In vivo studies of the rheological behavior of blood in the microcirculation were conducted by direct in situ measurements in cat mesentery. Upstream to downstream pressure drops were measured in unbranched arterioles, capillaries, and venules, with diameters from 7 to 58 micrometer. Simultaneous measurements of red cell velocity and vessel geometry facilitated computation of bulk velocity, pressure gradient, apparent viscosity, wall shear stress, and resistance. Arteriovenous distributions of these parameters revealed the following. Maximum pressure gradient (0.015 cm H20/micrometer) occurs in the true capillaries (7 micrometer in diameter); intravascular wall shear stress averaged 47.1 dynes/cm2 in arterioles and 29.0 dynes/cm2 in venules. Extreme values as great as 200 dynes/cm2 were observed in a few shunting arterioles. Apparent viscosity averaged 3.59 cP in arterioles, 5.15 cP in venules, and 4.22 cP overall. Intravascular resistance per unit length of microvessel varied with luminal diameter as a power law function with exponents of -4.04 for arterioles, -3.94 for venules, and -3.99 for all vessels combined. This apparent maintenance of Poiseuille's law is attributed to the opposing processes of hematocrit reduction and decreasing shear rate as blood is dispersed in successive arteriolar segments, and the converse action of these processes in the venous confluences which lessen the extent of network variations in apparent viscosity. Reductions in bulk velocity from the normal flow state to below 0.5 mm/sec resulted in increases in apparent viscosity by a factor of 2 to 10, which are attributed primarily to obstruction of the lumen by leukocyte-endothelium adhesion.

SUMMARY Direct intravital microscopy was used to analyze microclrculatory changes in the exteriorized spinotrapezius muscle of spontaneously hypertensive rats (SHR). The animals were anesthetized with a mixture of chloralose-urethane, and measurements made of pressure, flow, and resistance in vessels ranging in size from 50 to 5 Mm. Tbe vascular changes in SHR were compared with matched Wistar-Kyoto (WKY) strain animals for both young (5-6 weeks old) and mature (12-13 weeks) rats. Distinctive changes in tbe distribution and levels of pressure, flow, and resistance were seen in tbe entire mlcrovascular network during both stages of the syndrome. There was no significant increase in the resistance of the conduit arteries just proximal to tbe muscle proper. Blood pressure in hypertensives was brought down to normal and even below normal at the level of the capillaries and postcapiiiaries irrespective of tbe height of the pressure in the major artery supplying the muscle. The greater drop in pressure across the arteriolar branchings of tbe hypertensives was seen as early as at 5-6 weeks of age; this difference Is much more striking in 12-13 week-old mature hypertensives. The reduction in pressure was proportionately greatest in the region of the smallest (10-15 /im) precapillaries of hypertensives. Resistance values were below normal in the confluent capillaries and postcapiiiaries in both young and mature hypertensives. Volumetric flow, which was marginally higher throughout the arteriolar branchings, fell below normal on the postcapillary side. Since an increased resistance developed at an early age (5-6 weeks) in all of the microvessels on the precapillary side, tbe suggestion is advanced that hypertension is associated with a generalized effect on tbe muscular arterioles below 30 urn, an effect that becomes more pronounced with time and at 12-13 weeks begins to involve larger sized arterioles (

No abstract