1912.] Relation between Secretory and Capillary Pressure. 317 



the same. In the case of the brain, the cerebro-spinal pressure, the cerebro- 

 capillary pressure, and the cerebro-venous pressure are one and the same, 

 a fundamental fact which has been proved.* In the secreting glands, 

 confined by a capsule, the same conditions must hold good. The capillary, 

 venous, and tissue fluid pressures must be the same, adjusted one to the 

 other, excepting where, as in the salivary gland, membrame proprise are 

 drawn taut by the height of the secretory pressure when the duct is 

 obstructed. In the case of the salivary gland, the capacity to produce so 

 high a pressure may be correlated with the need which arises, under certain 

 circumstances, for a very rapid secretion of saliva, e.g. to wash out an 

 irritant. The same high secretory pressure is produced by the sweat glands. 

 A rapid secretion of sweat may be required to prevent heatstroke. Here, 

 again, the membranse propriae play the same part as in the salivary glands, 

 the secretory force possibly being increased by the layer outside the secreting 

 cells, which is supposed to be contractile. Both the flow of sweat and of 

 saliva may be temporarily obstructed by pressure applied to the ducts. 



In the case of the kidneys a lower secretory pressure suffices. The pelvis 

 and the bladder act as reservoirs, the abdominal wall and the diaphragm 

 regulate the intra-abdominal pressure. The whole kidney, just as the 

 salivary gland, is confined by a capsule which allows a certain expansion 

 and no more. Within the capsule, from time to time, there may be more 

 blood and less kidney substance (including tissue, fluid and secretion), or 

 more kidney substance and less blood. Suppose the arterioles of the 

 kidney dilate, the volume of the blood increases and the whole kidney 

 expands. Suppose, moreover, the renal tubules are inactive, not swollen, 

 and the membrane propriee are slackened, then there will be within the 

 capsule less kidney substance and more blood. The whole kidney substance 

 will be at the lowest fluid pressure, namely, that of the veins. The same 

 holds good for the salivary gland. Suppose, on the other hand, the tubules 

 are swollen, actively secreting, with the membranae propria? expanded, there 

 will be less room within the capsule for blood, the veins will be narrowed, 

 the renal venous-capillary pressure raised, the vascular system approximated 

 to a rigid system, the velocity of the flow increased. The whole kidney will 

 still be at the lowest fluid pressure, viz. that in the veins, but now it 

 will feel tense, for the venous-capillary pressure, under these conditions, is so 

 much higher. It is clear that the salivary or renal cells, swollen and 

 in an active state of secretion, not only produce a secretory pressure 

 in the tubules, but by narrowing the veins influence the height of the 

 blood pressure within and the rate of blood flow through the capillaries and 

 * Leonard Hill, ' The Cerebral Circulation,' 1896. J. and A. Churchill, London. 



