= Postcapillary venule
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>.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section20.1, Venules and vessel-wall organization]{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.<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; Figure3 and Video1]{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.
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