1200 PHYSIOLOGY 



are absorbed in different proportions determined by their normal values in 

 the plasma. The tubules absorb from the glomerular filtrate a slightly 

 alkaline fluid containing sugar, amino-acids, chlorides, sodium and potassium 

 in approximately the same proportions as they are present in normal plasma. 

 " Thus the tubules, out of the glomerular filtrate, return to the blood the 

 fluid best adapted for the tissues, and allow the rest to escape. If the plasma 

 is too rich in sugar or chloride, the filtrate also contains those substances 

 at or above the threshold value. The tubules however return them at the 

 optimal or threshold concentrations and the remainder passes into the 

 ureters. If much water has been ingested and the filtrate is correspondingly 

 dilute, the subtraction of the optimal solution leaves the excess water in 

 the urine along with the urea and other waste products " (Cushny). 



The power of absorption possessed by the cells of the tubules is not 

 indefinitely large, and the urine can therefore never exceed a certain con- 

 centration at which its osmotic pressure just equals the absorptive power 

 of the cells. This limiting concentration differs in different animals, the cat 

 being able to absorb against a resistance of fifty to sixty atmospheres, while 

 the human, kidney cannot concentrate against a resistance of more than 

 twenty-five atmospheres. The presence of any inabsorbable substance in 

 the glomerular fibrate, e.g. urea, sodium sulphate, or phosphate, must 

 therefore limit the absorption owing to the osmotic resistance they offered 

 to the absorptive powers of the cells. These substances will therefore act 

 as diuretics. In the same way the threshold substances will act as diuretics, 

 provided that they are present in the plasma in proportions above the 

 plasma, so that they can no longer be absorbed by the cells of the tubules. 

 It has been objected by Heidenhain and others to this view that, if we 

 exclude the occurrence of secretion by the cells of the tubules, we must 

 assume that, of the seventy litres passing the glomeruli in the course of 

 twenty-four hours, no less than sixty-eight litres must be reabsorbed in the 

 tubules in the formation of two litres of urine. But Cushny points out that 

 we have many analogies to this process in the body. Thus the. liver throws 

 into the duodenum 500 c.c. of fluid in twenty-four hours, all of which is re- 

 absorbed with the exception of a little pigment. The urine of birds passes 

 down the ureter as a clear fluid, but in the cloaca almost all the water is 

 absorbed, leaving a thick paste of urine. Nor is the work out of proportion 

 to the mechanism provided. In a cat fed on meat, 100 c.c. of urine con- 

 tained as much solids as twelve litres of plasma filtrate, so that for twelve 

 litres filtered through the glomeruli 119 were reabsorbed in the tubules. 

 Since each kidney contains about 16,000 glomeruli, the amount of fluid 

 filtered by each glomerulus would amount to about '055 c.c. per hour. Of 

 this more than '0144 c.c. was absorbed in passing along 3 cm. of tubule, 

 leaving less than 1 mg. per hour from each capsule to enter the collecting 

 tubule (Cushny). This cannot be regarded as too severe a task either for 

 tha glomeruli or for the tubules. 



