Venules
Venulae
Also known as: venule, postcapillary venules, high endothelial venules
Venules are small veins, about 10–100 µm across, that collect blood from capillaries. The smallest of them, the postcapillary venules, are a key site of fluid exchange and the main place where leukocytes leave the blood during inflammation.
Overview
Postcapillary venules have an endothelium and pericytes but no true media. Their cell-to-cell junctions are the loosest in the circulation. Inflammatory mediators such as histamine, bradykinin and leukotrienes make the endothelial cells contract, opening gaps between them. Plasma and proteins leak out, which causes inflammatory swelling. This is also where leukocytes roll along the wall on selectins, stick through integrins and squeeze through into the tissue (diapedesis).
Muscular venules, wider than about 50 µm, gain one or two layers of smooth muscle and join to form small veins.
High endothelial venules are specialised venules in lymph nodes and Peyer's patches. Their tall, cuboidal endothelial cells allow lymphocytes to leave the blood and enter the lymphoid tissue.
Anatomy
- Histology
Endothelium and pericytes, with smooth muscle appearing in the larger (muscular) venules. A thin adventitia. The lumen is wider and the wall thinner than in an arteriole of the same size.
Function
- Collect blood from capillary beds.
- Act as the main site of plasma leakage and leukocyte emigration in inflammation.
- Contribute to venous capacitance.
Clinical significance
Inflammatory oedema and urticaria
When histamine opens gaps between endothelial cells in postcapillary venules, fluid leaks into the skin. This produces the wheal of urticaria and the swelling of angio-oedema. The triple response of Lewis (a red line, a surrounding flare and a wheal) shows the same process after the skin is stroked firmly.
Leukocytoclastic vasculitis
Small-vessel vasculitides, such as IgA vasculitis, target the postcapillary venules. They produce palpable purpura.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.