CH. XXVI.] THE GASES OF THE BLOOD 361 



overcome by the muscles in the act of inspiring 200 cubic inches of 

 air is more than 450 Ibs. 



It is possible that the contractile power which the bronchial tubes 

 and air-vesicles possess, by means of their muscular fibres, may assist 

 in expiration ; but it is more likely that the chief purpose of this 

 muscular tissue is to regulate and adapt, in some measure, the 

 quantity of air admitted to the lungs, and to each part of them, 

 according to the supply of blood : the muscular tissue also contracts 

 upon and gradually expels collections of mucus, which may have 

 accumulated within the tubes, and which cannot be ejected by forced 

 expiratory efforts, owing to collapse or other morbid conditions of the 

 portion of lung connected with the obstructed tubes (Gairdner). 



The Gases of the Blood. 



Before the student can study either the chemistry of respiration 

 or its regulation, which is in part a chemical process, it is necessary 

 that he should have an adequate conception of the fundamental laws 

 which regulate the retention of oxygen and carbonic acid in the 

 blood ; and as the blood presents many complications, it will be best 

 at the outset to consider the solution of gases in such a simple 

 medium as water. 



Solution of Gases in Water. 



If water is shaken up with oxygen, a certain definite amount of 

 oxygen will become dissolved in the water. Under the same condi- 

 tions the same quantity of oxygen would always be dissolved, and in 

 the following argument it is assumed throughout that the tempera- 

 ture remains constant. The amount dissolved depends then upon two 

 circumstances, each of which can be measured. The first is the 

 pressure of the oxygen to which the water is exposed when shaken ; 

 the second is a property of the oxygen itself, namely, its solubility 

 in water. The solubilities of different gases differ very much ; some 

 (for instance, oxygen) are not readily soluble in water, whilst others, 

 such as carbonic acid, are very soluble. 



If a cubic centimetre of water was introduced into a large air- 

 tight bottle containing pure oxygen at the atmospheric pressure, and 

 another cubic centimetre of water was similarly placed in a bottle 

 containing pure carbonic acid at the same pressure, the former would 

 be found to have dissolved 0-04 c.c. of oxygen, the latter 1 c.c. of 

 carbonic acid. These figures represent the degrees to which the 

 two gases are soluble in water under similar circumstances, and 

 are called their coefficients of solubility. The coefficient of solu- 

 bility of gas in a liquid is therefore the volume of gas which 1 c.c. 

 of the liquid will dissolve at 760 mm. of mercury, that is, atmospheric 

 pressure. 



