Left Atrium Anatomy

The left atrium (LA) is one of the four chambers of the human heart and plays a crucial role in cardiac physiology. Its primary function is to receive oxygenated blood from the pulmonary veins and deliver it to the left ventricle, which then pumps the blood to the systemic circulation. Understanding the anatomy of the left atrium is fundamental for medical students, cardiologists, and surgeons alike, as this chamber is often involved in a variety of cardiovascular disorders such as atrial fibrillation, mitral valve diseases, and left atrial enlargement.

In this detailed guide, we will explore the anatomy of the left atrium from gross structure to microscopic histology, development, anatomical variations, relations, blood supply, innervation, and clinical relevance. This article aims to be a comprehensive reference with around 2900 words.

Overview of the Left Atrium

The left atrium is a thin-walled, posteriorly located heart chamber that lies superior and posterior to the left ventricle. It is roughly cuboidal in shape but can vary slightly depending on cardiac cycle and individual anatomy. It serves as a reservoir for pulmonary venous return during ventricular systole and as a conduit and active pump during ventricular diastole. Its volume and pressure changes significantly influence left ventricular filling and overall cardiac output.

Gross Anatomy of the Left Atrium

Location and Orientation

The left atrium is located posteriorly in the heart, forming most of the base of the heart. It lies just in front of the esophagus, which is why transesophageal echocardiography (TEE) provides excellent imaging of this chamber. The chamber is bordered by the right atrium on the right, the aortic root and left ventricular outflow tract anteriorly, and the descending thoracic aorta posteriorly and to the left.

Shape and Size

The left atrium is somewhat rectangular or cuboidal in shape, with an average volume of around 40–60 mL in adults under normal conditions. Its wall is thinner than that of the ventricles but slightly thicker than the right atrium. Left atrial size is clinically significant, as enlargement often indicates chronic pressure overload or volume overload, commonly seen in mitral valve disease.

External Features

Externally, the left atrium has several key features:

Left Auricle (Left Atrial Appendage): A small, ear-shaped muscular pouch projecting from the superior aspect of the chamber. It is a remnant of the embryological primitive atrium and has a rough trabeculated interior. The left atrial appendage is clinically important as it is the most common site for thrombus formation in atrial fibrillation.
Pulmonary Vein Openings: The posterior wall of the left atrium contains openings for four pulmonary veins (two from each lung). These veins bring oxygenated blood to the heart. Some individuals may have variant anatomy with three or five pulmonary veins.
Interatrial Septum: The medial wall of the left atrium contains the fossa ovalis, a thin depression that is a remnant of the fetal foramen ovale. Incomplete closure may result in a patent foramen ovale (PFO).
Mitral Valve Orifice: The anterior-inferior part of the left atrium opens into the left ventricle through the left atrioventricular orifice, which is guarded by the mitral valve.

Internal Features

Internally, the left atrium is smoother than the right atrium, particularly in its posterior wall. The walls are lined by smooth endocardium except for the left auricle, which has pectinate muscles. The transition between smooth and trabeculated portions is marked by the so-called “vestibule” of the mitral valve.

Relations of the Left Atrium

The left atrium’s anatomical relations are clinically significant because of their relevance to imaging, surgical approaches, and pathology spread.

Anterior: Left atrium is related to the transverse pericardial sinus, ascending aorta, and pulmonary trunk.
Posterior: Closely related to the esophagus, descending aorta, and thoracic vertebrae. Esophageal enlargement or pathology can compress the left atrium, and vice versa, left atrial enlargement can cause dysphagia.
Superior: Related to the right pulmonary artery.
Inferior: Related to the coronary sinus and left ventricle.
Right: Related to the right atrium and interatrial septum.

Microscopic Anatomy (Histology)

The left atrium is composed of three main layers:

Endocardium: Inner lining made of endothelial cells, resting on a thin layer of connective tissue. This layer is important for smooth blood flow and prevention of thrombus formation.
Myocardium: The muscular layer of the atrium composed of atrial myocytes. These cells are smaller and have fewer T-tubules compared to ventricular myocytes. Specialized conduction fibers, including Bachmann’s bundle, traverse the atrial myocardium, coordinating atrial contraction.
Epicardium: Outer layer, continuous with visceral pericardium, containing connective tissue, fat, blood vessels, and nerves.

