EXCRETION 



183 



quantity of blood flowing through it. Anything which increases 

 this quantity of blood increases the quantity of urine secreted; 

 anything which diminishes it lessens the amount of urine 

 secreted. 



In the everyday experience of healthy people the activity 

 of the kidneys is chiefly affected by three things ; namely, 

 (1) external temperature more 

 urine is secreted on a cold than on 

 a warm day; (2) the quantity of 

 water drunk ; and (3) the quantity 

 of food, and especially of protein 

 food, eaten. All three of these 

 agents, however, produce their re- 

 sults, largely if not entirely, because 

 of their influence upon the blood 

 flow through the kidney. Thus ex- 

 posure of the skin to cold causes a 

 constriction of the arterioles of the 

 skin and a compensating dilation of 

 those of internal organs, the kid- 

 neys included. More blood flows 

 through the kidneys and more urine 



is Secreted. Much the Same thing open, on the papillae (B, B}, into 



is true of the absorption of water " pel (c '> o the uretl 

 and of protein food, for both these conditions cause a widen- 

 ing of the arterioles of the kidney. 



Changes in the quantity of the urine secreted are, generally 

 speaking, only changes in the amount of water rather than in 

 the amount of urea and other dissolved wastes. Certain con- 

 stituents of the urine, however, are not very soluble, so that it 

 is not well to have water, the only solvent of these substances 

 in the urine, unduly diminished. A scanty secretion of urine 

 during the day is, in general, a distinct indication, especially 

 in warm weather, that insufficient water is being taken. Many 

 persons drink too little water rather than too much. 



FIG. 82. Vertical section of 

 the kidney. Diagrammatic 



The tubules (A) of the gland 



