An increase in the internal diameter of blood vessels, especially arterioles or capillaries, due to relaxation of smooth muscle cells that line the vessels, and usually resulting in a decrease in blood pressure.

Vasodilation is where blood vessels widen.[1] It results from relaxation of muscle cells within blood vessel walls. The process is essentially the opposite of vasoconstriction, which is the narrowing of blood vessels.

Diagram of blood vessels.

When vessels widen, blood flow is increased. This in turn decreases blood pressure. Drugs that cause vasodilation are called vasodilators.


  1. "Definition of Vasodilation". 27 April 2011. Retrieved 13 January 2012.