RESPIRATION 121 



dioxide is almost constant for the same individual, while the 

 oxygen percentages are liable to great variation. The mean 

 amount of carbon dioxide in alveolar air is found to be 5' 16 per 

 cent, in men ; it was less in women. Later on we shall better 

 appreciate the value of this constancy. 



During fast paces the respirations increase in frequency, 

 and as they do so they decrease in depth. The actual bulk of 

 an inspiration is less during fast paces than during repose, 

 though the total amount of air respired in a given time is 

 very much greater owing to 'the increase in the number of 

 respirations. 



The influence of work on chest-ventilation in increasing the 

 frequency and decreasing the depth of the respiration is dealt 

 with on p. 139. 



The Respiratory Exchange. — The respiratory exchange which 

 takes place is of two kinds : an external respiration between the 

 air and the blood through the medium of the lungs, and an 

 internal respiration between the blood and the tissues. Both 

 these are complex questions, and far from being settled. 



We have seen that the air leaving the lungs has gained over 

 4 per cent, in carbon dioxide, and lost nearly 5 per cent, of its 

 oxygen. The excess of oxygen absorbed over carbon dioxide 

 produced is explained by the fact that oxygen is required not 

 only for the purpose of oxidising carbon to carbon dioxide, 

 but also hydrogen to water. In addition to the above changes, 

 the expired air is found to be warmer than the inspired, and 

 to be saturated with water vapour. This indicates a loss of body 

 heat and likewise a loss of water. We have also learnt that 

 in the alveoli of the lungs the composition of the air is not the 

 same as in the dead space, and that in effect it contains more 

 C0 2 and less 2 . It is in the alveoli that the blood gets rid of 

 its carbon dioxide, and takes up its oxygen, and this process 

 must be examined in some little detail. 



The Gases of the Blood are oxygen, carbon dioxide, and 

 nitrogen. These may be extracted from the blood in the 

 vacuum of a mercurial pump, or more conveniently by chemically 

 expelling the oxygen by means of potassium ferricyanide, and 

 the carbon dioxide by means of tartaric acid, and estimating their 

 amounts. The proportion of gases found is liable to considerable 

 variation, depending on the condition of the animal, the vessel 

 from which the blood is drawn, the activity of the tissues, and 

 even the length of time elapsing between collecting and analysing 

 the sample. 



The gases of venous and arterial blood are necessarily different. 

 The figures given in the following table are only convenient 

 averages, and represent the gases in 100 volumes of blood, 



