The heart is a muscular organ that consists of four chambers: the right atrium, the left atrium, the right ventricle and the left ventricle. It is contained in a sac-like membrane called the pericardium where the inner layer of this, the epicardium, is in direct contact with the heart. During the cardiac cycle the right atrium receives deoxygenated blood coming from the superior and inferior vena cava (this blood has been transported from various parts of the body). The deoxygenated blood is pumped through to the right ventricle and transported to the lungs through the pulmonary arteries to be oxygenated during pulmonary circulation. This is permitted by the tricuspid valve which opens and allows blood to flow from the right atrium to the right
