Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom.
capillaries are tiny branched blood vessels that links arterioles to venules and allows exchange of nutrients and gases between the blood and its surrounding tissues
