530 FLOW OF BLOOD THROUGH KIDNEY. [Book n. 



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

 induced by section of the nerves of the renal plexus, whereby 

 the paths of all vasco-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 flow 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 flow 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 counter- 

 balancing 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 (§ 151), 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. 



§ 332. 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 discus- 

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

 approximately made out. Vaso-constrictor fibres for the kidney 



