The atria have thin walls because they only have to pump their load of blood downhill into the ventricles. The right ventricle is thicker because it has to pump blood to the lungs and back to the heart. The left ventricle is the thickest and strongest because it has to pump the blood throughout the whole body.
Because of gravity the blood moves. If there's no gravity they will not move.
Ventricles are more important than atria