542 DIURETICS [BOOK n. 



of blood and at the same time yields a fuller flow of urine 

 when water is injected into the blood, we may infer that the 

 blood diluted by the absorption of water acts directly on the 

 kidney. We may further suppose that it is the glomerular 

 mechanism which is thus especially increased in activity, though 

 it may be that the epithelial secretion is also augmented. 



When however fluid is taken simply as a proper accompani- 

 ment of solid food, the increase of urine which results has prob- 

 ably 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 promi- 

 nently 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. 



342. 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 promot- 

 ing a flow of urine when taken into the body are called " diu- 

 retics." 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 



,, . J i 

 oi others is complex. 



Such agents as sodium acetate and potassium nitrate appear 

 to produce their effect chiefly by acting directly on the kidney. 

 They induce, as we have seen, 334, local vascular dilation and 

 probably act by stirring up, after the fashion of urea, the epi- 

 thelium 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, though 

 it may also increase the glomerular secretion. 



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 effec- 

 tive as a diuretic in cardiac disease, there is great doubt whether 

 it really acts as a diuretic in health ; in cardiac disease it prob- 

 ably 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 particular 



