COMPOSITION AND CHARACTERS OF URINE 1241 



constituents of the urine and to summarise their properties, prepara- 

 tion, and normal significance. 



The urine of man is a clear yellow fluid which froths when shaken. 

 On standing, a cloud of mucus is deposited, consisting of a very small 

 amount of nucleoprotein derived from the epithelial lining of the 

 bladder and urinary passages. In concentrated urine a deposit occurs 

 on cooling. This deposit dissolves when the urine is warmed, and 

 consists of urates. Under certain circumstances urine is turbid as it 

 is passed, but in this case the turbidity generally consists of earthy 

 phosphates and is not cleared up by heating. 



The colour of the urine varies with its concentration. After severe 

 sweating the amount of water excreted by the kidneys is small, and 

 the urine is therefore concentrated and of high colour. After copious 

 draughts of liquid the urine may be very pale and dilute. 



Ordinary urine has an aromatic odour, but this varies largely with 

 the character of the food. Many food substances give characteristic 

 odours, which may depend on alterations undergone by them in their 

 passage through the body. 



The specific gravity of the urine is proportional to its concentra- 

 tion. Normally it is 1016 to 1020, though it may rise as high as 1040 

 or sink as low as 1002. 



The molecular concentration of the urine is almost always greater 

 than that of the blood. Its osmotic pressure may be measured by 

 determining the depression of freezing-point. The A of urine normally 

 varies between 0'87 and 2*71 (A of blood = 0*56). After copious 

 draughts of water the depression of freezing-point in the urine may be 

 less than that of serum, and may be as small as 0*25. 



The reaction of urine is generally described as acid. It is acid 

 to litmus and to phenolphthalein. This is due to the fact that neutral 

 constituents of the food give rise to acid end-products in metabolism. 

 The sulphur of proteins is converted into sulphuric acid and the 

 phosphorus of lecithin into phosphoric acid. There is thus a pre- 

 dominance of acid radicals over bases in ordinary urine. This state- 

 ment, however, only applies to man and to carnivora. In the food 

 of herbivora there is a predominance of alkaline bases .< Vegetable 

 acids, e.g. tartaric, malic, and citric acids, undergo oxidation to 

 carbonic acid in the body, so that their bases leave the body as alka- 

 line carbonates. The urine of such animals therefore contains an 

 excess of alkaline carbonates, and is alkaline in reaction and froths 

 on the addition of an acid. If a herbivorous animal be starved, so 

 that it has to live on its own tissues, it becomes for the time, so to 

 speak, carnivorous, and its urine becomes clear and acid. The urine 

 of man can be made alkaline by the ingestion of large quantities of 

 vegetables or fruits. Under such circumstances the urine as passed 



