Blood vessels are essential components of the cardiovascular system, serving as conduits for blood flow throughout the body. There are five main types of blood vessels — arteries, arterioles, capillaries, venules, and veins. Arteries are elastic vessels that carry blood away from the heart to all organs. All arteries except the pulmonary arteries transport oxygenated blood. Branching off from the arteries are small blood vessels called arterioles. They carry blood toward the smaller branches called the capillaries, which are less than one cell in thickness. Capillaries facilitate the exchange of oxygen and nutrients with the surrounding tissues and cells. The deoxygenated blood from these capillaries is carried away by small vessels known as the venules. They merge to form veins that carry blood back to the heart. All veins except the pulmonary veins carry deoxygenated blood. Unlike most arteries, many veins possess valves that prevent the backflow of blood and keep it flowing in a single direction.