Atria: The heart is divided into four chambers that are connected by heart valves. The upper two heart chambers are called atria. Atria are separated by an interatrial septum into the left atrium and the right atrium. The lower two chambers of the heart are called ventricles. Atria receive blood returning to the heart from the body and ventricles pump blood from the heart to the body. The heart is located between the lungs an under the sternum and is protected by the rib cage.