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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.
Vasodilation is the opposite of vasoconstriction, which is the narrowing of blood vessels. When blood vessels dilate , the flow of blood is increased due to a decrease in vascular resistance and increase in cardiac output [ further explanation needed ] .
Hypoxic pulmonary vasoconstriction (HPV), also known as the Euler–Liljestrand mechanism, is a physiological phenomenon in which small pulmonary arteries constrict in the presence of alveolar hypoxia (low oxygen levels).
Vascular resistance is the resistance that must be overcome for blood to flow through the circulatory system.The resistance offered by the systemic circulation is known as the systemic vascular resistance or may sometimes be called by another term total peripheral resistance, while the resistance caused by the pulmonary circulation is known as the pulmonary vascular resistance.
Excessive vasoconstriction leads to high blood pressure, while excessive vasodilation as in shock leads to low blood pressure. Vascular smooth muscle cells also play important roles during development, e.g. driving osteocyte differentiation from undifferentiated precursors during osteogenesis. [1]
Vasodilation and vasoconstriction are complex phenomena; they are functions not merely of the fluid mechanics of pressure and tissue elasticity but also of active homeostatic regulation with hormones and cell signaling, in which the body produces endogenous vasodilators and vasoconstrictors to modify its vessels' compliance.
The pathophysiology of pulmonary heart disease (cor pulmonale) has always indicated that an increase in right ventricular afterload causes RV failure (pulmonary vasoconstriction, anatomic disruption/pulmonary vascular bed and increased blood viscosity are usually involved [1]), however most of the time, the right ventricle adjusts to an overload in chronic pressure.
The hunting reaction or hunting response is a process of alternating vasoconstriction and vasodilation in extremities exposed to cold. The term Lewis reaction is used too, named after Thomas Lewis, who first described the effect in 1930. [1] Vasoconstriction occurs first to reduce heat loss, but also results in strong cooling of the extremities.