rv THE GASEOUS INTERCHANGE 155 



the exit and entry of those gases from and into the liquid. 

 Further, since gases diffuse readily through thin porous 

 films, the statements we have just made will, broadly 

 speaking, hold equally good in the case wlien the surface 

 of the fluid is separated from the neighbouring gases by 

 a thin, moist, porous film. 



The air in the alveoli of the lungs is a mixture of gases 

 separated by the thin, moist, filmy wall of the capillaries 

 from the venous ))lood brought to them by the pulmonary 

 artery ; and what we have now to consider is whether, 

 in the absence of any other obvious cause, the differences 

 of partial pressure between the gases in that air and the 

 same gases in that blood are sufficient to account for 

 the interchange by which the venous blood becomes 

 arterial. 



Now the oxygen and carbonic acid in blood are not in 

 mere solution but largely in combination with certain 

 constituents of the corjiuscles and plasma. Hence the 

 pressures they exert are mucli less than they would,be if 

 they were in simple solution. But on the other hand the 

 compounds formed by these gases in the blood are very 

 unstable and easily dissociated or broken up, so that 

 a sufficient difference of partial jJressure on the surface of 

 the blood may still easily start the interchange between 

 the blood and the alveolar air and between the blood 

 and the tissues. 



The partial pressures of oxygen and carbonic acid in 

 alveolar air are usually about 100 and 40 mm. of mercury 

 respectively. By applying these data we find that venous 

 blood in contact with oxygen at the partial pressure it has 

 in alveolar air becomes arterialised as regards its ox5'gen. 

 And the entry of the oxygen is further assisted by the 

 fact that it is stowed away in loose chemical combination 

 in the red corpuscles. Similarly we may say that the exit 

 of carbonic acid is due to the difference between the 



