Capillaries are the smallest of a body’s blood vessels (and lymph vessels) that make up the microcirculation. Their endothelial linings are only one cell layer thick. These microvessels, measuring around 5 to 10 micrometres (µm) in diameter, connect arterioles and venules, and they help to enable the exchange ofwater, oxygen, carbon dioxide, and many other nutrients and waste chemical substances between blood and thetissues surrounding them. Lymph capillaries interconnect with larger lymph vessels to drain lymph collected in the microcirculation.

During early embryonic development new capillaries are formed through vasculogenesis, the process of blood vesselformation that occurs through a de novo production of endothelial cells followed by their forming into vascular tubes. The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already present endothelium which divides.