RESPIRATION 127 



this, carbon dioxide, by the process of diffusion, passes out of 

 blood in which its partial pressure is high, into the air of the 

 alveoli, where the partial pressure is low.* 



We have said that for some time past the law of diffusion and 

 chemical dissociation has been held inadequate to explain the 

 pulmonary exchange, the reason being that there are many 

 observations, none of which, perhaps, are entirely free from 

 error, which go to show that the alveolar air may contain a 

 higher partial pressure of carbon dioxide than is found in the 

 blood, while the oxygen tension of the alveolar air is below that 

 of the blood. Under these circumstances diffusion is out of the 

 question, and the only explanation which can possibly account 

 for oxygen passing into the blood and carbon dioxide passing 

 out of it is that which attributes these to a secretory activity. 



It is no part of a work of this character to examine the rival 

 physical and vital theories of pulmonary respiration, but no 

 matter how difficult it may be to explain the secretion of oxygen, 

 the fact remains that, reasoning by analogy, there is no difficulty 

 in understanding the secretion of oxygen by the cells of the 

 alveoli. 



The so-called ' swim bladder ' of deep-sea fishes contains a 

 high percentage of oxygen. Cods brought up from a depth of 

 45 feet have shown 52 per cent, of oxygen in their gas reservoir, 

 and gas drawn off on succeeding days has shown 79 per cent, 

 and 84 per cent, of oxygen in the bladder. The production of 

 this gas by secretion is the only possible explanation of its 

 existence. As a matter of fact, there is a vascular area in the 

 swim bladder from which it is formed ; and to render the secre- 

 tory theory more complete, nerves supplied by the vagus have 



* If a mixture of gases be absorbed by a fluid, it is found that the 

 volume of each gas forming the mixture is absorbed as perfectly as if it 

 were the only gas present ; no more and no less is absorbed, whether the 

 gas be by itself or whether it form only a proportion of the mixed gases 

 present. This is explained as resulting from the fact that one gas does 

 not exercise any pressure upon the other gases with which it forms a mix- 

 ture. The term used by Bunsen to define the pressure exerted by one gas 

 in a mixture of gases is ' partial pressure.' For example, 100 volumes of 

 air contain at freezing-point o° C, and standard barometric pressure 

 760 mm. (30 inches), 21 volumes of oxygen and 79 volumes of nitrogen. 

 What is the partial pressure exerted by each gas in this mixture ? 



760 x 21 ^159/6 mm. (6' 3 inches) of mercury, which is the 



100 partial pressure Of the oxygen ; 



and 



760 x 79_6oo mm. (2 yy inches) of mercury, the partial pres- 



100 sure of the nitrogen. 



The term ' partial pressure ' occurs so constantly in all discussions of 

 the nature and causes of gaseous exchanges of the animal body, that the 

 above may help to make the matter clear. 



