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 pro- 

 cesses 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 the respiratory quotient, i.e. 



C0 2 expired. 



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 tidal air amounts to 500 c.c. If he breathes seventeen times 

 a minute the total pulmonary ventilation during the hour will be 500 x 17 

 X 60 = 510,000 c.c. per hour. This will contain 300 x 70 c.c. = 21,000 c.c. 



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