| − | Tiny, thin-walled blood vessels that allow the exchange of gases and nutrients between the blood cells.<ref>Wile, Dr. Jay L. ''Exploring Creation With General Science''. Anderson: Apologia Educational Ministries, Inc. | + | Tiny, thin-walled blood vessels that allow the exchange of gases and nutrients between the blood cells.<ref>Wile, Dr. Jay L. ''Exploring Creation With General Science''. Anderson: Apologia Educational Ministries, Inc. Capillaries are the smallest blood vessels. |