Left Atrium

Internally, the heart is divided into four hollow chambers, two on the left and two on the right. The upper chambers, called "atria," have relatively thin walls and receive blood returning through the veins. The left atrium chamber of the heart receives blood from the lungs through four "pulmonary veins" - two from the right lung and two from the left lung. Blood passes from the left atrium into the left ventricle through the atrioventricular opening, which is guarded by a valve. This valve has two leaflets or cusps, and is called the "bicuspid valve." It prevents blood from flowing back into the left atrium from the ventricle and is attached to papillary muscles by the "chordae tendinae."