Venule

Overview
A venule is a small blood vessel that allows deoxygenated blood to return from the capillary beds to the larger blood vessels called veins.

Structure
Venule walls have three layers: 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.