Tiny blood vessels that connect arteries to veins and allow oxygen and nutrients to pass into your body's tissues.
From Latin 'capillaris' meaning hair-like, from 'capillus' (hair). Scientists named these vessels after hair because they're so thin and delicate they resembled threads of hair under early microscopes.
Your capillaries are so small—about 5 micrometers wide, thinner than a red blood cell—that blood cells have to squeeze through single-file, which is exactly how they exchange oxygen for waste products in a brilliantly efficient biological design.
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