Venule

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Venules refer to the smallest veins that drain blood from capillaries within the microvascular bed, channeling it into larger veins. There are three primary subtypes of venules, which can be classified in order of increasing diameter as follows: postcapillary venules, collecting venules and muscular venules.

Postcapillary venules are formed when two or more capillaries unite. They typically range from 10–30 μm in diameter. Their walls, comprising mainly endothelium and occasional pericytes, enable the exchange of fluid, nutrients and other substances between blood and the interstitial fluid. These vessels are the primary sites of white blood cell (leukocyte) migration into interstitial fluid of surrounding tissues, particularly in the setting of infection or tissue injury. 

Collecting venules are larger venules that drain postcapillary venules. These share structural similarities with postcapillary venules, having thin walls composed of endothelium with more numerous pericytes, and a few scattered smooth muscle cells. Collecting venules feed into progressively larger vessels which, upon gaining a muscular layer, become muscular venules.

Muscular venules typically have a caliber of about 50–100 μm and possess a three-layered wall. The inner layer (tunica intima) comprises endothelium. The middle layer (tunica media) consists of two or three smooth muscle layers which aid in regulating blood flow. The final layer, the outer tunica adventitia is composed of connective tissue and some elastic fibers. Pericytes are typically absent in muscular venules.

At greater than 100 μm, venules become small veins which then drain into larger veins. These ultimately return deoxygenated blood back to the heart via the superior vena cava and inferior vena cava, which are the largest veins in the body.