692 FLOW OF BLOOD THROUGH KIDNEY. [BOOK n. 



which are supplied by the artery. This again must not be 

 accompanied by a too great constriction elsewhere. 



2. By a lowering of the general blood-pressure, brought about 

 (a) by diminished force, &c. of the heart's beat, (6) by a general 

 dilation of the small arteries of the body at large, or by a dilation 

 of vascular areas other than the area in question. 



Applying these considerations to the blood vessels of the 

 kidney, we should expect to find the following. 



A rise in general blood-pressure, and that means a rise of 

 pressure in the abdominal aorta at the mouth of the renal artery, 

 will cause a greater flow of blood through, and so an expansion of 

 the kidney, provided that the renal arteries themselves are not 

 unduly constricted at the same time. This is well shewn, as we 

 have seen, in the curve given above, where the increase of pressure 

 due to each heart-beat, as well as that due to each respiratory 

 movement, being of central origin and not due to arterial constric- 

 tion and being unaccompanied by any compensating constriction 

 of the renal artery, leads to expansion of the kidney, that is, to a 

 greater flow of blood through the kidney. 



If, however, the rise of general blood-pressure be due to events 

 which at the same time cause a constriction of the renal arteries, 

 the flow through the kidney may not only not be increased but 

 even be diminished ; the kidney may shrink instead of expanding. 

 Thus if dyspnoea be brought about, as by stopping artificial 

 respiration during an experiment, the kidney at once shrinks ; the 

 too venous blood stimulates the vaso-motor centre, and probably 

 also by direct action on the blood vessels leads to a general 

 arterial constriction and so to a rise of blood-pressure ; but the 

 renal vessels are involved in this constriction, so much so that 

 their constricted condition more than counterbalances the general 

 rise of blood-pressure, and less blood flows through the renal 

 vessels. So also when the spinal bulb or spinal cord is directly 

 stimulated by induction shocks (the animal being under urari so 

 as to eliminate the complications due to contractions of the skeletal 

 muscles) the renal vessels share so fully in the arterial constriction 

 which results that, in spite of the great rise of mean pressure 

 which is induced, less blood than normal passes through the renal 

 vessels, and the kidney shrinks. Or if the splanchnic nerves be 

 stimulated, since as we shall see these carry vaso-constrictor fibres 

 for the kidney, in spite of the rise of blood -pressure which follows, 

 the kidney shrinks on account of the great constriction of the renal 

 vessels. 



On the other hand if a rise of blood-pressure be for any 

 reason not accompanied by a compensating constriction of the 

 renal arteries, that rise, whether it be brought about by general 

 constriction of arteries other than the renal or by an increase 

 of the cardiac delivery, causes the kidney to swell, shewing a 

 greater flow of blood. Such a condition of things may be induced 



