DRUGS ACTING ON THE URINARY ORGANS 53 



(a) Four medullary centres. 



Vasomotor | Vasodilator centres 



(controlling renal vessels) f Vasoconstrictor centres 



Secretory (Stimulating cells o£ 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) Renal 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 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. Buchu, turpentine, juniper and cantharides 

 are said to act locally on the kidneys to contract the efferent 

 vessels of the malpighian bodies. Digitalis, strophanthus, 

 squill, caffeine and strychnine stimulate the heart and 

 contract the vessels generally, while locally they are believed 

 to cause contraction 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. 



