The heart pumps blood through the human circulatory system.
This muscular organ lies in the chest between the lungs, slightly left of the body's midline. Its rhythmic contractions move blood through two linked circuits: pulmonary circulation, which carries blood to the lungs for gas exchange, and systemic circulation, which supplies the rest of the body.
The human heart has four chambers. The right side receives oxygen-poor blood and sends it to the lungs, while the left side receives oxygen-rich blood and pumps it into the systemic circulation. Valves help keep blood moving in one direction.
The heart's contractions are coordinated by electrical signals generated and conducted by specialized cardiac tissue. The sinoatrial node normally initiates each heartbeat, which is why it is often called the natural pacemaker. The heart itself receives oxygen through the coronary arteries.