Quantitative Studies of Microcirculatory Structure and Function: I. Analysis of Pressure Distribution in the Terminal Vascular Bed in Cat Mesentery

A modified version of the Wiederhielm electronic servo system for micropressure recordings was used to establish the details of the pressure drop across successive segments of the microcirculation from arterioles (150μ) to venules (200μ) in the mesentery of the cat; most of the readings were made in vessels less than 25jz in diameter. Micropressures correlated with central pressure only in arterial vessels larger than 50μ. Frequencydistribution plots of micropressures showed a wider spread in the vessels that were 50–1Oμ in diameter than they did in capillary vessels that were less than 20μ in diameter. Therefore, it appears that classification of vessels on the basis of diameter, which is at present the only practical way of comparing pressure measurements in presumably identical segments of a continuously branching system, is not adequate and makes statistical analysis difficult. In the present study, micropressures remained pulsatile throughout the bed, although the pulse pressure could be narrowed to 2–4 mm Hg in the capillaries. Simultaneous measurement of pressure in two vessels showed that the greatest pressure drop occurred in the region where arterioles (40-50)u) distributed numerous precapillary branches (10–15/x); in this region arteriolar pressures were reduced from a level of 45–65 mm Hg to about 30–35 mm Hg. These data suggest that application of the resistance term for Poiseuille flow in the microcirculation should take into account the relative sizes of the parent vessel and the branch, the entry dimensions into the branch and the length of the neck or junctional segment. Evidence found in this investigation indicated that precapillary branching configurations were self-regulatory. A small number of low-resistance, shunt pathways between arterioles and venules were regularly present, as shown by the extremely small pressure drop in 15–18% of the vessels. The data indicate that much more caution is needed in interpretation of pressure-related phenomena in the microcirculation because of their strong dependence on the anatomical and physical features characteristic of individual tissues.