 Venule (source code)

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= 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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section20.1: flow sequence and Venules]{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.<Intraluminal crawling of neutrophils to emigration sites: a molecularly distinct process from adhesion in the recruitment cascade by Mia Phillipson and others>[Abstract and Results: intraluminal crawling after adhesion]{ref}

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

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