venula

Venula, plural venulae, refers to a very small vein that drains blood from capillary beds and feeds into small collecting veins. In the venous system, venulae form part of the microcirculation along with capillaries and arterioles, providing a transition from the circulation’s exchange vessels to larger venous channels.

Structure and variation: Venules are thin-walled vessels composed mainly of a single layer of endothelium with a basal lamina, often accompanied by a sparse layer of smooth muscle cells or pericytes. They are commonly divided into postcapillary venules, muscular venules, and collecting venules, with increasing smooth muscle investment as they join larger veins. Their small size and flexible walls facilitate the collection of blood from capillary networks.

Function: The primary role of venulae is to drain deoxygenated blood from capillary beds and channel it into small veins. They contribute to fluid exchange between blood and interstitial tissue and serve as a key site for immune cell trafficking. Postcapillary venules, in particular, are important in inflammation where endothelial adhesion molecules enable leukocyte rolling, adhesion, and diapedesis.

Clinical and physiological relevance: Venular permeability can increase in inflammatory states, contributing to edema. Abnormal venular structure or function can be involved in various vascular or inflammatory disorders. Understanding venulae helps explain the early venous drainage patterns from tissues and their role in immune responses and fluid balance.

Etymology: The term venula is Latin for “little vein.”