Spreading out from your heart, a network of arteries, veins and tiny capillaries delivers oxygen and nutrients to every cell in the body, while veins return carbon dioxide and waste materials to the heart. The circulatory system was first described in the 17th century by British physician William Harvey, who concluded that blood flows through the body in a continuous circuit. Prior to that it was believed that the blood, produced in the liver, was dispatched by the heart to reservoirs, where it waited until it was needed.
While the main function of blood is to act as the body's transport system, it also plays a major role in the defense against infections.
The circulatory system (heart and blood vessels) consists of two main parts: the systemic circulation, which comprises the blood supply to the entire body except the lungs, and the pulmonary circulation to the lungs, which is responsible for reoxygenating the blood and removing carbon dioxide.