Any number of body functions, substances, and medical conditions can cause the muscle walls of the blood vessels to narrow and constrict. This constriction is known as vasoconstriction.

Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular walls of the vessels, particularly the large arteries, small arterioles and veins.  The process is particularly important in staunching hemorrhage and acute blood loss.

Vasoconstriction is triggered by stimulation of vasomotor constriction center in the medulla. Impulses from this center travel along sympathetic nerve fibers and cause contraction of the smooth muscle layers of arteries, arterioles, and to a lesser extent venules and veins bringing about constriction of these vessels.

Vasoconstriction is also induced by vasomotor pressure reflexes, chemicals reflex, the medullary is chemic reflex, and impulses from the cerebral cortex and the hypothalamus.

When blood vessels constrict, the flow of blood is restricted or decreased, thus retaining body heat or increasing vascular resistance.
Vasoconstriction is one mechanism by which the body regulates and maintains mean arterial pressure.

Substances causing vasoconstriction are called vasoconstriction are called vasoconstrictors. Generalized vasoconstriction usually results in an increase in systemic blood pressure, but it may also occur in specific tissues causing a localized reduction in blood flow.

The extent of vasoconstriction may be slight or severe depending on the substance or circumstance.

The mechanism that leads to vasoconstriction results from the increased concentration of calcium within vascular smooth muscle cells. However, the specific mechanism for generating an increased intracellular concentration of calcium depends on the vasoconstrictor.

Two common stimuli for eliciting smooth muscle contraction are circulating epinephrine and activation of the sympathetic nervous system that directly innervates the muscle.

Causes of vasoconstriction can vary. Both internal forces as well as exterior variables may induce the response. Vasoconstrictors are different chemical compounds that can trigger the condition. These can be in the form of medications, such as ADHD treatment stimulants, amphetamines, caffeine, antihistamines, and decongestants.

Vasoconstriction can also occur if a person is exposed to severe cold.

Different nervous system process, hormone circulation, and other body system function may also encourage the blood vessels to compress.
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