Any of the smallest blood vessels where blood circulates within organ tissues. Microvessels include terminal arterioles, metarterioles, capillaries, and venules (but exclude lymphatic capillaries). Arterioles carry oxygenated blood to the capillaries, and blood flows out of the capillaries through venules into veins. [ http://en.wikipedia.org/wiki/Microcirculation http://orcid.org/0000-0002-7073-9172 ]