Vasodilators

n. something which causes the dilation of blood vessels

A vasodilator is a drug or chemical that relaxes the smooth muscle in blood vessels, which causes them to dilate. Dilation of arterial blood vessels (mainly arterioles) lead to a decrease in blood pressure.
See more at Wikipedia.org...

Vasodilators are medicines that act directly on muscles in blood vessel walls to make blood vessels widen. [more]Vasodilators - Community and Resources

Noun
1. a drug that causes dilation of blood vessels
(synonym) vasodilative
(hypernym) dilator
(hyponym) amlodipine besylate, Norvasc

Agents that act as blood vessel dilators (vasodilators) and open vessels by relaxing their muscular walls. For example, nitroglycerin is a vasodilator. So are the ACE (angiotensin converting enzyme) inhibitors.