43 



TEXT-BOOK OF PHYSIOLOGY. 



As a rule, this latter pressure is at a minimum. If the urine should 

 accumulate in the ureter and tubules either from ligation or mechan- 

 ical obstruction until its pressure approximates that of the blood, the 

 secretion would be diminished if not abolished. It is difficult to 

 determine the average pressure or velocity of the blood in the 

 glomerular capillaries, though they both must be greater than in 

 capillaries in other parts of the body, from the fact that the 

 efferent vessel is narrower than the afferent, and therefore offers 



great resistance to the outflow 

 of blood, a condition most favor- 

 able to the production of a 

 high pressure in the glomerulus. 

 The pressure of the blood in 

 the glomeruli may be raised and 

 the velocity increased : 



1. By an increase in blood-pres- 



sure generally. 



2. By an increase in the pressure 



of the renal artery alone. 

 The first condition may be 

 brought about by an increase in 

 either the force or frequency of 

 the heart's action or by a con- 

 traction of the arterioles of vas- 

 cular areas in any or all parts of 

 the body, excepting, of course, 

 the renal vascular area. The 

 second condition is brought about 

 by a dilatation of the renal artery 

 alone and possibly by a contrac- 

 tion of the efferent vessels of the 

 glomeruli. 



The pressure of the blood in 



(After Morat and Starling.) the glomeruli may be diminished 



and the velocity decreased - 



1. By a decrease in the blood-pressure generally. 



2. By a decrease in the pressure of the renal artery alone. 



The first condition is brought about by a decrease in either the 

 force or frequency of the heart's action or by a dilatation of the arteri- 

 oles of large vascular areas in any or all parts of the body. The 

 second condition is brought about by contraction of the renal artery 

 alone and possibly by a dilatation of the efferent vessels of the glom- 

 eruli. The effect of the contraction and relaxation of either the 

 afferent or efferent vessels on the pressure within the glomerulus 

 is shown in figure 195. 



Capsu.e 



FIG. 195. To ILLUSTRATE THE EFFECT 

 or ACTIVE CHANGES IN THE VASA 

 AFFERENTIA AND EFFERENTIA ON 

 THE PRESSURE IN THE GLOMERULAR 

 CAPILLARIES. A. Renal arteries. 

 G. Glomerular capillaries. C. 



Tubular capillaries. V. Vein. The 

 short thick lines represent the vasa 

 afferentia and efferentia. The con- 

 tinuous heavy line represents the 

 mean average pressure. If the vas 

 afferens dilates and the vas efferens 

 contracts separately or conjointly, 

 the pressure will rise, as indicated by 

 the upper dotted line. If the vas 

 afferens contracts and the vas 

 efferens dilates separately or con- 

 jointly, the pressure will fall, as in- 

 dicated by the lower dotted line. 



