THE URINARY SYSTEM. 383 



In hot weather or during exercise, when evaporation of sweat from the 

 skin is considerable, the urine is decreased in amount and is propor- 

 tionally concentrated. When the secretion of sweat is scanty, for 

 example on a cold day, a larger proportion of water is excreted by 

 the kidneys, and the urine is abundant and of low specific gravity : 

 copious draughts of water produce the same effect. In diabetes the 

 presence of sugar may raise the specific gravity to 1040 or more, while 

 in some forms of renal disease the specific gravity is always low (1005 

 to 1015). 



Reaction. The acid reaction of normal urine is due to acid sodium 

 phosphate (NaH 2 P0 4 ) ; no free acid is present. The bases and acid 

 radicles mentioned in the foregoing table are combined to form salts, and 

 are derived from the food. Sulphuric and phosphoric acid are formed by 

 the oxidation of the sulphur and phosphorus contained in protein, and 

 when the food contains much protein the amount of these acids is 

 increased in the urine, which becomes strongly acid in reaction. 

 Vegetable foods contain organic salts, such as citrates and tartrates 

 of potassium and sodium, in abundance, and in the body these organic 

 acids are completely oxidised, whereas the bases are excreted in the 

 urine. Hence in herbivorous animals, and in man on a vegetarian diet, 

 the urine is neutral or alkaline, though a starving herbivorous animal 

 which is living on its tissue proteins, and is for the time being car- 

 nivorous, excretes an acid urine. 



Colour. The colour of urine is almost entirely due to a pigment, 

 urochrome. of uncertain origin, the spectrum of which shows no 

 absorption bands. In addition, urine may contain three other pig- 

 ments, namely, (1) urobilin, (2) uroerythrin, and (3) hgematopor- 

 phyrin. 



Urobilin is formed in the digestive tract from bilirubin by bacterial 

 action, and after absorption into the blood is excreted into the urine 

 chiefly as a colourless chromogen, which can be converted into urobilin 

 by the addition of an acid. Urobilin itself occurs in urine in consider- 

 able quantity when the amount of bile pigment formed in the liver is 

 increased by an unusually rapid destruction of red cells in the body, 

 for instance in pernicious anaemia. It shows an absorption band at the 

 junction of the green and blue part of the spectrum, and gives a green 

 fluorescence with zinc chloride and ammonia. 



Uroerythrin occurs in combination with deposits of urates, giving 

 them a pink colour, which is changed to green on the addition of an 

 alkali ; its composition is unknown. 



Hcematoporphyrin (p. 162) normally occurs in minute traces, but 

 may be present in large amount in sulphonal poisoning. 



