The heart is a vital muscular organ about the size of a clenched fist, located in the chest between the lungs, slightly to the left of the breastbone

. It functions as the main pump of the cardiovascular system, continuously circulating blood throughout the body to deliver oxygen and nutrients to tissues and remove waste products like carbon dioxide

. Anatomically, the human heart is divided into four chambers: two upper atria and two lower ventricles. The right side of the heart receives deoxygenated blood and pumps it to the lungs for oxygenation, while the left side receives oxygen-rich blood from the lungs and pumps it out to the rest of the body

. The heart's pumping action is regulated by an electrical conduction system that controls the heartbeat rhythm

. The heart walls consist of three layers: the epicardium (outer layer), myocardium (muscular middle layer), and endocardium (inner layer)

. It is enclosed in a protective sac called the pericardium

. Valves within the heart ensure one-way blood flow between chambers and into major arteries

. In summary, the heart is a powerful, fist-sized muscle that pumps blood through a network of vessels, supporting life by maintaining the circulation of oxygenated blood and nutrients throughout the body