Venules

Venules are tiny veins that collect oxygen-poor blood from the capillary beds.

Their diameter may range between 8 and 100 μm. The postcapillary venules, present next to the capillary bed, are the smallest in the human body.

A typical venule is characterized by an endothelium, a sparse tunica media made of smooth muscle cells, and a thin tunica externa.

Venules are very porous, making them highly permeable, facilitating efficient nutrient and waste exchange, as well as, white blood cell migration.

However, the venules situated away from the capillary beds acquire one or two layers of smooth muscle cells, turning them into larger muscular venules spanning 50 to 200 μm in diameter.

These thick-walled muscular venules restrict the exchange of nutrients and waste between the blood and the surrounding cells.

Venules are among the most expandable components of the vascular system, serving as effective blood reservoirs.