continually 

 being formed. 



