1650s, "of or pertaining to the hair," from Latin capillaris "of hair," from capillus "hair" (of the head); perhaps related to caput "head" (but de Vaan finds this "difficult on the formal side" and "far from compelling, since capillus is a diminutive, and would mean 'little head', which hardly amounts to 'hair'"). Borrowed earlier as capillar (14c.). Meaning "taking place in capillary vessels" is from 1809. Capillary attraction attested from 1813. As a noun, "capillary blood vessel," from 1660s.
capillary cap·il·lary (kāp'ə-lěr'ē)
Of or relating to the capillaries.
Relating to or resembling a hair; fine and slender.
The tiny blood vessels throughout the body that connect arteries and veins. Capillaries form an intricate network around body tissues in order to distribute oxygen and nutrients to the cells and remove waste substances. (See circulatory system.)