Blood vessels

• Arteries carry blood away from the heart.
• Veins carry blood into the heart.
• Blood vessels contain: elastic fibres, smooth muscle and collagen.
• elastic fibres are made of elastin, allowing the vessel to stretch and recoil.
• smooth muscle contracts and relaxes involuntarily and changes the size of the lumen.
• collagen provides structural support, and maintains the shape and volume of the blood vessel.
• Endothelium: the tissue which forms a single layer of cells lining various organs and cavities of the body, especially the blood vessels, heart, and lymphatic vessels. they are smooth and reduce blood friction.
• arteries carry blood under high pressure, therefore they need a narrow lumen to maintain the pressure, thick elastic and muscle layers for the vessel to expand and recoil and a tough collagen layer to withstand the pressure
• veins have less elastic tissue, less collagen and less smooth muscle as the blood pressure is lower.
• tunica intima: inner layer of the blood vessel. Contains elastic tissue for stretching and recoil.
• tunica media: middle layer of the vessel wall, composed of smooth muscle to change lumen size.
• tunica externa: outer layer of the vessel, contains collagen and elastic fibers providing strength to withstand pressure.