Chap, hi.] ELIMINATION OF WASTE PRODUCTS. 529 



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

 about — (a) by diminished force, &c. of the heart's beat, (b) 

 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 ques- 

 tion. 



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 them- 

 selves 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 constriction 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 medulla 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 abdominal splanch- 

 nic 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 



34 



