I read that it begins with the oxygen-depleted blood arriving at the heart through the two large veins—the superior (top) and inferior (bottom) venae cavae. These veins empty into the first chamber of the heart, the right atrium. The right atrium then squeezes the blood into a more muscular chamber, the right ventricle. From here the blood goes to the lungs through the pulmonary trunk and the two pulmonary arteries—the only arteries carrying oxygen-depleted blood. This is normally done by veins.
While in the lungs, blood releases carbon dioxide and absorbs oxygen. It then flows down into the heart’s left atrium through the four pulmonary veins—the only veins carrying oxygen-rich blood. The left atrium empties into the heart’s most powerful chamber, the left ventricle, which pumps oxygenated blood out through the aorta and into the body. The two atria contract together followed by the two ventricles, the dual sequence constituting a heartbeat. Four internal valves ensure a one-way flow of blood through the heart.