44 GENERAL ACTION OF DRUGS 



DRUGS ACTING ON THE URINARY ORGANS. 



Diuretics are drugs increasing the, secretion of urine. Four factors 

 are concerned with urinary iiow. (1) The composition and viscosity of 

 the blood. (2) The state of local (renal) and general blood pressure, 

 (3) The activity of the renal cells. (4) Reabsorption or rapidity of 

 &0W of urine in the constricted tubes. The renal mechanism influenced 

 by drugs comprises: 



(1) The malpighian glomerules, which excrete alkaline fluid, urea 

 and salts (chlorides, sulphates and phosphates), and other substances 

 from the blood by osmosis. Their activity depends upon their blood 

 supply, which is increased by agents causing dilatation of their afferent 

 arterioles, thus accelerating the velocity and flow of blood to the glo- 

 merules ; and by drugs increasing general blood tension without corre- 

 sponding constriction of the renal vessels. Drugs augmenting the mass 

 of blood and tension in the malpighian bodies enlarge the surface of 

 cubical epithelium covering the capillary loops and promote osmosis or 

 filtration of fluid into the cavity of the malpighian capsule. There is 

 also some true secretion by the cells lining the glomeruli. Sugar, peptone 

 and egg albumin inj ected into the blood are excreted by the glomeruli. 



(2) The nucleated polyhedral cells lining the convoluted tubes. — 

 These secrete the solid products resulting from the retrograde metamor- 

 phosis of nitrogenous bodies circulating in the blood, as urea, uric acid, 

 creatin, and other organic substances, pigment, phosphates and water. 

 Also in animals having an acid urine, the change in reaction from alka- 

 line secretion of the malpighian bodies to acid occurs in convoluted tubes, 

 unless the urine is hurried along too rapidly by diuretics. 



(3) The constricted tubes. — These regulate the urinary secretion by 

 either impeding its passage by constriction of their walls, thus aiding 

 absorption, or by their active peristalsis facilitating the flow' of urine. 

 The chief function of the constricted tubes is to cause concentration of 

 the urine. 



(4) Nervous mechanism. — This governs the calibre of the vessels of 

 the malpighian bodies, and possibly controls the unstriped muscle of the 

 constricted tubes. No secretory nerves, such as those controlling the 

 secretory cells of the salivary glands, have been discovered in the kid- 

 neys. Variations in the blood-supply are apparently sufficient to account 

 for the secretion of urine. 



The flow of urine is therefore chiefly regulated by the vasomotor 

 system, with centres in the medulla and thoracic and lumbar cord. The 

 constrictor and dilator fibres run in the splanchnics, through the renal 

 plexus, enter the kidney at the hilum, and accompany the arteries to their 

 final endings. We may classify diuretics broadly in two groups: (1) 

 Those acting chiefly upon the glomerules. (2) Those affecting mainly 

 the renal cells of the tubules. 



(1) Diuretics increasing the glomerular fluid. 



(a) By increasing the flow through the renal arteries. 



(b) By lessening osmotic pressure of the blood. 



Water is the chief diuretic. Without an abundance of water diuresis 



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