The organ in the human body primarily responsible for pumping blood is the heart. This muscular organ is located in the thoracic cavity between the lungs and is roughly the size of a fist. The heart functions as a pump that circulates blood throughout the body via a complex system of arteries and veins. It consists of four chambers: the right and left atria and the right and left ventricles, which work together to ensure efficient blood flow. The heart's rhythmic contractions, controlled by electrical signals, are vital for delivering oxygen and nutrients to tissues while removing waste products.