The heart has four chambers — two on top and two on bottom:
The two bottom chambers are the right ventricle and the left ventricle. These pump blood out of the heart. A wall called the interventricular septum between the two ventricles.
The two top chambers are the right atrium and the left atrium. They receive the blood entering the heart. A wall called the interatrial septum between the atria.
The atria are separated from the ventricles by the atrioventricular valves: The tricuspid valve separates the right atrium from the right ventricle. The mitral valve separates the left atrium from the left ventricle. Two valves also separate the ventricles from the large blood vessels that carry blood, leaving the heart:
The pulmonic valve is between the right ventricle and the pulmonary artery, which carries blood to the lungs.
The aortic valve is between the left ventricle and the aorta, which carries blood to the body.