<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> 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>
what are the walls of the heart composed of
what are the walls of the heart composed of
three layers : epicardium , myocardium , and endocardium
three layers : epicardium , myocardium , and endocardium