1142 



PHYSIOLOGY 



obtained pari passu with the increased urinary flow ; and a consideration 

 of certain peculiarities in the renal circulation must prevent us from laying 

 too much stress on apparent exceptions to the rule. To the blood entering 

 the kidneys by the renal arteries two ways are open. The blood may pass 

 through the vasa afferentia, through the glomeruli and tubular capillaries, 

 back to the renal vein. On the other hand, it may escape the glomeruli 

 altogether, and pass through the vasa recta directly into the intertubular 

 capillaries and so into the renal veins. It is a common experience, in 

 injecting the blood-vessels of the kidneys post-mortem, to find the renal 

 arteries, intertubular capillaries, and veins filled to distension with the 

 injected mass, but hardly any in the glomeruli. One must assume in such 

 a case that there has been spasmodic contraction of the muscular coats of 

 the vasa afferentia (cp. Fig. 534). The normal amount of blood might 



Bowman's 

 Capsule 



FIG. 534. 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 represented 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. 



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 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 function 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, reaction, and osmotic pressure, 

 since the glomerular filtrate hurried through the tubules would have very 

 little time to undergo any changes resulting in its concentration. If, on the 

 other hand, the diuresis, produced by salt or sugar solutions, is to be ascribed 



