49 6 EXCRETION 



that the glomerular epithelium, when the difference of pressure 

 on its two surfaces is increased by hydraemic plethora, will use the 

 increased facility of nitration to rapidly excrete a portion of the 

 water. But who will believe that the addition of a tumbler of 

 water, absorbed from the alimentary canal, to 4 or 5 litres of blood 

 circulating in a system of vessels whose capacity can and does vary 

 within wide limits, should cause in the capillaries of the kidney 

 an increase of pressure exactly proportional to the increase in the 

 elimination of water in the urine, lasting for the same time and 

 disappearing at the moment when the normal composition of the 

 blood is restored ? Nor is it easier to explain on any mechanical 

 hypothesis how it is that in a starving animal the quantity of 

 inorganic substances eliminated in the urine drops almost to zero, 

 while the proportional amount in the blood and tissues is little, if 

 at all, affected. In a rabbit rendered poor in sodium chloride by 

 feeding it with salt-free food, the injection of a solution of sodium 

 chloride isotonic with the blood produces no diuresis for a con- 

 siderable time, but, on the contrary, a diminished flow of urine, 

 while a similar solution injected into the veins of a rabbit previously 

 fed with salted food causes an immediate and considerable diuresis. 

 When small quantities of isotonic solutions of various salts are 

 injected, those not normally present in the blood produce a greater 

 diuresis than normal constituents. Sodium chloride, which is 

 present in normal plasma in greater amount than any other salt, 

 causes the smallest diuresis of all (Haake and Spiro). 



Such facts suggest that the secreting cells of the kidney are stimu- 

 lated or inhibited by the contact of blood or lymph in which the 

 normal constituents are present in too great or in too small amount, 

 and that the intensity of the action is proportional to the degree of 

 deficiency or excess. The greater the velocity of the circulation 

 in the kidney, the more effective will be the stimulation produced 

 by any given substance present in excess, and therefore the greater 

 the total amount of it eliminated in a given time. For in making 

 the round of the renal circulation the concentration of the sub- 

 stance in any given portion of blood will fall less, and therefore the 

 average stimulation exerted by it during the round will be greater 

 the faster the blood flows. It is quite in agreement with this that 

 when plethora is occasioned by transfusion of blood there is little 

 or no diuresis, although the increase of arterial, capillary, and 

 venous pressure, and the dilatation of the kidney, are evident. 

 For the rapid passage of liquid out of the vessels would lead to a 

 great increase in the relative proportion of corpuscles to plasma 

 that is to say, to an abnormal condition of the blood. On the other 

 hand, when plethora is produced by injection of serum diuresis 

 occurs (Cushny). This, again, is what we should expect, since the 

 elimination of the superfluous liquid will restore the normal pro- 



