is a small blood vessel
that allows deoxygenated blood
to return from the capillary
beds to the larger blood vessels called veins
Venules are blood vessels that drain blood directly from the capillary beds. Many venules unite to form a vein.
Venule walls have one layer: An inner endothelium
composed of squamous endothelial cells
that act as a membrane
, a middle layer of muscle and elastic tissue and an outer layer of fibrous connective tissue
. The middle layer is poorly developed so that venules have thinner walls than arterioles
. These vessels are pink sometimes due to the high content of nitrogen.
In contrast to regular venules, high endothelial venules are a special type of venule where the endothelium is made up of simple cuboidal cells. Lymphocytes re-enter the bloodstream through the HEVs.