The Microvascular Network Pattern and Cell Morphology
The microvascular network topology in hypertensive and normotensive animals is overall the same but differs in quantitative terms. For example, in skeletal muscle the microvascular branching pattern formed by feed arterioles. by arcade arterioles, and by their regular side branches, the terminal (previously designated also as transverse) arteri-oles, is the same. The terminal arterioles form asymmetric dichotomous trees, which give rise to the capillary network. The SHR has a higher density of arcade arterioles with smaller trees forming the terminal arterioles. The capillaries form bundles with a modular pattern of alternating terminal arterioles and collecting venules, which in turn feed into the arcade venules and discharge into the central circulation through the draining veins. Compared to WKY rats, the SHR exhibits on average a lower capillary network density although individual capillaries have on average greater length (between bifurcations) and diameter. Apart from the fact that collecting venules of SHR are narrower while arcade venules are wider in lumen diameter than in the WKY rats, the two strains exhibit no differences in venular network topology.
The innervation of microvessels in skeletal muscle extends to the terminal arterioles in form of adrenergic
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fibers. The nerve fibers are positioned at the interface between smooth muscle media and adventitia down to the endings of the terminal arterioles. The density of adrenergic fibers in the SHR is significantly higher compared to that of the WYK rats. Capillaries or venules have no adrenergic innervation.
The walls of capillaries in hypertensives and normoten-sives consist of endothelial cells with pericytes. Venules have a thinner wall structure than their arteriolar counterpart, with attenuated endothelial thickness, pericytes and smooth muscle cells in the media, and fibroblast in the adventitia. Ultrastructural examination of adult capillaries and venules in hypertensives often reveals morphological damage not found in normotensives, e.g. in form of membrane bleb formation.
The elevated blood pressure in arteries of hypertensives is reduced in arterioles and in terminal arterioles to values which are similar to those in normotensive animals (Figure 1). Apart from the fact that the pressure drop on the venular side is small in both normotensives and hypertensives, there are no significant differences in blood pressure values in venules.
Both the cardiac output and the average local flow rates in different hierarchies of microvessels in hypertensive and normotensive microvascular networks are almost indistinguishable (Zweifach et al., 1981). However, within each microvessel hierarchy the hypertensives have larger variations of flow rates among individual vessels.
Estimates of the average hemodynamic resistance derived from micro-pressure and flow measurements indicate a higher resistance in arcade and terminal arterioles of the hypertensives without such significant differences in the venular counterparts (Boegehold, 1991).
The control of the hemodynamic resistance involves smooth muscle contraction and restructuring of the arteri-oles. In addition, also blood rheological mechanisms serve to control the hemodynamic resistance in capillaries and venules. In spite of the relatively small number of circulating leukocytes compared to significantly faster moving ery-throcytes, in capillaries with single file of blood cells the larger and stiffer leukocytes have an important influence on apparent viscosity and capillary resistance. The mechanism is due to hydrodynamic interaction of slower moving leukocytes with more flexible erythrocytes, which in capillaries displaces the erythrocytes away from their center-line position and leads to an elevated apparent viscosity. The effect is sensitive with respect to the exact erythrocyte and leuko-
Figure 1 Micropressure (top) and flow rate (bottom) and flow grouped according to vessel diameter in mature rat spinotrapezius muscle, according to Zweifach et al., 1981, in matched WKY and SHR. Capillaries are in the diameter range of about 5 mm on the abscissa. To the left are the diameters of the arterioles and to the right the diameters of the venules. Means and standard deviations are shown. While there is an elevated blood pressure in arterioles of the SHR, their capillaries and venules have no elevation of the blood pressure. Compared with the WKY rats, the SHR exhibit no differences in average flow rates.
cyte counts, and does not require membrane attachment to the endothelium (Helmke et al., 1997).
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