422 ACTION OF DIURETICS. 



of the epithelium of the uriniferous tubules. When the arterial anaemia is kept up for a long 

 time, the whole renal tissue dies (Litten). After long-continued ligation of the renal artery, 

 the epithelium of the glomeruli becomes greatly changed (Eibbcrt). 



5. Most diuretics act in one or other of the above mentioned ways. 

 [Some \liuretics act by increasing the general blood-pressure (digitalis and the action of cold 

 on the skin), others may increase the blood-pressure locally within the kidney, and this they 

 may do in several ways. The nitrites are said to paralyse the muscular fibres in the vasa 

 afferentia, and thus raise the blood-pressure within the glomeruli. But some also act on the 

 secretory epithelium, such as urea and caffein. Bran ton recommends the combination of 

 diuretics in appropriate cases, and the diuretics must be chosen according to the end in view 

 as we wisli to remove excess of fluids from the tissues and serous cavities, or as we wish to re- 

 move injurious waste products, or merely to dilute the urine.] 



[6. The amount of urine also depends upon the composition of the blood. Drink- 

 ing a large quantity of water, whereby the blood becomes more watery, increases 

 the amount of urine, but this is true only within certain limits. It is not merely 

 the increase of volume of the blood acting mechanically which causes this increase, 

 as we know that large quantities of fluid may be transfused without the general 

 blood-pressure being materially raised thereby.] 



[Heidenhain argues, that it is not so much the pressure of the blood in the 

 glomeruli as its velocity, which determines the process of the secretion of water in 

 the kidney. He contends that, while increase of the pressure in the renal artery 

 causes an increased flow of urine, ligature of the renal vein, whereby the pressure 

 in the glomeruli is also increased, arrests the secretion altogether. In both cases 

 the pressure is increased within the glomeruli, and the two cases differ essentially 

 in the velocity of the blood-current through the glomeruli.] 



Pressure in the Vas Afferens. The pressure in each vas afferens must be 

 relatively great, because (1) the double set of capillaries in the kidney offers con- 

 siderable resistance, and (2) the lumen of the vas efferens is narrower than that of 

 the vas afferens. Hence, owing to the high blood-pressure in the capillaries of the 

 renal glomeruli, filtration must take place from the blood into the Malpighian 

 capsules. When the vasa afferentia are dilated, the filtration-pressure is increased, 

 while, when they are contracted, the secretion is lessened. When the pressure 

 becomes so diminished as to retard greatly the blood-stream in the renal vein, the 

 secretion of urine begins to be arrested. Occlusion of the renal vein completely, 

 suppresses the secretion (//. Meyer, v. FrericJis). Ludwig concluded from this 

 observation, that the filtration or excretion of fluid could not take place through the 

 renal capillaries proper, as, owing to occlusion of the renal vein, the blood-pressure, 

 in these capillaries must rise, which ought to lead to increased filtration. Such an 

 experiment points to the conclusion that the filtration must take jdoice through the 

 capillaries of the glomeruli. The venous stasis distends the vas efferens, which 

 springs from the centre of the glomerulus, and compresses the capillary loops against 

 the wall of the Malpighian capsule, so that filtration cannot take place through 

 them. It is not decided whether any fluid is given off through the convoluted 

 urinary tubules. 



Venous congestion in the kidneys diminishes the quantity of urine and the urea. The NaCl 

 remains constant, but pathological albumin is increased (Senator and Munk). 



Pressure in Ureter. As the blood-pressure in the renal artery is about 120 to 140 mm. Hg, 

 and the urine in the ureter is moved along by a very slight propelling force, so that a counter- 

 pressure of from 10 (Lbbcll) to 40 mm. of Hg is sufficient to arrest its flow, it is clear that the 

 blood-pressure can also act as a vis a tergo to propel the urine through the ureter. The 

 pressure in the ureter is measured by dividing the ureter transversely and inserting a manometer 

 in it. 



(B) Secretory Activity of the Renal Epithelium. The degree of concentra- 

 tion of the urine depends upon the quantity of the dissolved constituents which 

 has passed from the blood into the urine. The secretory cells of the convoluted 

 tubules, by their own proper vital activity, seem to be able to take up, or secrete, 



