The heart is a muscular organ in humans which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients and also assists in the removel of waste. The heart is located in the middle of the mediastinum in the chest. In humans the heart is divided into four chambers: upper left and right atria and lower left and right ventricles. The right atrium and ventral are referred together as the right heart and their left counterparts s the left heart. In a healthy heart blood flows one way through the heart due to heart valves which prevent backflow. The heart is enclosed in a protective sac, the pericardium, which also contains an amount of fluid; the wall of the heart is made up of three layers:
