Right Atrium
It is one of the four chmaber in the heart. It receives deoxygenated blood from the superior and inferior venae cava, the coronary sinus, and the anterior and smallest cardiac veins, and pumps it into the right ventricle through the tricuspid valve. Also called right atrioventricular valve.

Left Atrium
Is about the same thickness as the right atrium and forms most of the base of the heart. It receives blood from the lungs through four pulmonary veins. Blood passes from the left atrium into the left ventricle through the bicuspid (mitral) valve which, as its name implies has two cusps. Also called left atrioventricular valve.

Right Ventricle
Is about 4-5mm ( 0.16-0.2 in) in average thickness and forms most of the anterior surface of the heart. Internally, the right ventricle is separated from the left ventricle by a partition called the interventricular septum. Blood passes from the right ventricle through the pulmonary valve ( pulmonary semilunar valve) into a large artery called the pulmonary truck, which divides into right and left pulmonary arteries.

Left Ventricle
Is the thickest chamber of the heart, averaging 10-15 mm (0.4-0.6 in) and forms the apex of the heart. The left ventricle connects to the rest of the body through its intrinsic function of supplying all of the organs with nutrition and oxygen in the blood, which it pumps with every heartbeat.