THE SECRETION OF URINE 1285 



is at any rate one factor in determining the substances absorbed. It 

 is not, however, the only factor. The function of the kidney is to // 

 preserve the normal constitution of the body fluids by turning out ^ 

 those substances which are abnormal or present in too great an amount. 

 The behaviour of the tubule cells with regard to any given substance 

 will therefore depend to a certain extent on the previous nutritive 

 history of the body. )1 T ^ 



If, for instance, in consequence of the administration of sodium 

 chloride in large quantities to the animal during the few days preceding 

 the experiment, the body is overloaded with this salt, it becomes an 

 abnormal constituent and the kidney secretes a urine far richer in 

 sodium chloride than is the blood-plasma. Moreover, when diuresis 

 is produced in such an animal by the injection of equivalent quantities 

 of sodium chloride and sodium sulphate, there is no diminution of 

 the NaCl in the urine towards the end of the diuresis, but its per- 

 centage rises steadily as the rate of urinary flow diminishes. On the 

 other hand, a total deprivation of sodium chloride extending over 

 several days, although not altering to any large extent the percentage 

 amount of this salt in the blood-plasma, leads to a total disappearance 

 of the salt from the urine, the whole of the sodium chloride present 

 in the glomerular transudate being absorbed on its way through the 

 urinary tubules. 



It has been suggested that the effects of certain diuretics on the 

 kidney, such as caffeine, diuretine, or theocine, may be largely condi- 

 tioned not so much by their influence on the glomerular circulation as 

 by a paralytic effect on the absorptive functions of the tubules. 

 According to Lowi, on injection of caffeine or diuretine, the increase 

 of total amount of urine is not accompanied by any diminution in the 

 percentage amount of Nad. Perhaps, however, the strongest evidence 

 in this direction is afforded by an experiment of Pototzky. A rabbit 

 had been fed on a diet almost totally devoid of chlorides, and was 

 therefore excreting a urine containing only -08 per cent. NaCl. 

 Under the influence of diuretine the urine was increased and the con- 

 centration of the NaCl rose to 0-64 per cent. The same increase in the 

 percentage amount of sodium chloride in the urine has also been 

 observed after the injection of theocine, which has therefore been 

 specially recommended as a diuretic in cases of dropsy, where a 

 diminution of the salt content of the body is a valuable means for the 

 diminution of the dropsical fluid present in the tissue spaces. 



THE RENAL MECHANISM 



What conclusions can we draw from this mass of experimental 

 data as to the functions of the kidney as a whole, and as to the part 

 played by its various constituent elements in the secretion of urine ? 



