302 HUMAN PHYSIOLOGY 



the alveoli is not renewed by direct ventilation from without, 

 but by a process of diffusion (p. 287). For this reason the 

 amount of oxygen in the alveoli must be much smaller, the 

 amount of carbonic acid much larger, than in the respired air. 



By catheterisation, samples of air have been withdrawn 

 from the deeper part of the lungs and have been analysed. 



Such analysis tends to show that in the alveolar air there is 



Oxygen about 10 per cent, at a partial pressure of 76 

 mm. Hg. 



Carbon dioxide about 4 per cent, at a partial pressure of 

 about 30 mm. Hg. 



The difference of the partial pressure of these gases on the 

 two sides of the membrane in the alveolar air and in the 

 blood may be represented as follows : 



Blood 



This shows that when the blood reaches the lungs the 

 distribution of pressure in the gases is such that, by the laws 

 of diffusion, oxygen will pass from the alveolar air into the 

 blood and carbon dioxide from blood to air ; but, before the 

 blood has left the lungs, the distribution is such that oxygen 

 should, by the laws of diffusion, pass from blood to lungs 

 and carbon dioxide from lungs to blood, which is exactly the 

 reverse of what occurs. The passage of oxygen to the blood 

 and the passage of carbon dioxide from the blood is much 

 greater than could be accounted for by diffusion. 



We must, therefore, conclude that the exchange of gases 

 between the alveolar air and the blood is not due entirely 

 to diffusion, but is in part, at least, brought about by the 

 activity of the cells lining the vessels and the alveoli. These 

 conclusions are confirmed by the observations of Haldane, 

 that when air containing 0*045 per cent, of CO is shaken 

 with blood, 31 per cent, of the haemoglobin is combined with 

 that gas, while, if an individual breathes such air, only 25 per 

 cent, of the haemoglobin of the blood is combined. In some 



