THE MAMMALS 143 



vironment' of the body— in a man of 70 kg. (154 pounds) 

 amounts to about n liters: excluding the blood cells, 

 proteins, and fats, which do not pass through the glo- 

 merular membranes, this entire internal environment is 

 filtered through the glomeruli only to be reabsorbed by 

 the renal tubules, some 16 times per day. 



Once the glomerular filtrate is in Bowman's capsule 

 surrounding the glomerular capillaries, it is literally out- 

 side the body, because this capsule drains freely into 

 the renal tubules, the renal tubules into the collecting 

 ducts, the collecting ducts into the renal pelvis, the pelvis 

 into the ureters and the bladder. What engineer, wish- 

 ing to regulate the composition of the internal environ- 

 ment of the body on which the function of every bone, 

 gland, muscle, and nerve depends, would devise a 

 scheme that operated by throwing the whole thing out 

 sixteen times a day— and rely on grabbing from it, as it 

 fell to earth, only those precious elements which he 

 wanted to keep? Only nature can be so extravagant, and 

 only in the light of historical perspective can we under- 

 stand her extravagance. 



It is scarcely necessary to emphasize again that the 

 substances present in largest amounts in the filtrate, and 

 reabsorbed in largest amounts by the tubules, are sodium 

 chloride and water. The evidence from experimental ani- 

 mals and man indicates that some 85 per cent of the 

 sodium chloride is actively reabsorbed in the proximal 

 segment of the tubule, and that an equal fraction of 

 water is simultaneously reabsorbed by passive diffusion, 

 so that the urine throughout the length of this segment 

 retains the same osmotic concentration as the blood. 



One of the most notable advances in renal physiology 

 of recent years is the demonstration of the role of the 

 thin segment and the loop of Henle in making an os- 

 moticaUy concentrated urine. This is achieved by the 

 reabsorption of sodium chloride in the ascending portion 

 of the thin segment in the loop; this sodium chloride, 

 transferred to the interstitial fluid, increases the osmotic 



