SECTION II 

 THE CHEMISTRY OF RESPIRATION 



THE energy of the body is derived almost entirely from the oxidation 

 of the carbon and hydrogen of the food-stuffs. An adult man during 

 the twenty -four hours produces on the average 250 c.c. of carbon 

 dioxide per kilo per hour. A man of 60 kilos will therefore excrete 

 250 x 60 x 24 = 360,000 c.c. carbon dioxide in the course of twenty- 

 four hours. During sleep the output of carbon dioxide is lowered 

 with the diminution in all the metabolic processes of the body and 

 amounts to only 160 c.c. per kilo per hour. If we assume that eight 

 hours of the twenty -four are given to sleep, this will leave 295 c.c. 

 per kilo per hour as the average excretion of carbon dioxide during 

 the waking hours. Since the access of oxygen to the body and the 

 removal of carbon dioxide is effected by the pulmonary ventilation, 

 the expired air will differ from the inspired air in containing more 

 carbon dioxide and less oxygen. The oxygen intake is not, however, 

 absolutely proportional to the carbon dioxide output. This is owing 

 to the fact that carbon is not the only element which leaves the body 

 in an oxidised condition. Fats, for example, contain a number of 

 unoxidised atoms of hydrogen, which in the metabolic processes of 

 the body are fully oxidised, to be excreted as water. Oxygen will 

 also leave the body in combination with carbon and nitrogen in the 

 urine, so that a certain amount of oxygen which is taken in does not 

 reappear as carbon dioxide in expired air. There is thus an absolute 

 diminution in the volume of expired air as compared with that of 

 inspired air. This diminution, due to loss of oxygen, is greater in 

 carnivora, whose food consists mainly of proteins and fats, than in 

 herbivora, which feed principally on carbohydrates, and depends on 



, . ., ,. C0 2 expired 



the respiratory quotient, i.e. the ratio - 



2 inspired. 



In man the average respiratory quotient can be taken as 0'85. 

 On this basis the amount of oxygen which will be taken in during 

 the waking hours will be 347 c.c. per kilo per hour. Taking round 

 figures, we may say that, when awake, a man takes in 350 c.c. 

 oxygen and gives out 300 c.c. carbon dioxide per kilo per hour. 

 From these figures we can calculate the normal composition of expired 

 air when a man is breathing quietly. Under these conditions the 



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