<Tr> <Td_colspan="2"> Problems playing this file? See media help . </Td> </Tr> <P> The heart is a muscular organ in most animals, which pumps blood through the blood vessels of the circulatory system . Blood provides the body with oxygen and nutrients, as well as assists in the removal of metabolic wastes . In humans, the heart is located between the lungs, in the middle compartment of the chest . </P> <P> 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 . </P> <P> The heart pumps blood with a rhythm determined by a group of pacemaking cells in the sinoatrial node . These generate a current that causes contraction of the heart, traveling through the atrioventricular node and along the conduction system of the heart . The heart receives blood low in oxygen from the systemic circulation, which enters the right atrium from the superior and inferior venae cavae and passes to the right ventricle . From here it is pumped into the pulmonary circulation, through the lungs where it receives oxygen and gives off carbon dioxide . Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta to the systemic circulation − where the oxygen is used and metabolized to carbon dioxide . The heart beats at a resting rate close to 72 beats per minute . Exercise temporarily increases the rate, but lowers resting heart rate in the long term, and is good for heart health . </P>

What are the walls of the heart composed of