It would be manifestly impossible to follow an individual molecule of water through all its interchanges, although it provides a wide scope for the fertile exercise of a vivid imagination. It will suffice if after its absorption, which will be more minutely described in connection with the salt-free diet, we take cognisance of the various channels by which water makes its escape from the body. As practically all the excreta with the exception of the faeces are in solution, this provides an interesting survey. About 50 per cent. of the water we swallow is eliminated by the kidneys, holding in solution urea, uric acid, ammonia, sulphates, and other waste matters from the breaking down of the proteins; 28 per cent. is excreted by the skin, from which it passes off as perspiration and sebaceous matter, containing some of the fats; 20 per cent. by the lungs in the shape of watery vapour, holding in solution waste matters from the katabolism of the carbohydrates and some of the fats; 2 per cent. by the bowels, where it tends to soften the faeces.

On a mixed diet from 60 to 120 grams of water are excreted daily in the fasces, whereas on a vegetarian diet as much as 300 grams may escape in this way.

From 1,350 to 1,500 c.c. of water are excreted each day by the kidneys, but the quantity varies inversely with the amount evaporated by the skin in perspiration. From 1.5 to 2.5 litres may be thus evaporated during moderate work and 3 litres or more with hard work. In the 15 cubic metres of air expired daily by persons at rest, about 400-500 grams of water are evaporated, so that if 930 grams of water be lost daily during repose by evaporation, 530 grams, or 60 per cent., would come from the skin, and 380 grams, or 40 per cent., from the lungs. Exercise and exposure to the sun's rays increases the evaporation. Increase of subcutaneous fat lessens the water lost at low temperatures, but increases it at high temperatures. Increase of drinking water increases the amount of urine, but leaves the evaporation from the skin and lungs unchanged.

Evaporation of 1 litre of water causes a loss of 580 calories of heat. When the loss of heat occurs too slowly, then the temperature of the body rises: one degree of elevation of the body temperature in a man of 70 kilograms is caused by the retention of 60 calories. One of the objects of training, therefore, is to effect the excretion of sweat before the temperature of the body has risen unduly.

Diuresis produced by an increased consumption of water is accompanied by an increased excretion of nitrogen in the urine, but this is only a washing out of what is stored up in the tissues and not due to abnormal disintegration of the protein. The body clings with avidity to its extractives, which is surely an indication that they are not quite so useless as has been supposed.

If no fluid be drunk at all, then only 500 c.c. of water from the food and 300 c.c. oxidation water are available for excretion. The excretion of urine may, therefore, amount to no more than 200 c.c, and the calculated perspiration - slightly different to the evaporation of water from skin and lungs - may fall from 1,000 grams to a few hundred grams, the loss being chiefly due to diminished cutaneous activity.

For further discussion of this subject, reference should be made to the chapter dealing with dietetic theories associated with water.