The heart is a muscular organ composed of four main chambers: the right atrium, right ventricle, left atrium, and left ventricle. The atria are the upper chambers that receive blood, while the ventricles are the lower chambers that pump blood out of the heart. The heart also contains valves, such as the tricuspid, pulmonary, mitral, and aortic valves, which ensure unidirectional blood flow. Surrounding the heart is the pericardium, a protective sac that helps anchor the heart in place. The heart's walls consist of three layers: the epicardium (outer layer), myocardium (muscle layer), and endocardium (inner layer). These structures work together to maintain efficient circulation throughout the body.