DIURETICS 389 



tially different from the serum albumin of the blood, but the serum albumin 

 is not excreted. These are both non-dialyzable compounds. Sugar and 

 urea, both readily dialyzable, present the same comparison, i.e., urea is ex- 

 creted, while sugar is not. If, however, the percentage of sugar is high, 0.25 

 per cent or more, it is then eliminated. The excretion of the highly diffusible 

 sodium chloride bears a similar quantitative relation to excretion. If present 

 in the blood in relatively low amounts it is not secreted, while if the concentra- 

 tion is slightly greater it may be quickly eliminated. Other inorganic salts, 

 present only in traces, are meanwhile rapidly eliminated. Even the rapid 

 elimination of a slight excess of water in the blood can scarcely be explained 

 on purely physical grounds. To discharge the water across the glomerulus 

 from the blood to the urine requires an expenditure of osmotic pressure much 

 greater than that balanced by the blood pressure. That is, the epithelial cells 

 must do work, and the energy is dependent on metabolism in the cells. At 



FIG. 303. Curve Taken by Renal Onkometer Compared with that of an Ordinary Blood- 

 pressure Curve, a, Kidney curve; b. blood -pressure curve. (Roy.) 



least one substance, hippuric acid, is built up chemically by the renal cells 

 and secreted as such. 



It would seem, therefore, that the separation of urine in the kidney is a 

 secretory process dependent on the protoplasmic activity of the living renal 

 cells, that the apparent selective property of the cells is a manifestation of 

 such activity, and that even water is secreted. 



Diuretics. Certain substances increase the flow of urine and are 

 called diuretics. They act directly on the renal epithelium, for example, 

 urea, or indirectly on the circulatory system to increase the flow of blood. 

 Digitalis is a well-known diuretic which increases the efficiency of the circula- 

 tion. It also stimulates the renal epithelium with the production of a marked 

 increase in the flow of urine. Caffeine diuresis can best be explained on an 

 assumed stimulating action on the renal epithelium. Urea introduced into 

 the blood produces a copious secretion of urine. Both urea and the saline 

 diuretics induce a flow of urine out of all proportion to the osmotic changes 

 produced, and they may be regarded as direct stimulators of the renal epithe- 

 lium. 



