662 FLOW OF BLOOD THROUGH KIDNEY. [BOOK n. 



section of the nerves of the renal plexus, whereby the paths of 

 all vaso-constrictor impulses to the kidney are blocked. After 

 this has been done a rise of general pressure whether by dyspnoea, 

 or by direct stimulation of the spinal cord, or by stimulation 

 of the abdominal splanchnic nerves, leads to a greater rlow 

 through the renal vessels and an increased expansion of the 

 kidney. 



A rise of general blood-pressure then may be accompanied 

 by either a shrinking or a swelling of the kidney, by either a 

 greater or a less How of blood through the kidney, according 

 to the concomitant condition of the renal vessels ; or indeed may 

 under certain circumstances be accompanied by no change at all 

 in the renal circulation, the local effects exactly counterbalancing 

 the general ones. 



Conversely, in a similar way, a fall of blood-pressure leads to a 

 lesser flow through the renal vessels and a shrinking of the kidney 

 unless it be accompanied by a dilation of the renal vessels out of 

 proportion to the general fall. Thus when the spinal cord is 

 divided below the medulla the fall of general blood-pressure is, as 

 we have seen ( 173) very marked, being due to an abolition for the 

 time being of wonted constrictor impulses. The pressure in the 

 aorta falls rapidly, and at the same time, owing to the more open 

 pathway through the region of peripheral resistance in the body 

 generally, the pressure in the vena cava is increased ; the difference 

 of pressure between the mouth of the renal artery in the aorta and 

 the mouth of the renal vein in the vena cava is so largely reduced 

 that in spite of the concomitant relaxed condition of the renal 

 vessels themselves the flow of blood through the kidney is largely 

 diminished. 



It will of course be understood that, the general blood-pressure 

 remaining the same, the flow through the kidney will at once be 

 on the one hand increased by dilation and on the other decreased 

 by constriction of the renal vessels themselves. The constricted 

 or dilated condition of the renal vessels can by themselves produce 

 but little effect on the pressure either in the aorta or in the 

 vena cava ; and the difference between the pressure at the 

 mouth of the renal artery and that at the mouth of the renal 

 vein remaining the same, the more open passages of the dilated 

 renal vessels must lead to a fuller, and the narrower passages 

 of the constricted renal vessels to a scantier flow, through the 

 kidney. 



412. By means of the oncometer, watching the shrinking 

 and swelling of the kidney and thus judging of the flow of blood 

 through it, the results being always interpreted with reference to 

 the general blood-pressure on the lines of the above discussion, 

 the paths of vaso-motor impulses to the kidney have been 

 approximately made out. Vaso-constrictor fibres for the kidney 

 are supplied from what we have previously ( 169 and elsewhere) 



