The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. The blood provides the body with oxygen and nutrients, as well as removing metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest. In humans, other mammals and birds the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. Fish in contrast have two chambers, an atrium and a ventricle, while reptiles have three chambers. 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 a small amount of fluid. The wall of the heart is made up of three layers: epicardium; myocardium; and endocardium. The heart pumps blood through both circulatory systems. Blood low in oxygen from the systemic circulation enters the right atrium from the superior and inferior vena cavae and passes to the right ventricle.