The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers. The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum. This distinction is visible also on the surface of the heart as the coronary sulcus.There is an ear-shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage.The right atrium and the right ventricle together are sometimes referred to as the right heart. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart.The ventricles are separated from each other by the interventricular septum, visible on the surface of the heart as the anterior longitudinal sulcusand the posterior interventricular sulcus.

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.