REMOVAL OF THE END-PRODUCTS OF METABOLISM 165 



may shrink somewhat or may be kept near the standard as 

 a result of the water drinking which is stimulated. The 

 output of water from the skin during the hot weather and 

 also during muscular activity does not serve primarily as a 

 vehicle of waste, but rather as a means of ridding the body 

 of heat. This matter will be discussed at length at an- 

 other time. The evaporation from the respiratory tract 

 probably varies less widely than the perspiration. The 

 body seems bound to saturate all the air that is breathed, 

 so that this loss increases with the volume of breathing 

 and is greater when the air is dry than when it is humid. 



The Kidneys and the Urine. The chief significance of 

 the kidneys is their function of excreting the distinctive 

 products of protein metabolism,. A secondary service is 

 the disposal of inorganic salts. The two glands are placed 

 to the right and left of the spinal column at the level of the 

 lower ribs. Each kidney receives a large short artery from 

 the aorta which passes between them. Each returns a 

 large vein, not to the portal system, but to the chief venous 

 trunk of the body. The kidneys, in consequence of this 

 arrangement, constitute short cuts or "shunts" in the cir- 

 culation, and are perfused by an exceptionally large quan- 

 tity of blood. No portion of the blood can long escape 

 their influence. The urine discharged by the many tubu- 

 lar units of the kidneys is conveyed through the ureters, 

 contractile vessels which lead to the bladder. This is a 

 saccular organ capable of accommodating much urine when 

 dilated, and of contracting again to nearly complete expul- 

 sion of its contents. Its walls of muscle (the same type 

 found in the alimentary canal) are obviously under ner- 

 vous control and much subject to reflexes. 



Urine of average composition is a complex solution con- 

 taining some 3 or 4 per cent, of dissolved solids. The 

 leading substance is urea, the chief nitrogenous waste of 

 the system, and the index, according to Folin, of the 

 exogenous metabolism. Its origin has been discussed. 

 Evidently the duty of the kidney is less to manufacture 

 urea than to select and remove from the blood the urea 



