THE SECRETION OF URINE 



1203 



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 

 injection 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. 549). The normal amount of blood might 



FIG . 549. 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 bo 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 of 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. 



On the other hand, it seems probable that many diuretics of which 

 caffeine may be one act by altering the activity of the tubules. If we accept 

 the idea that the main function of these structures is that of secretion, we 

 may assume that the diuretics increase their secretory power. It is more 

 simple however to assume that any action these substances possess on the 

 tubules is one of paralysis, complete or partial, of their powers of absorption. 

 Thus the action of phlorhizin may be assumed to paralyse the absorptive 

 powers of the tubular cells for glucose i. e. to reduce glucose for this par- 

 ticular kidney to the state of a no-threshold substance. The glucose in the 



