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TEXT-BOOK OF PHYSIOLOGY 



The Influence of Blood-pressure. Whether the elimination of thfe 

 urinary constituents is entirely secretor (physiologic) in character or not there 

 can be no doubt 4;hat the whole process is largely determined by the pressure 

 and velocity of the blood in the glomerular capillaries, or, to state it more ao 

 curately, on the difference of pressure between the blood in the capillaries and 

 the urine in the capsules. As a rule, this latter pressure is at a minimum. li 

 the urine should accumulate in the ureter and tubules either from ligation 01 

 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. 



Glomerular Pressure. 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 vessel and the capillaries beyond. 



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

 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 oa 

 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 bj 

 a dilatation of the renal artery alone and 

 possibly by a simultaneous contraction 

 of the efferent vessels of the glomeruli, 

 The pressure of the blood in the 

 renal artery and therefore in the glomer- 

 uli may be diminished and the velocit} 

 decreased : 



1. By a decrease in the blood-pressure 

 generally. 



2. By a decrease in the blood-pressure 

 of the renal artery alone. 



The first condition may be caused by a decrease in either the force 01 

 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- 



FIG. 211. 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- 

 sents 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.) 



