EXCRETION. 



481 



the urine should accumulate in the ureter and tubules either from ligation or 

 mechanical obstruction until its pressure approximated that of the blood, the 

 secretion should 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 effer- 

 ent vessel and the capillaries beyond. 



The pressure of blood in the renal artery may be augmented and 

 the velocity of the blood stream increased : 

 i. By an increase in blood-pressure generally. 

 2. By an increase in the blood-pressure 

 of the renal artery alone. 



The first condition may be caused 

 by an increase in either the force, or 

 frequency of the heart's action or by 

 a contraction of the arterioles of the 

 vascular areas in any or all parts of the 

 body, excepting, of course, the renal vas- 

 cular area. Should this condition arise, 

 the blood would be forced into the renal 

 artery in larger volumes and in conse- 

 quence its pressure would be increased. 

 The second condition is brought about 

 by a dilatation of the renal artery alone 

 and possibly by a contraction of the 

 efferent vessels of the glomeruli. 



The pressure of the blood in the 



renal artery and therefore in the glomer- sents the mean average pressure. If the vas 

 lili mav be HiminUripH anH the veloritv afferens dilates and the vas efferens con tractr 



6 Velocity ratel or con j oin tl y , the pressure will rise 



decreased: 



1. By a decrease in the blood-pressure 

 generally. 



2. By a decrease in the blood-pressure line. (After Morat.} 

 of the renal artery alone. 



The first condition may be caused by a decrease in either the force or 

 frequency of the heart's action or by a dilatation of the arterioles of large 

 vascular areas in any or all parts of the body. Should this condition arise, 

 the volume of blood delivered to the kidney in the unit of time would be 

 diminished and hence its pressure would fall. The dilatation of the cutane- 

 ous vessels in summer, the result of the high temperature leads to a di- 

 minished blood supply to the kidney and a diminution in the amount of urine 

 secreted. The second condition is brought about by contraction of the 

 renal artery alone and possibly by a dilatation of the efferent vessels of the 

 glomeruli. Moreover the pressure in the vessels of the glomeruli may be 

 varied according to the degree of contraction or relaxation of the muscle 

 31 



Bow 

 Capsule 



FIG. 221 .To ILLUSTRATE THE EFFECT 

 OF ACTIVE CHANGES IN THE VASA AFFER- 

 ENTIA AND EFFERENTIA ON THE PRESSURE 

 IN THE GLOMERULAR CAPILLARIES. A. Re- 

 nal arteries. G. Glomerular capillaries. 

 C. Tubular capillaries. V. Vein. The short 

 thick lines represent the vasa afferentia and 

 efferentia. The continuous heavy line repre- 

 ean average pressure. If the vas 

 aff erens dilates and the vas efferens contracts 



mjo 



as" indicated by the up'per 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 



