274 APPLIED PHYSIOLOGY 



tion in the distribution of the blood, which results in the 

 kidney getting more or less than its usual share. 



On the other hand, the amount of urine is also influ- 

 enced by alterations in the general blood-pressure, pro- 

 vided the kidney participates in them. If the general 

 pressure falls, the amount of water excreted by the 

 kidney is lessened, and if the fall in the renal capillaries 

 be great enough (down to 40 millimetres), excretion stops 

 altogether. In conditions of shock the urine may become 

 very scanty from the fall of blood -pressure, which results 

 from ' pooling ' of the blood in the abdominal veins. 

 The polyuria of chronic nephritis, on the other hand, is 

 probably due, in part at least, to a marked rise of general 

 blood-pressure. One must distinguish between the 

 polyuria which results from alterations in the blood- 

 supply of the kidney, and which consists mainly in an in- 

 creased excretion of water, and a true diuresis, in which 

 the specific organic constituents of the urine are also 

 voided more freely. The latter can only be brought 

 about by an increased activity of the epithelial cells, but 

 of the factors which determine such an increased activity 

 we know little, except that one of them may be the pres- 

 ence in the blood of an unusual amount of waste material 

 to be eliminated. Some diuretic drugs, however, such 

 as caffeine, appear to possess the power of stimulating 

 the renal cells, and it is in such a property that their 

 chief virtue as medicines resides. 



Important effects upon the renal circulation in certain 

 circumstances are brought about by the fact that the 

 capsule of the kidney is comparatively inelastic. When, 

 therefore, the epithelium of the organ swells, as it does, 



