Postkapilláris

Postkapilláris, in physiology and anatomy, refers to the postcapillary venules—the small veins that drain blood from the capillary networks in tissues. These vessels are a key segment of the microcirculation, lying immediately downstream of capillaries and upstream of collecting venules and larger veins.

Anatomy and histology: Postcapillary venules have diameters around 10–50 μm. Their walls consist of a single endothelium with a basal lamina; pericytes may be present. They lack a strong smooth-muscle layer, contributing to high compliance and permeability, and they drain capillary beds.

Physiology: Compared with arterioles, postcapillary venules experience relatively low shear and slower flow, facilitating immune surveillance and exchange. Their endothelium is relatively permeable, allowing diffusion of fluids and proteins. In response to inflammatory mediators, endothelial cells upregulate adhesion molecules and create openings that permit leukocyte extravasation.

Role in inflammation: Postkapilláris are the main site of leukocyte margination and diapedesis. Neutrophils and monocytes adhere to and migrate through the endothelium via selectins and integrins in response to chemokines. This transmigration enables immune cells to reach sites of tissue injury or infection.

Clinical relevance: Increased vascular permeability in postcapillary venules contributes to tissue edema during inflammation. Dysregulated leakage is observed in sepsis and various inflammatory diseases, and therapies sometimes target adhesion molecules or endothelial function to modulate leukocyte trafficking.

Terminology: The term postkapilláris is used in Hungarian medical literature to denote postcapillary venules, the venous segment downstream of the capillary bed.