Pressure and flow-dependent tone in resistance arteries. Role of myogenic tone.

Pre-capillary arteries are often called resistance arteries as they are responsible for a large proportion of peripheral vascular resistance. Resistance arteries possess a permanent vasoconstrictor tone providing the capacity to further contract or dilate. This property allows an efficient control of local blood flow, and this tone is called the "vasodilation reserve". Physiologically, this basal tone is also the functional limit between resistance and large compliance arteries. Microvascular tone is controlled by the metabolic needs of the surrounding tissue, the nervous system, local and circulating hormonal systems and mechanical factors. Pressure and flow are the main mechanical factors influencing microvascular tone. Increases in blood pressure induce a contraction called myogenic tone whereas increases in blood flow induce a rise in shear stress triggering dilation (flow-mediated dilation). Myogenic tone can be considered as a background tone preparing the contractile apparatus to contract more if agonists are produced, or less if the endothelium generates vasodilators. Such a basal tone allows a rapid and efficient control of vascular tone and local blood flow. The role of myogenic tone in pathological situations is not yet clearly known. Nevertheless, in hypertension or diabetes a blunted myogenic responsiveness in renal pre-glomerular arterioles may account for the pressure-induced damages in glomeruli. On the other hand, an increased myogenic tone may participate in the occurrence of vasospasm.