This sac protects the heart from rubbing against the lungs or the wall of the chest. The structure and action of the heart are designed to serve the two loops of circulation. Blood moves through each side of the heart systematically. Venous blood, which is lower in oxygen content, is delivered into the right atrium. It then enters the right ventricle, from where it is pumped out into the pulmonary artery and to the lungs.
Oxygenated blood returning in the pulmonary vein flows into the left atrium.
This blood enters the left ventricle and is then pumped into the aorta for circulation. The flow of blood in each side of the heart is controlled by a series of valves. The pumping action of the heart is achieved by the contraction of the cardiac muscle of which the heart is largely composed. The rhythm of the heartbeat is regulated by bursts of electrical impulses sent out by a concentration of specialized heart tissue called the sinoatrial node, which is the natural pacemaker of the heart. Under the influence of the pacemaker, the heart of an adult at rest beats at a rate of 60 to 100 beats a minute.