venules

Venules are small blood vessels that drain capillary beds and transport blood from the microcirculation into larger venous channels that return blood toward the heart. They form the transition between the thin-walled capillaries where gas and fluid exchange occur and the veins that carry blood back to the heart. The walls of venules are relatively thin and are primarily composed of endothelial cells with a basement membrane; smooth muscle is sparse in the smallest venules and more apparent in larger, muscular venules.

There are two main categories of venules: postcapillary venules and muscular venules. Postcapillary venules immediately drain capillaries and are especially important in inflammatory responses. Their endothelium supports leukocyte rolling and diapedesis, allowing white blood cells to exit the bloodstream and enter tissues. Muscular venules have a somewhat thicker wall with a modest smooth muscle layer, providing greater control over venous tone and blood flow.

Functionally, venules contribute to fluid balance and microvascular resistance. They participate in the exchange of fluid and solutes with the interstitial space and respond to vasoactive mediators that can increase permeability or alter tone. During inflammation, increased venular permeability can lead to edema as plasma proteins and fluid accumulate in tissues.

Clinically, venular function and permeability are relevant in conditions such as inflammatory responses and sepsis, where microvascular leakage can contribute to tissue edema and organ dysfunction. In histology and imaging, venules are identified as small, thin-walled vessels that bridge capillaries and veins and play a key role in immune cell trafficking and microcirculatory regulation.