470 TEXT-BOOK OF PHYSIOLOGY 



The Influence of Blood-pressure. Glomerular Pressure. Whether 

 the elimination of the glomerular filtrate is entirely secretor (physiologic) 

 in character or not, there can be no doubt that the whole process is 

 largely determined by the pressure and velocity of the blood in the glom- 

 erular capillaries, or, to state it more accurately, on the difference of 

 pressure between the blood in the capillaries and the filtrate in the capsules. 

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

 accumulate in the ureter and tubules either from ligation or mechanical 

 obstruction until its pressure approximated 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 

 favorable to the production of a high pressure in the glomerulus. 



The pressure of the blood in the glomeruli is the resultant of the 

 pressure in the renal artery and the resistance to the outflow of blood 

 through the efferent vessels and the capillaries beyond. 



Bowmn' 

 C*p*ul 



FlG. 208. TO ILLUSTRATE THE EFFECT OF ACTIVE CHANGES IN THE V~ASA 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 repre- 

 sent the vasa afferentia and efferentia. The continuous 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 conjointly, the pressure will fall, as indicated 

 by the lower dotted line. (After Morat.) 



The pressure of blood in the renal artery and therefore in the glom- 

 eruli, may be augmented by an increase in the inflow of blood. This 

 effect may be the result: 



1. Of a dilatation of the renal artery and its branches, and possibly a 



simultaneous contraction of the efferent vessel of the glomerulus. 



2. Of an increase in the rate and force of the heart's action or an increase 



in the contraction of the arterioles of large vascular areas in any or all 

 parts of the body excepting, of course, the renal vascular area. Should 

 either of these conditions arise the blood would be forced into the 

 renal artery in larger volumes and in consequence its pressure would 

 be increased. The contraction of the cutaneous vessels in the early 

 winter, the result of a falling temperature, leads to an increased 

 blood supply to the kidney and hence an increase in the amount of 

 urine secreted. 



