198 THE REMOVAL OF WASTE PRODUCTS 



reservoir, the bladder. At both points of contact be- 

 tween the capillaries and the renal tubules, water and 

 certain waste substances in solution are removed from 

 the blood. 



Excretion. Since the waste products taken from the 

 blood are to be ejected from the body, the process of 

 removing them from the blood is called excretion. The 



Beginning of 



urinary tubule 

 Tuft of capillaries 



Artery 



Vein to tubule 

 Vein from tubule 



Urinary tubule- 

 Capillary mesh 

 over tubule 



Artery 

 Vein 



FIG. 109. Structure of kidney, showing the secreting mechanism (diagrammatic). 

 The arrows indicate the direction of flow of the fluids. 



excreted fluid 1 is constantly drained from the kidneys 

 into the bladder, where it is temporarily stored. The 

 amount secreted by the kidneys in twenty-four hours is 

 normally about three pints. 



Effect upon blood. The kidneys, through their power 

 of eliminating water from the blood, prevent the blood 



1 The total amount in 24 hours is 1500 grams. Its composition 

 is water, 1440 gms. ; total solids, 60 gms. (consisting of urea, 35 

 gms.; uric acid, 0.75 gms.; sodium chloride, 16.5 gms.; and other 

 substances as salts of potassium, magnesium and calcium, 7.75 

 gms.). 



