Chap, xxiii.] EXCHANGE OF GASES IN LUNGS. 293 



exchange that may be effected will depend on the 

 partial pressures of each gas in the liquid and in the 

 atmosphere. Suppose the liquid to contain O at a 

 less and C0 2 at a greater pressure than the atmo- 

 sphere, then O will pass from the atmosphere into the 

 liquid, and C0 2 from the liquid into the atmosphere. 



The exchanges in the lung's between the blood 

 and the air cells, is, to a large extent, a physical problem 

 to be solved by the application of the laws that have 

 been stated. The delicate walls of the air cells and of 

 the pulmonary capillaries form a septum, separating, on 

 the one side, blood containing oxygen and carbonic acid 

 gas and nitrogen, from air on the other side, containing 

 the same gases. Disregarding the nitrogen, the pres- 

 sures of O and C0 2 in the two cases are found to be 

 very different, as the following tables show : 



PRESSURE or OXYGEN. 



In the Pulmonary In the Air of Differ- 



Capillaries. the Air Cells. ence 



Inspiration (calm) . . 44 129 85 



Inspiration (deep) . . 44 140 96 



Expiration (calm) . . 44 121 77 



Expiration (deep) . . 44 110 66 



PRESSURE OF CARBONIC ACID GAS. 



In the Pulmonary In the Air of Differ- 



Capillaries. the Air Cells. ence. 



Inspiration (calm) . . 82 30 52 



Inspiration (deep) . . 82 7 75 



Expiration (calm) . . 82 38 44 



Expiration (deep) . . 82 67 15 



(Beaunis.) 



Supposing for the moment the blood to be in direct 

 contact with the air in the air cells, the differences of 

 pressure show that oxygen would be passing from the 

 air cells into the blood during expiration as well as 

 during inspiration, though less freely in the former case, 