Blood Supply and Lymphatic Drainage

The left atrium is supplied mainly by branches of the left coronary artery:

Left Circumflex Artery: Provides atrial branches to the posterior wall.
Right Coronary Artery: May contribute small branches to the interatrial septum.

Venous drainage occurs through small atrial veins that drain into the coronary sinus and indirectly into the right atrium. Lymphatic drainage follows the coronary vessels to the tracheobronchial lymph nodes.

Nerve Supply

The left atrium is innervated by the autonomic nervous system:

Parasympathetic: Supplied by vagus nerve fibers, which slow atrial conduction and reduce contractility.
Sympathetic: Postganglionic fibers from cervical and upper thoracic sympathetic ganglia increase atrial contractility and conduction.
Intrinsic Cardiac Nervous System: Contains ganglionated plexi near the pulmonary vein ostia, which play a crucial role in atrial fibrillation initiation.

Embryology of the Left Atrium

The left atrium develops from two main components:

Primitive Atrium Contribution: Forms the trabeculated portion, including the left auricle.
Pulmonary Venous Contribution: Forms the smooth-walled portion of the left atrium. Initially, a single pulmonary vein drains into the primitive atrium, which is gradually incorporated into the left atrial wall, forming four separate pulmonary vein openings.

Defects in this process can result in anomalous pulmonary venous return.

Anatomical Variations

Some common variations include:

Variation in the number of pulmonary veins (common ostium or supernumerary veins).
Differences in the size, shape, and orientation of the left atrial appendage.
Patent foramen ovale (PFO) or atrial septal defects affecting the interatrial septum.

These variations are clinically significant in interventional procedures like catheter ablation for atrial fibrillation or device closure of septal defects.

Physiological Role of the Left Atrium

The left atrium has three major functions:

Reservoir Function: During ventricular systole, the mitral valve is closed, and blood from the pulmonary veins fills the left atrium.
Conduit Function: During early diastole, the mitral valve opens, allowing passive flow into the left ventricle.
Booster Pump Function: During late diastole (atrial systole), the left atrium actively contracts, contributing 15–30% of left ventricular filling.

Imaging of the Left Atrium

Several imaging techniques are used to assess left atrial anatomy:

Echocardiography: Both transthoracic (TTE) and transesophageal (TEE) provide excellent assessment of size, thrombus presence, and valve relationships.
CT & MRI: Used for detailed anatomical mapping, particularly before ablation procedures.
Electroanatomical Mapping: Used in electrophysiology labs to identify conduction pathways and arrhythmogenic foci.

Clinical Relevance

Left Atrial Enlargement

Left atrial enlargement is a common echocardiographic finding and may result from:

Chronic mitral valve disease
Left ventricular dysfunction
Hypertension
Chronic atrial fibrillation

It is an independent predictor of cardiovascular events.

Atrial Fibrillation

Atrial fibrillation (AF) often originates from triggers near the pulmonary vein ostia. Catheter ablation procedures frequently target these areas to isolate abnormal electrical activity.

Thromboembolism

The left atrial appendage is the most common site of thrombus formation in AF, which can lead to embolic stroke. Left atrial appendage occlusion devices may be implanted in high-risk patients.

Mitral Valve Disorders

Mitral stenosis and regurgitation directly affect left atrial pressure and size. Long-standing mitral stenosis can lead to massive left atrial enlargement, sometimes visible on chest X-ray.

Tumors and Masses

Left atrial myxoma is the most common primary cardiac tumor. It can obstruct the mitral valve or embolize.

Surgical and Interventional Considerations

Understanding left atrial anatomy is essential for:

Mitral valve repair or replacement
Maze procedure for AF
Pulmonary vein isolation techniques
Left atrial appendage occlusion or excision