<Tr> <Td> </Td> <Td> Normal heart sounds Normal heart sounds as heard with a stethoscope </Td> </Tr> <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>

Which side of the heart is the most muscular