Influence of pressure and flow on constriction of blood vessels.

Until recently the primary control of vessel diameter, or vascular resistance, has been focused on neuronal influences, circulating vasoactive hormones, and locally released substances such as endothelial derived relaxing factors (EDRF) and metabolites. It is now clear that physical forces interacting with endothelial and vascular smooth muscle cells play an important role in the control of vascular resistance. The forces are pressure which tends to stretch the cells and shear force which is a drag force on the inner surface of the vessel due to blood flow. These forces determine the sensitivity of smooth muscle to various vasoactive substances, cause the release of vasoconstrictor and vasodilator substances from endothelial cells, and alter the myogenic tone of smooth muscle.