The human heart is a muscular organ, roughly the size of a closed fist, located in the thoracic cavity between the lungs. It functions as a double pump, maintaining two separate circuits of blood flow: the pulmonary circulation, which carries blood to the lungs, and the systemic circulation, which supplies the rest of the body.
Deoxygenated blood returning from the body enters the right atrium through the vena cava and passes through the tricuspid valve into the right ventricle, which contracts to propel it along the pulmonary artery to the lungs, where carbon dioxide is released and oxygen is absorbed. Oxygenated blood then returns via the pulmonary vein to the left atrium, before moving through the bicuspid valve into the left ventricle. Because the left ventricle must generate sufficient pressure to force blood through the aorta to every organ, its muscular wall is considerably thicker than that of the right ventricle. Valves prevent backflow throughout, ensuring that blood travels in one direction only.