SECTION XI 

 EXCRETION OF MATTER FROM THE BODY 



1. EXCRETION BY THE LUNGS (see Respiration, p. 277) 

 2. EXCRETION BY THE KIDNEYS 



URINE 



THE water and waste nitrogen of the body are chiefly elimi- 

 nated in the urine, which is secreted by the kidneys. 



The testa for the various constituents of the urine must be 

 studied practically. (Chemical Physiology, p. 22 et seq.) 



I. Physical Characters. 



The characters of the urine depend largely on the 

 relative proportion of water and of solids which are excreted 

 in it : at one time it may be very concentrated, while at 

 another time it may be very dilute indeed. For this reason 

 its specific gravity, which depends upon the percentage of 

 solids in solution, varies within wide limits, being often as 

 high as 1030 and frequently as low as 1005 ; but the average 

 specific gravity is about 1020. It is possible from the 

 specific gravity to form a rough idea of the amount of solids 

 present, for by multiplying the last two figures by 2'22 the 

 amount of solids per 1000 parts is given. 



Since the percentage of pigments in the urine varies like 

 that of the other constituents, the colour of the urine shows 

 wide divergence in the normal condition. A concentrated 

 urine has a dark amber colour, while a dilute urine may be 

 almost colourless. Under average conditions the urine has 

 a straw yellow colour. 



The reaction of urine is normally acid in man, chiefly 

 from the presence of acid sodium phosphate, NaH 2 P0 4 , and 



394 



