Harvey N. Mayrovitz

Defining the precapillary sphincter

Abstract Past and present use and abuse of the term precapillary sphincter is reviewed. The advantages of a uniform and functional definition are described, and a definition that is consistent with current information is proposed.

Microvascular hemodynamic variations accompanying microvessel dimensional changes.

Abstract The bat wing is used as an experimental preparation and as a self-contained vascular bed. The number, dimensions, and distribution of the vessels of the real vascular bed are included into an analyzable, representative geometric configuration. Based on theoretical analysis and experimental data, equations are developed and utilized to characterize the pressure-flow relationships for each branching order of the vascular field. The geometric configuration and associated describing equations are used to determine the effects of microvessel diameter change on the distribution of resistance, pressure, and velocity throughout the microvasculature. Predicted hemodynamic changes are compared with available experimental data and evaluated in the light of current concepts concerning the significance of microvessel dimensional changes.

Analytical characterization of microvascular resistance distribution

The bat wing is used as an experimental preparation and as a self-contained vascular bed. The number, dimensions, and distribution of the vessels of the real vascular bed are included into an analyzable, representative geometric configuration. Based on theoretical analysis and experimental data, equations are developed and utilized to characterize the pressure-flow relationships for each branching order of the vascular field. The geometric configuration and associated describing equations are used to determine the resistance distribution of the microvascular bed. The predicted values are compared with experimentally determined quantities in normal and hypertensive animals.

Effects of vasoactive drugs on platelet aggregation in vivo and in vitro.

Abstract Based on the generalization that most vasoconstrictor agents promote platelet aggregation while most vasodilator agents inhibit it, experiments were designed to test this premise in the living animal. Platelet aggregates, induced by a laser beam in arterial vessels of the hamster cheek pouch and bat wing, were observed microscopically to determine changes in growth, embolization, and vessel wall adherence in the presence of epinephrine, norepinephrine, phenylephrine, vasopressin, serotonin, isoproterenol, dipyridamole and sodium nitroprusside. Concentrations of the drugs too weak to alter microvessel diameters were used. In vitro responses were determined by inspection of a platelet-rich plasma-drug mixture in a hemocytometer chamber. It was established that drugs which enhanced platelet aggregation in vivo also produced aggregates in platelet-rich plasma from the same animal except for epinephrine and norepinephrine where the results in vitro were not definitive. Phenylephrine and isoproterenol produced changes contrary to expectation. Phenylephrine, an alpha adrenergic stimulator, caused a reduction in the duration of activity of platelets at the site of a laser-induced aggregate. When mixed with platelet-rich plasma, no platelet aggregates were produced. Isoproterenol, a beta adrenergic stimulator, enhanced the duration of activity of the platelet aggregate. When mixed with PRP, small, evenly dispersed aggregates were seen in the hemocytometer chamber.

Red Cell Interactions with the Microcirculation

We have measured RBC velocity profiles for mammalian arterioles and venules from high-speed cinematographic motion pictures. Measurements were made at 320× and 400× optical magnification over an averaging time period of 10 ms. In vivo profiles are uniformly nonsymmetrical, the RBCs exhibit rotation, and they frequently deviate sidewise from the overall axial direction of motion. In general, this is more pronounced on the venous side. Since all of the profiles are time variant and the average values are synchronous with the midstream velocity, individual RBC velocities will vary about the average. Profiles become more blunted in vessels with smaller diameters. In vessels below 16 μm diameter, the velocity gradients between adjacent RBCs are quite small; for large vessels, recognizable profiles develop and become fully developed in blood vessels above 30 μm in diameter. This blunting is further affected by the midstream velocity and the local hematocrit; when the velocity is reduced below 1.2 mmls and/or an increased hematocrit is present, the profile becomes more blunted.

Blood Flow in Skeletal Muscle

It is well known that during their course from arteriole to venule the capillaries in skeletal muscle branch and anastomose repeatedly (Krogh, 1919; Hammersen, 1968). There appears, however, to have been little previous attempt to quantitate the extent and pattern of capillary branching, although Eriksson and Myrhage (1972) reported that the capillaries of the tenuissimus muscle of the cat were 1.015 mm long, with branches to adjacent capillaries every 0.20 mm along their length. We have studied the branching pattern of these interconnections in the sartorius muscle of the frog, vasodilated by injection of papaverine and perfused with a silicone elastomer (Microfil). Each terminal arteriole gave rise to an average of 2.71 ± 1.34 (SD) capillaries. Each capillary had a mean of 3.44 ± 1.84 (SD) interior branch points. More than 90% of capillary branchings were simple divergent or convergent bifurcations, the included angle having an approximate mean value of 60° in each case. The initial branchings were mainly divergent, but the proportion of convergences increased linearly at successive branchings along the path. Some branches anastomosed with a capillary path to which the vessel had been connected previously. The incidence of such “rejoins” rose to 35% by the third and fourth branches, maximizing interaction between adjacent capillaries and maintaining each group of vessels as a fairly distinct microvascular unit. From these numerical data, plus measurements of path and segment lengths, realistic models of the capillary network have been synthesized. The fact that the anastomoses are so extensive suggests that they may play a key role in maintaining oxygenation of muscle tissue.