The left atrium receives oxygenated blood from the pulmonary veins and delivers it to the left ventricle through the mitral valve. It forms the base of the heart and is the most posterior chamber.
Location and External Features
The left atrium is located at the base of the heart, posterior to the right atrium and superior to the left ventricle. It is the most posterior cardiac chamber and lies directly anterior to the esophagus. This anatomic relationship makes transesophageal echocardiography (TEE) an excellent modality for imaging the left atrium.
Left Atrial Appendage (Auricle): A finger-like, muscular pouch that extends anteriorly from the main atrial body. Unlike the right atrial appendage, the left appendage is narrower and has a more complex, multilobed shape. It lies adjacent to the pulmonary trunk and left coronary artery.
Pulmonary Vein Connections
The left atrium receives blood from four pulmonary veins (two from each lung):
Right Pulmonary Veins:
- Right superior pulmonary vein
- Right inferior pulmonary vein
Left Pulmonary Veins:
- Left superior pulmonary vein
- Left inferior pulmonary vein
The pulmonary vein orifices are located at the posterior and superior aspects of the left atrial cavity. The smooth wall between the right and left pulmonary vein ostia is known as the interpulmonary vein ridge.
Anomalous pulmonary venous return can occur, where one or more pulmonary veins drain into the right atrium or systemic veins instead of the left atrium.
Internal Anatomy
The left atrium has two distinct regions:
Smooth-Walled Portion
The majority of the left atrial wall is smooth, reflecting its embryologic origin from the pulmonary veins. This region includes:
- The posterior wall containing the pulmonary vein orifices
- The septal surface (interatrial septum) on the right side
- The anterior wall
The left side of the interatrial septum features a shallow depression corresponding to the fossa ovalis of the right atrium, though it is less distinct.
Muscular Portion (Appendage)
The left atrial appendage has a trabeculated internal surface with pectinate muscles. The appendage is narrower than its right counterpart and has a characteristic hooked shape.
Dimensions
- Normal diameter: 3-4 cm (measured in parasternal long-axis view on echocardiography)
- Volume: 22-52 mL (indexed to body surface area)
- Wall thickness: 2-3 mm
Function
The left atrium serves multiple functions:
- Reservoir: Collects blood from the pulmonary veins during ventricular systole
- Conduit: Passively transfers blood to the left ventricle during early diastole
- Booster Pump: Actively contracts at the end of diastole (atrial kick), contributing 15-30% of left ventricular filling
The atrial kick is particularly important in patients with left ventricular hypertrophy or diastolic dysfunction, where passive filling is impaired.
Relations of the Left Atrium
- Anterior: Aortic root and right ventricular outflow tract
- Posterior: Esophagus and descending thoracic aorta
- Right: Right atrium and interatrial septum
- Left: Left lung and pleura
- Superior: Bifurcation of the pulmonary trunk
Clinical Significance
Left Atrial Enlargement
Enlargement of the left atrium occurs in conditions associated with elevated left atrial pressure or volume overload, including mitral valve disease, hypertension, heart failure, and atrial fibrillation.
ECG findings include a prolonged P wave duration (P mitrale) and a negative terminal P wave in V1.
Atrial Fibrillation
The left atrium, particularly the pulmonary vein ostia, is the most common source of ectopic electrical activity triggering atrial fibrillation. Catheter ablation procedures target these areas to eliminate arrhythmogenic foci.
Left Atrial Appendage Thrombus
Blood stasis in the left atrial appendage during atrial fibrillation can lead to thrombus formation. Embolization of these thrombi is a major cause of stroke. Anticoagulation or appendage occlusion devices are used for prevention.
Mitral Stenosis
Narrowing of the mitral orifice (most commonly due to rheumatic heart disease) causes left atrial pressure overload, leading to progressive atrial enlargement, pulmonary hypertension, and atrial fibrillation.