The organ responsible for pumping blood in the human body is the heart. Located in the thoracic cavity, the heart is a muscular organ approximately the size of a fist. It functions as the central part of the circulatory system, which delivers oxygen and nutrients to tissues while removing carbon dioxide and waste. The heart has four chambers: the right atrium and right ventricle, which receive and pump deoxygenated blood to the lungs, and the left atrium and left ventricle, which distribute oxygen-rich blood to the rest of the body. Its rhythmic contractions are regulated by electrical impulses.