Blood vessel

Blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away from the tissues. Blood vessels are needed to sustain life, because all of the body's tissues rely on their functionality. There are five types of blood vessels: the arteries, which carry the blood away from the heart; the arterioles; the capillaries, where the exchange of water and chemicals between the blood and the tissues occurs; the venules; and the veins, which carry blood from the capillaries back towards the heart. The word vascular, meaning relating to the blood vessels, is derived from the Latin vas, meaning vessel. Some structures – such as cartilage, the epithelium, and the lens and cornea of the eye – do not contain blood vessels and are labeled avascular. artery: late Middle English; from Latin arteria, from Greek artēria, probably from airein ("raise") vein: Middle English; from Old French veine, from Latin vena. The earliest senses were "blood vessel" and "small natural underground channel of water". capillary: mid 17th century; from Latin capillaris, from capillus ("hair"), influenced by Old French capillaire. The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins: The inner layer, tunica intima, is the thinnest layer. It is a single layer of flat cells (simple squamous epithelium) glued by a polysaccharide intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel. The middle layer tunica media is the thickest layer in arteries. It consists of circularly arranged elastic fiber, connective tissue, polysaccharide substances, the second and third layer are separated by another thick elastic band called external elastic lamina.