676 DIURETICS. [BOOK n. 



conclude that in such cases the organism, striving, though too often 

 in vain, to free itself from the evils to which it is being subjected, 

 has recourse rather to the simpler glomerular mechanism than to 

 the more expensive tissue-wasting activity of the tubules ; and 

 the urine in such cases is probably discharged chiefly by the 

 method of dilating the renal vessels and thus throwing the 

 poisoned blood into the glomeruli. 



When however fluid is taken simply as a proper accompaniment 

 of solid food, the increase of urine which results has probably 

 another origin. As we have already said, and as we shall point out 

 more fully later on, the absorption of proteid material, which is 

 a constituent and generally a conspicuous constituent of every 

 meal, leads to a formation of urea; and urea, as we have seen 

 reason to believe, directly stimulates the epithelium of the tubules 

 to secretory activity. And what seems prominently true of urea 

 is probably true of many other products of digestion ; so that the 

 increased flow of urine which follows an ordinary meal accompanied 

 with not more than the ordinary amount of fluid, is the result of 

 the labours of the epithelium of the tubules as well as of the 

 fuller stream of blood through the glomeruli. 



422. What has just been said concerning the influence on 

 the kidney of food and water may be applied also to the action of 

 substances which being especially efficacious in promoting a flow 

 of urine when taken into the body are called " diuretics." The 

 several actions of various diuretics are very varied, and it would be 

 out of place to discuss them fully. We may however say that 

 while the action of some appears simple that of others is complex. 



Such agents as sodium acetate and potassium nitrate probably 

 produce their effect chiefly by acting directly on the kidney, 

 inducing, as we have seen, 414, local vascular dilation and so 

 working on the glomeruli, but probably at the same time also 

 stirring up, after the fashion of urea, the epithelium of the 

 tubules to secretory activity, the accompanying fuller stream of 

 blood through the whole kidney being, as in the case of the 

 salivary and other glands, a useful adjuvant. 



The diuretic effect of such an agent as digitalis is probably 

 more complex. By increasing the cardiac stroke, and at the same 

 time constricting many small vessels, digitalis raises the general 

 blood-pressure ; but the tendency of the increased blood-pressure 

 to increase the flow of urine may be counterbalanced by the 

 constriction of the renal vessels themselves. And while it is a 

 matter of common experience that digitalis is very effective as a 

 diuretic in cardiac disease, there is great doubt whether it really 

 acts as a diuretic in health ; in cardiac disease it probably raises 

 the blood-pressure by improving the cardiac stroke and not by 

 constriction of the blood vessels. But even in the absence of 

 cardiac disease, digitalis has been found in certain cases to act as 

 a powerful diuretic, and in these cases either it must act directly 



