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A postcapillary venule is a small venule immediately downstream of a capillary network. Its thin wall includes an endothelium supported by associated pericytes. It collects blood returning from exchange vessels and is an important site of leukocyte passage from the circulation into surrounding tissue.^[ref]

In a 2006 experiment, investigators followed neutrophils in inflamed mouse postcapillary venules using real-time and time-lapse intravital microscopy. After adhering, the neutrophils crawled along the luminal surface before leaving the vessel, sometimes moving against blood flow. Genetic perturbations separated effective adhesion from subsequent crawling.^[ref]

The time sequence matters: finding a neutrophil attached in one image does not establish that it has crossed the vessel wall. Following the same cell distinguishes arrest, active searching and exit, and shows why the number of adherent cells alone is an incomplete measure of recruitment.

A venule is a small blood vessel that collects blood leaving capillaries and conveys it toward larger veins. It lies on the returning side of the microcirculation: arterioles supply the capillary network, while venules drain it. The smallest postcapillary venules have thin walls lined by endothelium.^[ref]

A venule is also an important interface between flowing blood and surrounding tissue. In an experiment using time-lapse intravital microscopy, investigators followed neutrophils within inflamed mouse postcapillary venules. After adhering, the neutrophils crawled along the inner surface toward sites where they could leave the vessel. Observing movement over time distinguished this searching step from the initial arrest.^[ref]

A simple flow example illustrates the collecting role. If three incoming capillary branches each deliver $1nL/min$ and there is no local accumulation or exchange of fluid, the outgoing volumetric flow rate is $3nL/min$. This adds flow rates; the resulting speed also depends on the vessel's cross-sectional area.