Heart has 4 chambers- two Atria (atrium-singular) and two Ventricles. The atria have thinner walls compared to those of ventricles which are thick and muscular. The atria receive blood and ventricles pump blood. The wall that separates left and right atria is known as Inter Atrial Septum (IAS) and the one that separate ventricles is called Inter Ventricular Septum (IVS).
Right Atrium receives blood from all over the body through two large veins. Superior Vena Cava (SVC) brings blood from upper body while Inferior Vena Cava (IVC) drains the lower body. IVC contributes 60% of the blood return. Apart from this, one major route of blood return is the Coronary Sinus (CS), which returns blood from the veins of heart itself.
Left Atrium receives pure blood from the lungs via 4 pulmonary veins. Two on Right (Right Superior and Inferior pulmonary veins) and two on the left (Left Superior and Inferior pulmonary veins).
The right ventricle pumps its blood into large pulmonary Artery while left ventricle ejects into Aorta- the biggest artery in the body.