Describe the structure of the heart and its function in the human circulatory system.

The heart is a muscular organ composed of 4 chambers; the left atrium and right atrium (pl. atria) and the left ventricle and right ventricle (pl. ventricles). The left and right sides of the heart are separated by a septum. The right atrium receives deoxygenated blood through the vena cava. After passing through the tricuspid valve, the right ventricle pumps this deoxygenated blood out through the pulmonary artery to the lungs. This is called the pulmonary circuit. Simultaneously, the left atrium receives oxygenated blood, from the lungs, through the pulmonary vein. Once this oxygenated blood passes through the bicuspid valve and into the left ventricle, it is pumped out through the aorta to the rest of the body. This is called the systemic circuit. Valves act to partition the atria from the ventricles but most importantly prevent the backflow of blood. Structurally, the left wall of the heart is thicker hence maintains a higher blood pressure compared to the right side of the heart. All in all, humans have a double circulatory system through which blood is pumped by the heart to supply cells of the body.