LOCAL CONTROL OF VENOUS FUNCTION

Publisher Summary Alterations in venous diameter affect hemodynamics both at the local and the general level. Locally, the degree of opening of the veins is one of the factors determining the ratio between pre- and post-capillary resistance and, thus, the movements of fluid to the tissues. From the general hemodynamic point of view, changes in venous diameter help adjust the total capacity of the cardiovascular system and, thus, the return of blood to the right heart; in the skin, they contribute mainly to thermoregulatory adjustments. The diameter of each vein is at each moment the resultant of the distending pressure within it (passive changes) and of the degree of activity of the smooth muscle cells in its wall (active changes). Venous smooth muscle cells possess the morphological characteristics of contractile tissues; they contain sarcoplasmic reticulum, mitochrondria, and contractile proteins. The trigger for contraction is the change in membrane permeability that is associated with changes in membrane potential.