Venules: Microcirculation Return Pathways

Venules (10–50 μm) are the smallest veins, formed by capillary convergence, returning deoxygenated blood to the venous system.

Structure & Function

• Wall: Endothelium + sparse basement membrane + few pericytes/smooth muscle cells — thinner than arterioles.
• Permeability: High — primary site for leukocyte extravasation (inflammation) and fluid exchange.
• Pressure: Low (10–15 mmHg) — capacitance vessels.

Types

• Post-capillary venules: 10–30 μm; highest permeability; inflammation site.
• Muscular venules: 30–100 μm; 1–2 smooth muscle layers; regulate capacitance.

Clinical Relevance

• Inflammation: Endothelial activation → selectins/integrins → leukocyte rolling/adhesion/transmigration.
• Capillary leak syndrome: Venular hyperpermeability → edema, hypovolemia.
• Venous insufficiency: Valvular incompetence → venous hypertension → venular dilation, skin changes, ulceration.