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There are two types of blood vessels in the circulatory system of the body: arteries that carry oxygenated blood from the heart to various parts of the body and veins that carry blood towards the heart for purification.hey are tiny branches of arteries that lead to capillaries and help maintain the pressure and blood flow in the body. Venous valves prevent blood from flowing in the opposite direction. Connective tissues make up the outermost layer of a vein which is also known as-tunica adventitia or tunica externa.
Blood flows from the capillaries into very small veins called venules, then into the veins that lead back to the heart. Veins have much thinner walls than do arteries, largely because the pressure in veins is so much lower. Veins can widen (dilate) as the amount of fluid in them increases.rom the capillaries, blood passes into venules, then into veins to return to the heart. Arteries and arterioles have relatively thick muscular walls because blood pressure in them is high and because they must adjust their diameter to maintain blood pressure and to control blood flow.