Labels:daily OCR: LEFT ATRIUM OUR HEART is divided into four chaml bers, two of which are called atria (one IS an atrium The upper left cham ber the left atrium opening into lower cham ber, or ventricle. Oxygen-rich blood from the lungs enters the left atrium in large veins called the pulmonary veins. The heart muscle then contracts and squeezes blood from the left atrium into the eft ventricle From the ventricle, blood is pumped out of the heart. The walls of the atria are thinner than those of the ventricles because they only have squeeze blood short way into the next cham ber YOUR entricle
