Three-dimensional view of an arteriole (top left) on the left and a longitudinal section of the arteriole with two branching capillaries (top right) are shown. A cross section of the arteriole appears below them.

The arteriole has an intima composed of an endothelium sitting on a thin layer of connective tissue in which collagen fibrils (not shown) are associated with branching elastic fibres oriented longitudinally. Note that these elastic fibres do not form the internal elastic lamella that characterizes the arteries.

The media of arterioles is composed of one or two layers of smooth muscle cells arranged circularly. At the sites where the arteriole branches into capillaries, these smooth muscle cells form pre-capillary sphincters that regulate the blood flow in the capillary networks.

The thin adventitia is composed of connective tissue fibres or fibrils and of fibrocytes.

N.B.: The thin basement membrane under the endothelium and surrounding the smooth muscle cells is not shown on these drawings.