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Blood vessels are key components in the systemic and pulmonary circulatory systems and function to distribute blood throughout the body. There are three major types of blood vessels, arteries which carry blood away from the heart branching into smaller arterioles throughout the body and eventually form the capillary network which is closely associated with tissues of the body facilitating efficient chemical exchange between tissue and blood. Capillaries in turn merge into venules, then larger veins responsible for returning the blood to the heart. The junctions between vessels are termed anastomoses
Arteries and veins are comprised of three distinct layers whereas the much smaller capillaries are composed of a single layer.
The inner layer is termed the tunica intima and is the thinnest layer, formed from a single continuous layer of endothelial cells and supported by a sub-endothelial layer of connective tissue and supportive cells. In smaller arterioles or venules this sub-endothelial layer consists of a single layer of cells but can be much thicker, especially in vessels such as the aorta. The tunica intima is surrounded by a thin membrane comprised of elastic fibers running parallel to the vessel. Capillaries consist only of the thin endothelial layer of cells with an associated thin layer of connective tissue.
Surrounding the tunica intima is the tunica media, which is comprised of smooth muscle cells and elastic and connective tissues arranged circularly to around the vessel. This layer is much thicker in arteries than in veins and fiber composition also differs which veins containing fewer elastic fibers, and functions to control caliber of the arteries, a key step in maintaining blood pressure.
The outermost layer is the tunica externa, or tunica adventitia, composed entirely from connective fibers and surrounded by an external elastic lamina which functions to anchor vessels with surrounding tissues. The tunica externa is often thicker in veins preventing collapse of the blood vessel, and also as veins are often more superficially located providing protection from damage.
A major structural difference between arteries and veins is the presence of valves. In arteries the blood is pumped under pressure from the heart and so back-flow cannot occur. However, passing through the capillary network results in a decrease in blood pressure meaning that back-flow of blood is possible in veins. To counteract this veins contain numerous one-direction valves preventing back-flow.