The left atrium is a small upper cavity of the |heart|. Oxygen rich ~blood~ returns from the |lungs| through the four |pulmonary veins| into the smooth chamber of the left atrium. The chamber is constructed of two overlapping layers of muscle: a superficial layer and an inner layer, composed of many small bundles. The wall of the chamber is slightly thicker and more powerful than the right atrium. As the |heart| contracts (ventricular systole), ~blood~ flows into the ascending |aorta| through the |aortic arch|. As the |heart| relaxes (ventricle diastole), the ~blood~ flows through the |mitral valve| to the left ~ventricle~.
