The heart is a muscular organ that pumps blood throughout the body. It has four chambers: the two upper chambers, called atria, receive blood, while the two lower chambers, known as ventricles, pump blood out. The heart's rhythmic contractions are controlled by electrical signals, ensuring a steady flow of blood to deliver oxygen and nutrients to tissues. Blood circulates through two main pathways: the pulmonary circulation, which carries blood to the lungs for oxygenation, and the systemic circulation, which delivers oxygen-rich blood to the rest of the body. This continuous cycle is essential for maintaining overall health and supporting bodily functions.