~Blood~ vessels are tubes that carry ~blood~ to and from all parts of the body. The human circulatory system is made up of three types of ~blood~ vessels. An |artery| is a large ~blood~ vessel with muscular tissue in the wall that carries ~blood~ away from the |heart| to the organs and tissues of the body. |Arterioles| are small branches of the |artery| that are about 0.2mm in diameter. The |veins| and smaller |venules| are vessels that carry ~blood~ from the organs and tissues toward the |heart|. |Veins| have small |valves| that open to let ~blood~ through, and close to prevent it from flowing back. |Capillaries| are microscopic in size. They link the |arterioles| and |venules| through a network of metarterioles.
