The three main parts of the circulatory system are the heart, the blood and the blood vessels. The purpose of the circulatory system is to transport oxygen throughout the body.
In humans, the heart is a fist-sized muscle located slightly to the left of center in the chest. It acts as an engine for the circulatory system, pumping blood through the blood vessels. It will beat approximately three billion times in an average human lifespan.
The blood vessels are hollow tubes that conduct blood from the heart to the body’s tissues and back again. There are three types: arteries, veins and capillaries. Arteries carry oxygenated blood away from the heart and to the tissues. The one exception to this is the pulmonary artery, which carries deoxygenated blood from the heart to the lungs. They are thick-walled, elastic vessels that contain no valves. Veins carry blood back to the heart after the body tissues have used the blood’s oxygen. They have thinner walls than arteries and contain valves to keep the blood from flowing backward. Capillaries are tiny vessels that connect arteries to veins. Because their walls are so thin, they allow oxygen and other nutrients to pass from the vessels to the surrounding tissue.
Blood is made of two parts: plasma and blood corpuscles (or cells). Plasma is the liquid portion of blood and is made mostly of water. There are three types of corpuscles: red blood cells, white blood cells and platelets. The red blood cells carry oxygen and carbon dioxide, white blood cells attack germs and foreign bodies and platelets help to clot the flow of blood in cuts or injuries.