40 Physiology of the Kidney 



passes down the tubules valuable substances such as glucose, 

 sodium, chloride, amino acids, etc., are reabsorbed and re- 

 turned to the blood by various processes of tubular reabsorp- 

 tion. At the same time certain waste products and foreign 

 substances are taken from the blood by the tubule cells and 

 transferred to the tubular urine. These excreted substances 

 and such waste products and foreign compounds as are pres- 

 ent in the original filtrate but are themselves not reabsorbed, 

 remain in the tubular fluid to be excreted in the urine. Of 

 all substances reabsorbed by the tubules water is reabsorbed 

 to the greatest extent: out of the 125 cc. of filtrate formed 

 each minute, on the average 124 cc. of water are reabsorbed, 

 leaving only 1.0 cc. to be excreted as urine. In consequence 

 of this extensive reabsorption of water, such substances as 

 are filtered through the glomeruli but are themselves not re- 

 absorbed by the tubules appear in the final urine in a highly 

 concentrated form.^^ 



In inquiring how the renal tubule elaborates the glom- 

 erular filtrate into urine it will be noted that this tubule is 

 cytologically differentiated into three segments: a proximal 

 segment, an intermediate thin segment, and a distal segment 

 with drains into an arborized system of collecting tubules. 

 The proximal segment appears to be a jack-of-all-trades, cap- 

 able of reabsorbing valuable constituents, notably glucose 

 and chloride, from the glomerular filtrate, and at the same 

 time capable of transporting many waste products and for- 

 eign substances from blood to urine. On rather indirect evi- 

 dence it has been inferred that the thin segment is responsible 

 for the final reabsorption of water and the production of a 

 highly concentrated urine. The function of the distal seg- 

 ment remains something of a mystery, but there are reasons 

 to believe that it is responsible for the adjustment of the 



