The human circulatory system includes five types of blood vessels: arteries, arterioles, capillaries, venules and veins. The purpose of blood vessels is to carry blood to and from the heart, while delivering oxygen and nutrients to cells and removing carbon dioxide and other wastes.
An artery is a blood vessel that carries oxygenated blood away from the heart; however, the pulmonary arteries are the only arteries to carry deoxygenated blood, which is taken to the lungs. There are three main layers to the walls of an artery. The outermost layer consists of collagen, which is connective tissue. The middle layer is a thick layer of smooth muscle, and the innermost layer is elastic tissue. The three layers work together to manage blood flow by contracting and relaxing, as instructed by the sympathetic nervous system. In comparison to veins, arteries transport blood at a higher pressure.
Arterioles are tiny arteries that branch off from a main artery to carry blood to the cells. Also controlled by the sympathetic nervous systems, arterioles manage blood flow through contracting and relaxing smooth muscles. In fact, arterioles are the main blood vessels that control blood pressure. Arterioles also lead to, and carry blood to, capillaries.
Capillaries are extremely small blood vessels that work with the tissues to exchange gases, nutrients, water, salts and waste. Because capillary walls have the thickness of one cell, they are easily able to deliver oxygen and nutrients to the tissues while removing carbon dioxide from the tissues. Arterioles supply the blood in capillaries, and venules carry the waste-filled blood away. They are very narrow--approximately five to 20 micro-meters in diameter. Capillary beds, found everywhere in the body, explain why injuries anywhere in the body can result in bleeding.
Venules are tiny blood vessels, each one connected to many capillaries. Each capillary drains into a venule, allowing for the waste-filled blood to enter veins and be carried back toward the heart. The smooth muscle layer and the connective tissue layer of venule walls are not as thick as those in arteries.
A vein is a blood vessel that carries deoxygenated blood back to the heart; however, pulmonary veins carry oxygenated blood from the lungs to the heart. Venules drain into veins. Muscle contractions create a constant movement of blood toward the heart. In addition, one-way valves prevent blood from flowing in the opposite direction, back away from the heart. The one-way valves also work against gravity, helping to return blood in the lower body to the heart.