THE SECRETION OF URINE 



1277 



renal arteries, interlobular capillaries, and veins filled to distension 

 with the injected mass, but hardly any in the glbmeruli. One must 

 assume in such a case that there has been spasmodic contraction of the 

 muscular coats of the vasa afferentia (cp. Fig. 532). The normal amount 

 of blood might therefore circulate through the kidney without any 

 flowing through the filtering apparatus, i.e. the glomeruli. On the other 

 hand, a dilatation of the afferent vessels and a slight constriction of the 

 efferent vessels would cause a considerable rise of pressure in the 

 glomerular capillaries, and a consequent increased transudation, without 

 necessarily altering to any marked extent the total circulation of blood 

 through the whole organ. The changes in the afferent and efferent 

 vessels and the glomeruli are, however, beyond our control or powers 



Bowman's 

 Capsule 



FIG. 532. Diagram (after MORAT) to illustrate the effect of active changes 

 in the vasa afferentia and efferentia on the pressure in the glomerular 

 capillaries. If the vas afferens constricts, the pressure will be repre- 

 sented by the lower dotted line. On the other hand, constriction of 

 the vas efferens would raise the pressure in the glomerulus till it almost 

 equalled that in the renal artery, as is shown by the upper dotted line. 

 A, arteries ; G, glomerular capillaries ; c, tubular capillaries ; v, vein. 



of observation, so that it is impossible to devise at the present time 

 any crucial experiment which might decide the nature of the process 

 occurring in the glomeruli. 



THE COMPOSITION OF THE URINE. If the glomerular func- 

 tion is that of mere filtration, we should expect that the more rapidly 

 the process occurs, the more nearly would the urine which is turned 

 out into the ureters resemble the blood-plasma in composition, reac- 

 tion, and osmotic pressure, since the glomerular filtrate hurried through 

 the tubules would have very little time to undergo any changes result- 

 ing in its concentration. If, on the other hand, the diuresis, produced by 

 salt or sugar solutions, is to be ascribed to a stimulation of the renal 

 epithelium, the differences between blood-plasma and urine should be 

 greatest at the height of the diuresis, when the concentration of the 

 specific stimulant is also at its highest. The following experiment shows 

 that the more rapid the secretion of urine, the more closely does its 



