Arteriole

The arteriole is a small branch of an artery that forms part of the microcirculation. Arteries divide into progressively smaller vessels and eventually become arterioles. These arterioles give rise to metarterioles, which act as transitional channels between arterioles and capillaries. In larger arterioles, the vessel wall still contains the three typical layers found in arteries, but these layers become thinner as arterioles approach the capillary network.

The smallest arterioles consist mainly of an endothelial lining surrounded by a few smooth muscle fibers and a minimal amount of connective tissue. Both arterioles and metarterioles are specially adapted for vasoconstriction and vasodilation: their smooth muscle responds to autonomic nervous system impulses by contracting or relaxing. By changing their diameter, they play a critical role in regulating blood flow into the capillary beds.