CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 707 



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 

 on the tubular epithelium, or its effects in constricting the renal 

 arteries must be less than its effects on other small arteries or must 

 pass off before the influence of the heightened blood-pressure has 

 disappeared. 



