• These are thick-walled and deeply located blood vessels, which always carry the blood away from the heart to various, body parts. these have narrow lumens but do not have valves. in them, the blood flows at high pressure and high speed. these generally carry oxygenated blood.

Veins

• These are thin-walled and superficially located blood vessels, which always carry blood from various, body parts generally to the heart. these have wider lumens and valves in them to prevent the backflow of blood. in them, blood flows at low pressure and low speed.

Capillaries

• These are the thinnest blood vessels. and formed by division and re-division of arteriole inside the body organs. each is lined by a single layer of flat cells called the endothelium, which allows the exchange of materials between blood and the body cells through tissue fluid.

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• A blood vessel ( artery or vein) is formed of three layers., these are tunica Interna tunica media and tunica external.
• Tunica Interna is the innermost layer and is formed of the inner endothelium of flat cells and the outer elastic membrane of yellow fibrous tissue.
• Tunica media is the middle layer and is formed of circular smooth muscle fibres and elastic yellow fibres.
• Tunica Externa is the outermost layer and is formed of collagen-rich areolar connective tissue.