50 SECRETION 



so that it probably depends upon the living structure of the 

 epithelial cells. 



It is a very instructive condition when in the blood the 

 normal proteins and sugar of the blood circulate side by side 

 with egg albumin, peptone, and cane sugar. The kidney cells 

 then show a selective action, excreting the latter and not the 

 former. Likewise, urea and dextrose, both highly diffusible 

 substances, are simultaneously present in the blood reaching 

 the kidney. Urea is present to the extent of four parts per ten 

 thousand, but there is three times as much sugar. Yet the 

 kidney selects the urea for excretion and rejects the other. 



The separation of urine from blood implies the performance 

 of a large amount of work in overcoming differences of osmotic 

 pressure. The osmotic pressure of a liquid is readily deter- 

 mined by noting the depression of the freezing point. Plasma, 

 for instance, freezes at 0.6 C. This is expressed as follows: 

 i = 0.6 C. Likewise, A for urine is equal to 1.8 C. 

 When these values are translated into meters of water press- 

 ure it is found that the urine exerts 225 meters of water 

 pressure, while plasma exerts but 75 meters of water pressure. 

 The urine, therefore, considering this factor only, would tend 

 to diffuse back through the kidney cells into the blood with 

 a force equal to 150 meters of water pressure. This would 

 be opposed only by the normal hydrostatic pressure between 

 blood and urine equal to only 1.35 meters of water. The 

 passage of urinary constituents is, therefore, from a region 

 of low to a region of high osmotic pressure. 



The quantities of water lost through the kidneys and skin 

 stand in inverse proportion to one another. Since water lost 

 through the skin affects the normal constitution of the urine 

 through the medium of the blood, it is to be expected that 

 other substances in the circulation might have similar influence. 

 This, as a matter of fact, is true. A temporary! alteration of 

 the blood by the absorption of large quantities of water and 

 the presence of diuretics increases the flow of water from the 

 kidneys. If saline diuretics (potassium nitrate, sodium chloride, 

 urea, dextrose, etc.) are injected into the blood, an abundant 

 secretion soon takes place, which is accompanied by an en- 

 largement of the kidney and a slight rise of blood pressure. 

 It has been shown that the power of these diuretics is propor- 



