Introduction
Function of the heart
- to propel blood
- to maintain blood pressure
Function of the blood
- to transport oxygen and carbon dioxide
- to deliver nutrients and hormones
- to remove waste products
- to assist in defense against disease
Function of blood vessels
- to distribute blood throughout the body.
Blood vessels consist of:
- arteries, which carry blood from heart to capillaries
- capillaries, the smallest of the vessels and sites of exchange between blood and interstitial fluids
- veins, which return blood from capillaries to the heart
Arteries
Arteries are large diameter vessels that carry oxygenated blood away from the heart. Arteries gradually decrease in size to become arterioles, the smallest vessels of the arterial system. From here, blood enters the capillaries to serve local tissues.
Veins
Blood flowing toward the heart begins in the tissues and flows through venules, which are the smallest vessels of the venous system. The pathway increases to larger and larger vessels called veins, which carry non-oxygenated blood back toward the heart.
Arteries and veins often lay side by side in a narrow band of connective tissue.
The walls of arteries and veins contain three layers:
- tunica interna (tunica intima) is the innermost layer, which includes the endothelial lining and an underlying layer of connective tissue dominated by elastic fibers.
- tunica media is the middle layer of smooth muscle tissue in a framework of collagen and elastic fibers. When the smooth muscles contract, the vessels decrease in diameter. When they relax, the diameter increases. The tunica media of an artery contains more smooth muscle and elastic fibers than a vein does.
- tunica externa (tunica adventitia) forms a sheath of connective tissue around the vessel. Its collagen fibers may intertwine with those of adjacent tissues which stabilize and anchor the blood vessel.
