540 A MANUAL OF PHYSIOLOGY 



diet is 250 to 300 grammes. About 2,000 grammes of lean 

 meat would be required to yield this quantity of carbon ; and, 

 even if such a mass could be digested and absorbed, more than 

 three times the necessary nitrogen would have to undergo pre- 

 liminary cleavage and excretion as urea or be thrown upon the 

 tissues. 



Not only may carbon equilibrium be maintained for a short 

 time in a dog on a diet containing fat only, or fat and carbo- 

 hydrates, but the expenditure of carbon may be less than the 

 income, and fat may be stored up. But, of course, if this 

 diet is continued, the animal ultimately dies of nitrogen 

 starvation. 



So far we have spoken only of the income and expenditure 

 of carbon and nitrogen ; and from these data alone it is possible 

 to deduce many important facts in metabolism, since, knowing 

 the elementary composition of proteins, fats, and carbo-hydrates, 

 we can, on certain assumptions, translate into terms of proteins 

 or fat the gain or loss of an organism in nitrogen and carbon, 

 or in carbon alone. But the hydrogen and oxygen contained in 

 the solids and water of the food, and the oxygen taken in by 

 the lungs, are just as important as the carbon and nitrogen ; it 

 is just as necessary to take account of them in drawing up 

 a complete and accurate balance-sheet of nutrition. Fortu- 

 nately, however, it is permissible to devote much less time to 

 them here, for when we have determined the quantitative 

 relations of the absorption and excretion of the carbon and 

 nitrogen, we have also to a large extent determined those of the 

 oxygen and hydrogen. 



(3) Income and Expenditure of Oxygen and Hydrogen. 

 The oxygen absorbed as gas and in the solids of the food is given 

 off chiefly as carbon dioxide by the lungs ; to a small extent as 

 water by the lungs, kidneys, and skin ; and as urea and other 

 substances in the urine and faeces. The hydrogen of the solids 

 of the food is excreted in part as urea, but in far larger amount 

 as water. The hydrogen and oxygen of the ingested water 

 pass off as water, without, so far as we know, undergoing any 

 chemical change, or existing in any other form within the body. 

 But it is important to recognise that although none of the water 

 taken in as such is broken up, some water is manufactured in the 

 tissues by the oxidation of hydrogen. We have already con- 

 sidered (p. 242) the gaseous exchange in the lungs, and we have 

 seen that all the oxygen taken in does not reappear as carbon 

 dioxide. It was stated there that the missing oxygen goes to 

 oxidize other elements than carbon, and especially to oxidize 

 hydrogen. We have now to explain more fully the cause of 

 this oxygen deficit. 



