Left Atrium

After exchange of gases in the pulmonary capillaries, blood returns to the left atrium high in oxygen via one of the four pulmonary veins. Blood flows nearly continuously from the pulmonary veins back into the atrium which acts as the receiving chamber and from here through an opening into the left ventricle. The opening between the left atrium and ventricle is guarded by the the Left AV or bicuspid valve. The myocardium of the thick walled left ventricle pumps strongly creating enough pressure to force blood flow through the entire body. As the blood leaves the left ventricle, it passes through the aortic valve into the aorta. Although both sides of the heart will pump the same amount of blood, the muscular layer is much thicker in the left ventricle compared to the right.