Quantitative studies of microcirculatory structure and function. II. Direct measurement of capillary pressure in splanchnic mesenteric vessels.

A systematic study was made of capillary pressures in the microvascular bed of the mesentery of the cat. Simultaneous recordings were made with two micropipettes using a modification of the Wiederhielm servo-null system. All of the measurements were then referred to their specific location in the capillary bed by photographically reconstructing discrete networks. Pressures in different capillary branches within a given network ranged from 40 mm Hg to as low as 24 mm Hg. There was no obvious pattern in the distribution of high-pressure vessels relative to low-pressure vessels. When capillary pressure was measured in a selected vessel and averaged over a period of several minutes, it remained constant (±3 mm Hg), despite much greater fluctuations in the precapillary and postcapillary vessels. A number of different procedures was used to obtain average values for capillary pressure in a given network. Estimates of capillary pressure from venous wedge pressures in the mesentery were related to the actual capillary pressure, but the precise proportionality of the two pressures was not the same in different preparations. Possibly, the estimates of average capillary pressure based on direct measurements were higher than those obtained indirectly from calculations of fluid exchange because other factors affecting the latter were not adequately considered.