The human organ primarily responsible for pumping blood is the heart. It consists of four chambers: the right atrium and ventricle, which receive deoxygenated blood and pump it to the lungs for oxygenation, and the left atrium and ventricle, which receive oxygenated blood from the lungs and distribute it throughout the body. The heart functions continuously and is essential for maintaining blood circulation, supplying organs and tissues with oxygen and nutrients, and removing waste products. Its rhythmic contractions are regulated by electrical signals, ensuring that it meets the body's varying demands for blood flow.