Post-capillary venules have a single layer of endothelium surrounded by a basal lamina.

[1] They have 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.

They are porous so that fluid and blood cells can move easily from the bloodstream through their walls.

Lymphocytes exit the blood stream and enter the lymph nodes via these specialized venules when an infection is detected.