416 EXCRETION 



But Heidenhain's view is strengthened by Bowman's observation that in birds 

 crystals of uric acid are to be seen in the cells of the convoluted tubules, and 

 in the lumen adjacent. 



Only traces of the sugars and proteins of the blood are found in normal 

 urine, but when either cane sugar, peptone, or egg albumin is introduced into 

 the blood it is rapidly eliminated by the kidney. Egg albumin is not essen- 

 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, o . 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 



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

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



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. 



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 actively 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 w r ell-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 of the renal epithelium. Urea introduced into 

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



