184 ELEMENTARY PHYSIOLOGY LESS. 



with a very much smaller amount of uric acid, (ii) In- 

 organic salts, chiefly sodium chloride and suli)hates and 

 phosphates of sodium, potassium, calcium and magnesium, 

 (iii) Golonrimf matters, of which but little is known, 

 (iv) Gases, chiefly carbonic acid with a very small amount 

 of nitrogen and still less oxygen. 



An average healthy man excretes about 1,500 c.c. 

 (50 ounces or 2i pints) of urine each day. In this are 

 dissolved 33 grammes (1| oz. or about 2 per cent.) of urea 

 and not more than '5 grammes (10 grains) of uric acid. 

 The amount of salts is about half that of the urea, and of 

 this the larger part consists of sodium chloride. 



The quantity and composition of tiie urine vary greatly 

 according to the time of day ; the temperature and mois- 

 ture of the air ; the fasting or replete condition of the 

 alimentary canal ; the nature of the food ; and the amount 

 of fluid consumed. 



The qnantit]! depends on temperature and moisture of 

 the air, because, as we shall see (p. 198), these determine 

 the greater or less loss of water by the skin, and thus 

 leave less or more to be excreted by the kidneys. The 

 relationship of fluid consumed to the amount of urine 

 excreted is obvious. The composition varies with the 

 kind and amount of food, chiefly in respect of the amount 

 of urea excreted, for the nitrogen in urea represents 

 nearly all the nitrogen introduced into the body as 

 proteids. 



This relationship of the nitrogen in food to the nitrogen 

 of urea confers upon urea its supreme importance as a 

 constituent of urine. For the body cannot make good its 

 nitrogenous waste from any source other than the nitrogen 

 introduced into it in the form of proteids, and the nitro- 

 gen in this waste leaves the body again chiefly as urea, a 

 very small part reappearing in the form of uric acid. 

 Hence variations in the quantity of urea excreted thus 

 become the measure of the amount of nitrogen turned 

 over or " metabolised " in the body from time to time. 



