Left atrium

Abstract

The left atrium is one of the most structurally and physiologically complex chambers in the heart. The left atrium is divided into a posterior and an anterior half. The posterior half receives blood from the four pulmonary veins. The two superior pulmonary veins enter the upper posterior part, whereas the two inferior pulmonary veins open into the lower posterior left atrium. The anterior portion called the body is continuous with the left atrial appendage (LAA). The LAA is a blind-ending saccule projecting anteriorly and then to the right over the external surface of the pulmonary trunk. Its inner surface is covered by musculi pectinati. From the atrium, the blood is emptied into the left ventricle through the left atrioventricular or mitral valve. The interatrial septum lies in front of the left atrium as well as to its right.