The size of blood vessels corresponds to the amount of blood passing through the vessel. All blood vessels contain a hollow area called the lumen through which blood can flow. Arteries have three layers of thick walls and carry blood away from the heart. The walls of the arteries expand and contract as the heart beats; this rhythm is called pulsation and pushes the blood. The largest artery is the aorta, which carries blood to the rest of the body from the left ventricle. Arteries become arterioles, which eventually divide into a series of small vessels called capillaries. Capillaries have thin walls and are the point of exchange between blood and surrounding tissues. They are also narrow, slowing blood flow to give time for oxygen, vitamins, minerals, water and amino acids to pass through the cells lining the capillary into the tissue and for carbon dioxide and other waste to leave the tissues. the blood. Veins carry blood from the capillaries to the heart. The pressure in the veins is low, so they rely on muscle contractions to move the blood, and they have valves to stop the backflow. The largest vein is the vena cava and provides the entry point for deoxygenated blood into the right