Venule

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.