EXCRETION 209 



composition is maintained by the eliminating activity of the 

 kidneys. 



Even in the diuretic action of digitalis we see indications of 

 something more than an alteration of the hydrostatics of the 

 blood-supply of the kidney. The brisker circulation carries 

 waste-products to the liver ; the liver transforms nitrogenous 

 refuse into urea ; urea stimulates the renal epithelium. It 

 would be a mistake to lay too much stress upon the direct effect 

 of the drug upon the blood-pressure in the kidney. Other 

 illustrations throw the mere hydrostatics of the problem into 

 the background. Adrenalin (extract of suprarenal capsule) 

 causes a severe contraction of the small arteries, which raises the 

 general blood-pressure considerably ; but the increased blood- 

 pressure is not accompanied by diuresis, because the glomerular 

 arterioles share to a full extent, perhaps to a disproportionate 

 extent, in the general constriction. In migraine and certain 

 other disorders it frequently happens that the blood- pressure in 

 the aorta is unduly high, yet very little fluid enters the renal 

 tubules. If a " saline diuretic," potassic nitrate, sodic acetate, 

 or some other drug of the same kind, be administered, a copious 

 flow is established, the blood-pressure is relieved, the distressing 

 symptoms disappear. Then, again, certain diuretics, such as 

 " sweet spirits of nitre," tea, gin, etc., may bring about a flow 

 out of all proportion to the alteration they produce in the 

 hydrostatics of the circulation. The diuretic action of these 

 various drugs is clearly due to increase in permeability of the 

 renal epithelium. And, of all stimulants to secretion, urea, the 

 natural stimulant, is the most effective. If a kidney be 

 removed from the body, a cannula inserted into its artery, and 

 defibrinated blood caused to circulate under pressure through 

 the organ, water may or may not drip from the ureter. On 

 addition of urea to the blood, a copious excretion is set up. In 

 explaining the mode of working of the kidney, as, indeed, in 

 explaining that of every other organ of the body, the mechanical 

 aspects of the problem must be kept in the background. When 

 we are contemplating the plan of construction of the kidney, 

 the hydrostatics of the circulation attract attention ; but 

 alterations in hydrostatic conditions are not the initiating cause 

 of a greater or less flow of urine. The chemical condition of the 

 blood circulating through the kidney is the initiating cause. 



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