Inside the heart, the muscular wall (myocardium) makes space for four chambers: two upper atria (sing: atrium) and two lower ventricles. The portion of the myocardium that surrounds the left chambers is thicker than the myocardium that surrounds the right chambers.
A muscular wall called the interventricular_septum separates the left chambers from the right chambers.
AV_valves separate the upper heart chambers from the lower ones. The tricuspid valve lies between the right atrium and ventricle. The bicuspid (mitral) valve lies between the left atrium and ventricle. Collagenous cords known as chordae_tendineae attach to the bottom surface of each AV valve and prevent the valves from "washing" back up into the upper chambers and staying there. The chordae tendineae are attached to papillary_muscles that control their movements.
Semilunar_valves are located at the beginning of the aorta and the pulmonary trunk. These valves help prevent blood scheduled to leave the heart from dropping back into the heart.