METABOLISM OF FOODS AND DISPOSAL OF WASTES 

 glomerulus 



525 



renal vein 

 rcnol artcpy 



uret«r 



Henle's loop 



Fig. 20-3. A schematic view of the human kidney cut to show its internal structure and with a single excretory unit 



enlarged to show its detailed anatomy. 



very complex and extremely delicate organ. 



The cortex is made up of renal corpus- 

 cles, which are the tiny units where the 

 excretory process begins ( Fig. 20-3 ) . Each 

 consists of a minute ball of capillaries, the 

 glomerulus, surrounded by the double- 

 walled cup-like sac, Bowman's capsule. The 

 inner wall of the capsule closely adheres to 

 the glomerulus in order that substances 

 may diffuse readily from the blood stream 

 into the cavity and thence through the long 

 tubule to the pelvis of the kidney. The 

 medullary portion of the kidney consists 

 almost exclusively of these tiny tubules 

 which play an important function in the 

 business of excretion. 



Urine formation. The function of the kid- 

 ney has long been known but just how 

 urine forms has become clear only in recent 

 years. Following through the process as it 

 is now believed to take place, the blood 

 passes into the kidney through the renal 

 artery which immediately breaks up into 



smaller and smaller vessels until these 

 eventually form the glomeruli of tlie renal 

 corpuscles. As the blood passes through, all 

 of the substances in the blood except the 

 cells and proteins diffuse through Bowman's 

 capsule by filtration. This is purely a physi- 

 cal process which depends entirely on the 

 pressure in tlie blood vessels, and the 

 amount of fluid passing into Bowman's 

 capsule rises and falls with that pressure. 

 The vessels coming from the glomerulus are 

 slightly smaller than those going to it, so 

 that the pressure remains high in these 

 vessels. About 1 per cent of the blood vol- 

 ume is lost to the capsular filtrate as it 

 passes through tlie kidney. 



By some ingenious experiments with tiny 

 micro-needles. Prof. A. N. Richards was able 

 to examine the capsular filtrate and found 

 that it contained urea, sugar, amino acids, 

 salts, and so forth, in about the same con- 

 centrations as those in the blood plasma. 

 Obviously, if all of these valuable products 



