- The ventricles of the heart are responsible for pumping blood out of the heart to the lungs (right ventricle) and the rest of the body (left ventricle). - Pumping blood against the resistance of the circulatory system requires significant force, especially for the systemic circulation served by the left ventricle, which pumps blood throughout the entire body. - The atria, on the other hand, only pump blood into the ventricles, which requires much less force. Therefore, the muscular walls of the ventricles, particularly the left ventricle, are significantly thicker than the atrial walls to generate the necessary pressure for pumping blood to distant parts of the body.