402 RELATIONS TO BLOOD-PRESSURE. [BOOK n. 



concerned, the chief circumstance of the vascular condition of the 

 kidney which we have to consider is the extent of pressure present 

 in the small vessels of the renal glomeruli. The more the pressure 

 of the blood in these exceeds the pressure of the fluid in the 

 channels of the uriniferous tubules, the more rapid and extensive 

 will be the nitration from the one into the other. 



This local blood-pressure in the small vessels of the glomeruli 

 may be increased 



1. By an increase of the general blood-pressure, brought about 

 (a) by an increased force, frequency, &c. of the heart's beat, 

 (6) by the constriction of the small arteries supplying areas other 

 than the kidney itself. 



2. By a relaxation of the renal artery, which, as we have pre- 

 viously pointed out (p. 216), while diminishing the pressure in the 

 artery itself, increases the pressure in the capillaries and small 

 veins which the artery supplies. It need hardly be added that 

 this local relaxation must either be accompanied by constriction in 

 other vascular areas, or at all events must not be accompanied by a 

 sufficiently compensating dilation elsewhere. 



The same local pressure may similarly be diminished 



1. By a constriction of the renal artery and its branches, which, 

 while increasing the pressure on the cardiac side of the artery, 

 diminishes the pressure in the capillaries and veins which are 

 supplied by the artery. This again must either be accompanied by 

 dilation in other vascular areas, or at least not accompanied by a 

 compensating constriction. 



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 kidneys. 



Bearing these facts in mind, it becomes apparently easy to 

 explain many of the instances in which an increase or diminution 

 of urine is produced by natural or artificial means. Thus section 

 of the spinal cord below the medulla causes a great diminution, and 

 indeed in many cases a complete or almost complete arrest, of the 

 secretion of urine. This operation, as we have seen in discussing 

 the vaso-motor system, leads to a very general vascular dilation, in 

 consequence of which there ensues a great fall of the general blood- 

 pressure. At present it seems uncertain whether the renal arteries 

 really possess a normal tone like that of most other arteries ; and 

 we do not know whether they in consequence of the operation 

 share in the general dilation. Even if they do, their expansion 

 apparently is insufficient to compensate the great diminution of 

 general blood-pressure. It has been stated that the effect of section 

 of the medulla is so marked and constant that when, in the dog, the 

 blood-pressure sinks at least below 30 mm. mercury the secretion of 

 urine is invariably arrested. It would appear however that this is 

 not always the case, and that secretion is sometimes observed to 



