Any of the smallest blood vessels connecting arterioles with venules.
Comment:
[homology-note] "The appearance of Chordata and subsequently the vertebrates is accompanied by a rapid structural diversification of this primitive linear heart: looping, unidirectional circulation, an enclosed vasculature, and the conduction system.[well established][VHOG]" xsd:string {date_retrieved="2012-09-17", external_class="VHOG:0001253", ontology="VHOG", source="DOI:10.1196/annals.1341.002 Bishopric NH, Evolution of the heart from bacteria to man. Annals of the New York Academy of Sciences (2006)", source="http://bgee.unil.ch/"}; [structure-note] "capillaries lack tunica media and tunica adventitia; only the endothelial wall of the tunica intima belongs"