396 



ESSENTIALS OF PHYSIOLOGY. 



and of the urine are known. The freezing point of any substance in 

 solution in water is lower than that of pure water, and the osmotic 

 pressure of such a solution is proportional to the depression of the 

 freezing point below C. The freezing point of serum is - O56 C., 

 and that of urine may be - 4 '5 C. The osmotic pressure of urine, 

 therefore, is very much greater than that of blood, and the amount of 

 work done by the kidneys in producing urine of high osmotic pressure 

 from blood, of which the osmotic pressure is low, is extremely large. 



Absorption by the Tubules. When a mixture of sodium chloride 

 and sodium sulphate is injected into the circulation of an animal, the 

 percentage of chlorides in the urine gradually falls as the experiment 

 continues, and may become less than that in the blood plasma, while 

 the percentage of sodium sulphate in the urine remains high. This is 

 seen in the following table: 



Accepting the view that the glomeruli filter off from the blood a 

 fluid containing the same percentage of sodium chloride as that in 

 the blood, this result can only be explained by supposing that as the 

 glomerular filtrate passes down the tubules they absorb sodium chloride. 



Similar results have been obtained in frogs. When the kidneys 

 of the frog are perfused through the renal arteries with oxygenated 

 Ringer's solution, the urine, which in these experiments is formed 

 solely by the glomeruli, is more dilute than the perfusing fluid ; this 

 is due to the fact that, as the urine formed by the glomeruli passes 

 down the tubules, the latter absorb sodium chloride. When the 

 tubules are poisoned with corrosive sublimate, absorption no longer 

 takes place, and the urine is isotonic with the perfusing fluid. 



This process of absorption probably serves to prevent the loss of 

 salts which are needed in the body, especially when they are not 

 being replaced in the food. That this is the case is suggested by the 

 observation that, when animals are fed for some days on a diet free 

 from chlorides, their urine is almost free from chlorides, although the 

 percentage of sodium chloride in the blood plasma may be unaltered. 



In all probability water, and possibly other substances, are also 

 absorbed. The part of the tubule which carries out this process has 

 not been ascertained. 



