398 PHYSIOLOGY CHAP. 



aqueous vapour with which it is saturated also contributes. 

 When the expired air is again reduced to the temperature and 

 degree of moisture of that inspired, it presents in relation to the 

 latter a slight diminution of volume, as first noted by Lavoisier 

 (1777). 



The chemical composition of expired air varies between 

 sufficiently wide limits, not merely in different classes and species 

 of animals, but also in the same individual under different external 

 and internal conditions, according to the frequency and depth of 

 the respiratory movements. The slower and deeper these are, the 

 greater will be the output of carbonic acid and intake of oxygen. 

 The first portions of air given off in a respiratory act, which come 

 from the more superficial bronchial passages, contain less carbon 

 dioxide and more oxygen, in comparison with the later portions of 

 expired air, which come from the deeper bronchial tubes and the 

 alveoli. 



It is obvious that the more perfect the pulmonary ventilation 

 consequent on the deepest respiratory acts, the more rapid and 

 abundant will be the gas exchanges, and therewith the emission of 

 carbonic acid and absorption of oxygen in the time unit. Ex- 

 perience, however, shows that increased pulmonary ventilation 

 does not merely increase the gas exchanges, which would be a 

 temporary effect, but also increases the formation of carbonic acid, 

 i.e. the absolute quantity which is expelled in the unit of time. 

 This phenomenon depends on the fact that increased pulmonary 

 ventilation exacts more work from the respiratory muscles, and 

 naturally determines an increase of combustion and thus of 

 carbonic acid production. 



When a known volume of oxygen is converted by combustion 

 into carbonic acid, the original volume of gas is not altered. 

 Since, however, in respiration the volume of oxygen absorbed 

 exceeds that of carbonic acid exhaled, it follows that a greater 

 or less amount of oxygen must be applied to other oxidative 

 purposes. Among these, besides the combustion of hydrogen by 

 which water is formed, must be reckoned the combustion of 

 sulphur to form sulphates, and of phosphorus, which forms 

 phosphates. 



The ratio between the volume of carbonic acid exhaled and 

 the volume of oxygen absorbed is known as the respiratory 

 quotient. This quotient, expressed by the formula ?, is generally 

 lower than 1, and varies considerably in the different classes of 

 animals, and even in the same individual, according to the nature 

 of his food. 



In the combustion of the different food-stuffs outside the body 

 a different quantity of oxygen is required according to the 

 different chemical constitution of their molecules. Thus, each 



