354 EXCRETION 



renal capsules. Some observers, indeed, have extracted from the kid- 

 neys a substance (renin) that produces an elevation of blood-pressure 

 when injected into the tissues or bloodvessels of living animals, by 

 exciting the vasomotor centres. Renal extract has also been employed 

 in the treatment of uremia, it was thought with favorable results. The 

 entire question, however, demands further investigation. 



Work of the Kidney. The work of the kidney, as regards the 

 separation of water from the blood-plasma, has been estimated by 

 comparing the osmotic pressure of the urine with that of the blood- 

 plasma, expressed in percentage solutions of sodium chloride. In a 

 typical experiment by Dreser, two hundred cubic centimeters (about 

 6| fluidounces) of urine were secreted during the night, with a pres- 

 sure equal to a four per cent solution of sodium chloride. Compared 

 with the blood-plasma, with a pressure equal to a 0.92 per cent solution 

 of sodium chloride, the difference (equal to a 3.08 per cent solution of 

 sodium chloride) is equivalent to 267.88 foot-pounds (thirty-seven kilo- 

 grammeters). It must be admitted, however, that this view in regard 

 to the actual work of the kidneys in separating from the blood a given 

 volume of a liquid of a certain osmotic pressure is in some degree 

 speculative. It is not certain that the laws of osmosis can be applied 

 to the processes of secretion absolutely and without reserve, taking no 

 account of a cell-action, with the exact nature of which physiologists are 

 not acquainted. 



