acid, 



KIDNEY 51 



tional to their molecular weights, and it is, therefore, highly 

 probable that through their osmotic power they withdraw 

 water from the tissues to the blood. The diuresis which they 

 bring about lasts only as long as the blood pressure remains 

 above normal. 



Other diuretics are caffeine and digitalis. If one-half grain 

 of caffeine is injected into the circulation the kidney at first 

 contracts in volume and the secretion of water is stopped. 

 Soon, however, an expansion takes place and a copious urinary 

 flow results. The general blood pressure is also lessened and 

 then heightened. Caffeine seems to act on the renal vessels, 

 diminishing and then augmenting the flow of blood through 

 the glomeruli. Digitalis is rather uncertain in its action as 

 a diuretic. It slows and strengthens the beat of the heart 

 in certain subjects, increasing the arterial pressure and lower- 

 ing the venous pressure, which favors the flow of blood through 

 the kidney and produces an increase in the amount of water 

 in the urine. 



Diuretics may be considered as acting in the following ways : 



1. They may cause hydremic plethora by drawing water 

 from the tissues, thus increasing the blood pressure and favor- 

 ing filtration. 



2. The substances may act directly upon the cells of the 

 glomeruli. 



General anesthetics also affect the activity of the renal 

 epithelium. Ether increases while chloroform decreases the 

 secretion. From this point of view a mixture of ether and 

 chloroform would be the anesthetic to be preferred. 



It may be tentatively accepted as most nearly expressing 

 present ideas upon kidney secretion that the water and salts 

 of the urine are chiefly separated by the glomeruli; the process 

 is Jiot a mere physical filtration, but a physiological activity. 

 Physical forces and processes are not excluded, but they act 

 under conditions and produce results in a manner not con^ 

 ceivable under present conditions of knowledge, so that it 

 would be proper to call the activity a vital activity. Sub- 

 stances like sugar, peptone, egg albumin, and hemoglobin 

 when injected into the blood are probably excreted mainly 

 by the glomeruli; and so is the sugar in diabetes. Urea, uric 

 acid, and other constituents of normal urine, with a portion 



