Large arterioles in the control of blood flow: role of endothelium-dependent dilation

Although it is generally assumed that small arterioles form the major site of vascular resistance, microcirculatory studies revealed that 40-55% of th e total network resistance can reside in large arterioles and small arterie s. Thus, the mechanisms that control smooth muscle tone in these vessels ha ve a major impact on the overall conductance of the vascular network. These control mechanisms are different from those in small arterioles: Aside fro m an apparently reduced sensitivity to metabolites, the large resistance ve ssels are normally too far away from the capillary areas which they feed to be reached by diffusing metabolites from dependent cells within a reasonab le period of time. Rather, recent intravital microscopic studies suggest th at large resistance vessels are under tight control of endothelial factors such as nitric oxide and endothelium-derived hyperpolarising factor (EDHF). Nitric oxide opposes myogenic constrictions of large arterioles that poten tially would impair tissue perfusion and oxygenation. Moreover, nitric oxid e and EDHF play an important role in the co-ordination of large and small r esistance vessel behaviour that is pivotal for the adaptation of blood flow to altered tissue oxygen demands.