47 2 A MANUAL OF PHYSIOLOGY 



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 considered (p. 225) 

 the gaseous interchange 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. 



The Oxygen Deficit. The carbo-hydrates contain in themselves 

 enough oxygen to form water with all their hydrogen ; they account 

 for a part of the water-formation in the body, but for none of the 

 oxygen deficit. 



The fats are very different ; their hydrogen can be nothing like 

 completely oxidized by their oxygen. A gramme of hydrogen is 

 contained in 8-5 grammes of dry fat, and needs 8 grammes of oxygen 

 for its complete combustion. Oaly i gramme of oxygen is yielded 

 by the fat itself; so that if a man uses 100 grammes of fat in twenty- 

 four hours, rather more than 80 grammes of the oxygen taken in must 

 go to oxidize the hydrogen of the fat. 



The proteids also contribute to the deficit. In 100 grammes of 

 dry proteids there are 15 grammes of nitrogen, 7 grammes of 

 hydrogen, and 21 grammes of oxygen. The carbon does not concern 

 us at present. The 33 grammes of urea, corresponding to 100 

 grammes of proteid, contain 15 grammes of nitrogen, a little more 

 than 2 grammes of hydrogen, and a little less than 9 grammes of 

 oxygen. There remain 5 grammes of hydrogen and 1 2 grammes of 

 oxygen. But 5 grammes of hydrogen need for complete combustion 

 40 grammes of oxygen ; therefore 28 grammes of the oxygen taken 

 in must go to oxidize the hydrogen of 100 grammes of proteid. 

 Taking 140 grammes of proteid as the amount in the diet of a man, 

 we get 39 grammes as the required quantity of oxygen. This, a ided 

 to the 80 grammes needed for the hydrogen of the fat, makes a total 

 of, say, 120 grammes, equivalent to about 85 litres of oxygen. A 

 small amount of oxygen also goes to oxidize the sulphur of proteids. 

 With a diet containing less fat and proteid and more carbo-hydrate, 

 the oxygen deficit would of course be less. 



The Production of Water in the Body. One gramme of hydrogen 

 corresponds to 9 grammes of water. In 140 grammes of proteids 

 and 100 grammes of fat there are, in round numbers, 22 grammes 

 of hydrogen; in 350 grammes of starch, 21*5 grammes. With this 

 diet, 43-5 grammes of hydrogen are oxidized to water within the body 

 in twenty-four hours, corresponding to a water-production of 391 

 grammes, or 15 to 20 per cent, of the whole excretion of water. It 

 has been observed that during starvation the tissues sometimes 



