Blood Vessels

Arteries are blood vessels that carry oxygenated blood away from the heart (with the exception of the pulmonary artery). The oxygen the blood is transporting is what gives it a bright red colour. Arteries are designed to withstand the high pressure exerted from the blood as it is pumped from the heart. Arteries vary in size but all have thick walls consisting of three layers of tissue. The middle layer is a thick layer of muscle. Arterioles are smaller blood vessels that branch out from an artery that lead to capillaries.

Veins are the blood vessels that transport blood to the heart and carry deoxygenated blood (with the exception of the pulmonary vein). The blood is a dark purplish red as it is no longer carrying oxygen. The walls of the veins are much thinner than those of arteries and they have less muscle and elastic tissue. Muscular movement and breathing help to move blood along the veins in the body. The blood within veins is not under pressure unlike the blood flow in the arteries. Most veins contain
special valves which prevent blood from flowing back along the vein. The smallest veins are called venules.

Capillaries are the smallest blood vessels. They link the arteries (carrying blood away from the heart) back to the veins (to return blood back to the heart). Capillary walls are only a single cell layer thick, and are semipermeable. Substances such as oxygen, vitamins, minerals, water and amino acids are able to move easily from the capillaries into the surrounding tissues to nourish and feed the cells. Substances such as carbon dioxide, cellular waste and water pass back into the capillaries to
be removed. This simple process is known as capillary exchange. Due to their size, blood cells and large substances, such as plasma proteins, remain in the capillaries and cannot move out into the surrounding tissue unless the blood vessel is damaged.