The heart is the core organ of the human circulatory system, located slightly to the left in the center of the chest cavity, roughly the size of a person's fist. By regular contractions and relaxations, blood is pumped to various parts of the body, providing oxygen and nutrients to cells while carrying away metabolic waste. The heart is mainly composed of myocardium, which has automatic rhythmicity and can autonomously control the heart rate and intensity. The four chambers of the heart include two atria and two ventricles. The right atrium and right ventricle are responsible for receiving and pumping blood to the lungs for oxygenation, while the left atrium and left ventricle are responsible for delivering oxygenated blood throughout the body.