The blood vessels are part of the circulatory system and function to transport blood throughout the body. The major types of blood vessels are:

• arteries: smaller arteries are called arterioles
• capillaries
• veins: smaller veins are called venules

• Vasoconstriction is when blood vessels in the body constrict (grow smaller in cross-sectional area) by contracting the smooth muscle in the vessel walls.
  • A vasoconstrictor is any drug that acts to constrict blood vessels. Many vasoconstrictors act on specific receptors, such as vasopressin receptors or adrenoreceptors.

• Vasodilation is when blood vessels in the body dilate (grow wider) by relaxing the smooth muscle in the vessel walls.
  • Vasodilators