How blood vessels distribute blood throughout the human body

The blood vessels form part of the circulatory system. The circulatory system comprises a continuous system of tubes, which transports nutrients, oxygen, and water to millions of body cells and carries away waste material, including carbon dioxide (CO2) from the body. The major types of blood vessels include the arteries, which transport blood away from the heart; the capillaries, which facilitate the exchange of water and substances between the blood and the tissues; and the veins, which move blood from the capillaries back to the heart.

Facts about blood vessels

Blood vessels form part of a complex system of tubes that distributes blood throughout various parts of the human body. The size of blood vessels varies from a diameter of about 25 mm in the aorta to about 8 mm in the capillaries. The thickness of blood vessel walls varies tremendously with the largest found in the large arteries due to their closeness to the heart and that they are exposed to higher blood pressures. The smaller diameters are found in the veins. Vein walls are thinner than arterial walls and their lumens are larger. The walls of blood vessels are made of smooth muscle fibers that contract and dilate, allowing the flow of blood. There is an estimated 60,000 miles of blood vessels in the human body. Every heartbeat pumps 1/15 of a pint of blood.

There are three distinct types of blood vessels: the arteries, veins and capillaries. Arteries are elastic vessels that transport blood away from the heart. The two main types of arteries are pulmonary arteries and systemic arteries. Pulmonary arteries transport deoxygenated blood from the heart to the lungs where carbon dioxide is delivered and oxygen-rich blood is picked up and carried to the heart through the pulmonary veins. There are two main types of systemic arteries: elastic arteries and muscular arteries; elastic arteries are larger than muscular arteries. Systemic arteries carry oxygenated blood away from the heart to the entire body. The aorta and its major branches; the common iliac, left common carotid, left subclavian, and brachiocephalic arteries are elastic arteries.  

Arterioles are tiny branches of arteries that branch out into capillaries. Arterioles regulate blood pressure and deliver blood to the capillaries. Arteries and arterioles are controlled by the sympathetic nervous system. Capillaries are tiny networks of blood vessels in most organs and tissues of the body. Blood is supplied to capillaries via the arterioles and is drained by the venules. Capillary walls are very small, providing a relatively large surface area for the exchange of gases and nutrients to the surrounding tissue. Venules are tiny vessels that drain blood from capillaries and into the veins.

Veins transport blood from the capillaries to the heart. Veins carry deoxygenated blood from the tissues of the body back to the heart. Veins differ in structure and function from the arteries. The walls of veins are thinner and the lumens are larger than those in the arteries. They may carry a large volume of blood and may act as blood reservoirs. Therefore, veins and venules are known as capacitance vessels. Most veins comprise a system of valves that prevent blood from flowing back to the tissues of the body.

Blood vessels, except capillaries, include a similar three-layered structure consisting of an innermost layer called the tunica intima, which is composed of endothelial cells (endothelium). The tunica intima restricts the entry of substances into the vascular wall; regulates coagulation; and controls blood vessel diameter. The lumen is the area through which the blood flows. The middle layer is known as the tunica media, and is composed of smooth muscle fibers, collagen and elastic fibers. The tunica media confers strength, elasticity and contractile properties to the vessel wall. The tunica media is thicker in arteries than in veins.

On top of the tunica media is the tunica adventitia, which is the strong outer covering of arteries and veins, and is composed of connective tissue, collagen and elastic fibers that support fibroblasts cells (cells that secrete fibrous proteins, collagen and elastin), vasa vasorum (small blood vessels that supply oxygen and nutrients to the walls of large arteries and veins) and nerves. Elastic fibers allow the arteries and veins to stretch, preventing over-expansion when pressure is exerted on the blood vessel walls by blood flow.