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The Cardiovascular System: Blood Vessels

Blood vessels are responsible for the transport of blood around the body.  There are three main types of blood vessel:

Arteries

Arteries carry blood away from the heart.  They have thick, muscular, elastic walls, which enables them to withstand the high pressure that blood is under as it leaves the heart.  Arteries carry oxygenated blood to tissues such as muscle that are actively using oxygen.  The one exception to this rule is the pulmonary artery which takes deoxygenated blood from the heart to the lungs.  Arteries subdivide into smaller vessels in order to take blood to all the different body parts.  Arteries continue to subdivide until they reach their smallest size where they are known as arterioles.  Arterioles further divide and lead into capillaries where oxygen diffuses across the thin capillary surface to be delivered to the working tissue.

Veins

Veins carry blood back to the heart.  Because the blood is now under far less pressure, veins only have thin inelastic walls.  Veins carry deoxygenated blood away from the organs and tissues and back to the heart.  The one exception is the pulmonary vein which carries oxygenated blood from the lungs back to the heart.  The movement of blood, through veins, occurs primarily through the contraction of skeletal muscles, which constricts veins, forcing blood along the veins back towards the heart.  Veins contain non-returnable valves which will not allow blood to flow backwards away from the heart.  The smallest veins are called venules.  These collect blood from the capillaries.  These lead into other venules to form veins.  These connect with other veins and in this way the nearer you get to the heart the larger the veins are.

Capillaries

Capillaries are the smallest blood vessels.  There walls are just one cell thick, enabling oxygen to easily diffuse across them, from red blood cells across, and allowing carbon dioxide to easily diffuse across them into the blood.  Capillaries link arteries and veins and it is as blood passes from arteries through the capillaries that it looses most of its pressure.  As well as being the point of oxygen delivery to tissues they are also the point at which important nutrients, minerals, vitamins and hormones are taken up by muscles, and the point at which waste products are taken up by the blood.  Capillaries also play an important role in the regulation of temperature by regulating blood flow to the skin.  Under hot conditions blood flow to the skin is increased, through the dilation of capillaries in the skin, to aid heat loss, and under cold conditions capillaries in the skin constrict, reducing blood flow and therefore reducing heat loss.