The heart has four chambers that contract and thereby pump blood to the next chamber or blood vessel. The heart has two atria and two ventricles. Blood entering the heart always arrives at the atria, and blood that leaves the heart always departs from the ventricles. The left atrium pumps blood to the left ventricle and the right atrium pumps blood to the right ventricle. The heart is always pictured from the owners perspective.
The atria are small and have a relatively thin muscle wall surrounding the chambers. Ventricles however, are relatively large and have a thicker muscular wall. The left ventricle has a thicker muscle wall than the right ventricle. Ventricles constitute a larger volume than the atria. The atria are made up of smooth muscle. However, specialised cells that originate in the right atrium rapidly transmit electrical impulses throughout both atria leading to the coordinated contraction of both chambers, pumping blood into the ventricles.
The atria pump blood into each ventricle and the ventricles pump blood into their connecting blood vessels. The atria contribute relatively little with respect to the actual pumping. This is because 60-75% of the blood entering the atria flows directly into the ventricles. During the contraction the atria thus contribute only the last remnants of blood into the ventricles. Meaning if the atria were to cease coordinated contraction, the heart would still be able to maintain relatively good blood transport through the blood vessels. This is demonstrated by people living with atrial fibrillation.
The ventricles pump blood either to the lungs (from the right ventricle) or around the whole body (left ventricle). This explains why the left ventricle has a much thicker muscle wall than the right ventricle. Much more power is required to create the pressure that pushes blood to the whole body in contrast to the short distance to the lungs.