Venule

Venules are small blood vessels that receive blood from capillaries and transport it toward larger veins. They are a key part of the microcirculation, forming the link between the capillary bed and the venous system. In many tissues, postcapillary venules are the first venous segment after capillaries and are the major site of leukocyte trafficking and fluid exchange.

Structure and classification: Venules have thin walls composed mainly of endothelial cells, with a sparse surrounding supporting tissue. In postcapillary venules, walls are particularly simple and lack substantial smooth muscle, while muscular venules may contain a few smooth muscle cells that help regulate venous outflow. Pericytes may be present along the vessel wall in some tissues. Venules are typically grouped into postcapillary venules and collecting (muscular) venules, which transition toward small veins.

Function: The primary role of venules is to drain capillary beds and begin the return of blood to the heart. They contribute to tissue fluid homeostasis through the movement of fluid and proteins, and postcapillary venules are important sites for leukocyte extravasation during inflammation, aided by endothelial adhesion molecules and chemokines.

Clinical relevance: Venular dysfunction can accompany inflammatory states, sepsis, and chronic vascular diseases. Increased venular permeability can contribute to edema, and microvascular inflammation involving venules can affect tissue perfusion and healing. While larger veins are often implicated in venous insufficiency, the venular segment plays a critical role in early edema formation and immune cell trafficking.