What are the three types of blood vessels and their functions?
In this blog post we will look at each type of blood vessel and their different functions.
Arteries carry blood away from the heart. The largest is the aorta. The circulation to all the body except the lungs is called the systemic circulation and carries oxygenated blood. The circulation to the lungs is called the pulmonary circulation and carries deoxygenated blood.
Arteries divide into smaller arterioles which divide again and again and eventually become capillaries.
Arteries have thick muscular walls, which enable them to resist the pressure of blood flow. The innermost wall (the tunica intima) is a single layer of cells and provides a smooth lining which allows the least possible frictional resistance to the flow of blood. The middle layer (the tunica media) is made up of elastic tissue which can stretch when the heart beats. The outer layer (the tunica adventitia) is in the form of a thin covering.
Pressure within arteries is maintained by elastic stretch and recoil, the recoil keeping up a continuous pressure in the artery, which keeps the blood flowing evenly.
Veins are large blood vessels which carry blood back to the heart. Systemic veins carry deoxygenated blood. The largest veins are the superior and inferior vena cava, which return blood to the heart from the upper body and lower body respectively. Veins have a system of valves to prevent back-flow.
The flow of blood is aided by the action of muscles, especially the large muscles of the leg. Like the artery the vein has a single cell layer, the tunica intima, as its lining.
Unlike the artery the vein has a poorly developed middle layer, the tunica media. This is because the vein is not working under pressure and does not stretch in the same way as the artery. Unlike the artery the vein has a thick outer layer, the tunica adventitia. It needs this for strength, as it does not have muscle in its walls. The lumen (inside) of the vein is much larger than in an artery, reflecting the slower rate of blood flow.
Capillaries are the smallest blood vessels. The average diameter is eight micrometers with a very thin wall of 0.2 micrometers. The body has approximately 100,000 km. of capillaries. They are the site for exchange of gases, nutrients and waste between circulation and body tissues.
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