Vasoconstriction

Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vessels. The process is particularly important in staunching hemorrhage and acute blood loss. When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance. This makes the skin turn paler because less blood reaches the surface, reducing the radiation of heat. On a larger level, vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure.The mechanism that leads to vasoconstriction results from the increased concentration of calcium (Ca2+ ions) within vascular smooth muscle cells. However, the specific mechanisms for generating an increased intracellular concentration of calcium depends on the vasoconstrictor. Smooth muscle cells are capable of generating action potentials, but this mechanism is rarely utilized for contraction in the vasculature. Hormonal or pharmacokinetic components are more physiologically relevant. Two common stimuli for eliciting smooth muscle contraction are circulating epinephrine and activation of the sympathetic nervous system (through release of norepinephrine) that directly innervates the muscle. These compounds interact with cell surface adrenergic receptors. Such stimuli result in a signal transduction cascade that leads to increased intracellular calcium from the sarcoplasmic reticulum through IP3-mediated calcium release, as well as enhanced calcium entry across the sarcolemma through calcium channels. The rise in intracellular calcium complexes with calmodulin, which in turn activates myosin light-chain kinase. This enzyme is responsible for phosphorylating the light chain of myosin to stimulate cross-bridge cycling.Factors that trigger vasoconstriction can be of exogenous or endogenous origin. Ambient temperature is an example of the former. Cutaneous vasoconstriction will occur because of the body's exposure to the severe cold. Examples of endogenous factors include the autonomic nervous system, circulating hormones, and intrinsic mechanisms inherent to the vasculature itself (also referred to as the myogenic response).Vasoconstriction can be a contributing factor to erectile dysfunction. An increase in blood flow to the penis causes an erection.