DRUGS ACTING ON THE URINAET ORGANS • 53 



(a) Four medullary centres. 



Vasomotor | Vasodilator centres 



(controlling renal vessels) j Vasoconstrictor centres 



Secretory (Stimulating cells ot convoluted tubes) 



Inhibitory (Depressing activity of renal cells) 



(b) Subsidiary centres in the spinal cord, solar and renal plexus. 



(c) Secretory nerve-endings in kidneys. 



(d) Muscular fibre in renal vessels. 



(e) Benal cells. 



These structures may be either stimulated or depressed, 

 or the same drug may cause diuresis in various ways by a 

 combination of actions. 



Diuretics more commonly act in two ways. (1) By 

 influencing renal circulation ; (2) by stimulating the secre- 

 tory nerve-endings in the renal cells or the cells them- 

 selves. 



(1) Diuretics increasing general or. local {renal) blood tension. 



These drugs increase cardiac action in some cases, and 

 in others stimulate or depress the vasomotor centres, or the 

 terminations of the vasoconstrictors or dilators in the renal 

 vessels — to cause dilatation of the branches of the renal 

 artery carrying blood to the capillary loops in the Malpighian 

 bodies, or to produce contraction of the arterial branches 

 conducting blood away from the Malpighian tufts — in either 

 way increasing local blood tension and secretion of urine.' 



Local blood tension is thought to be augmented by the 

 nitrites and alcohol acting on the vasomotor centre or renal 

 vessels to dilate the afferent vessels of the Malpighian 

 glomerules, in addition to stimulating the heart. Buchu, 

 turpentine, juniper and cantharides are said to act locally 

 on the kidneys to contract the efferent vessels of the Mal- 

 pighian bodies. Digitalis, strophanthus, squill, caffeine and 

 strychnine stimulate the heart and contract the vessels 

 generally, while locally they are believed to cause contrac- 

 tion of the efferent vessels of the Malpighian bodies by 

 action on the vasomotor centres. The entrance of water 

 into the circulation increases the mass of blood, vascular 

 tension, and thus the flow of urine. 



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