Left atrium

At a glance

English

Left atrium

Latin (TA2)

Atrium sinistrum

System

Heart

Region

Trunk

Side

Single, on the midline

About the left atrium

The left atrium is the most posterior of the four heart chambers. It forms most of the base of the heart, the surface that faces backwards towards the vertebral column, and sits directly in front of the oesophagus, with the descending aorta and the left main bronchus close by. Because of this position an enlarged left atrium can press on the oesophagus, and the chamber is examined well from behind with an ultrasound probe passed down the gullet.

Its cavity is roughly cuboidal with smooth walls over most of its inner surface. The four pulmonary veins, usually two from each lung, open into its back wall without valves. From its upper front corner projects the left auricle, or left atrial appendage, a narrow, ear-shaped pouch that curls forwards round the left side of the pulmonary trunk. Unlike the rest of the chamber, the appendage is lined with ridged pectinate muscle. The interatrial septum forms its right wall, and here a faint mark shows the valve of the oval foramen that closes the fetal opening between the atria.

Below and in front, the left atrium leads into the left ventricle by way of the mitral valve. It works mainly as a reservoir and conduit: while the ventricle contracts it fills from the lungs, and when the ventricle relaxes the blood flows on. A final atrial contraction just before ventricular systole tops up the ventricle.

Clinically the left atrium matters most in atrial fibrillation. When it quivers instead of contracting, blood can pool and clot in the appendage, and a clot that escapes may travel to the brain and cause a stroke. For this reason the appendage is sometimes closed with a device or tied off during heart surgery. The atrium is also enlarged by mitral valve disease, and on a frontal chest X-ray a bulging appendage can straighten the left heart border.

Function and supply

Function

Collects oxygenated blood from the four pulmonary veins and delivers it through the mitral valve to the left ventricle, acting as reservoir, conduit and booster pump.

Nerve supply

Autonomic fibres from the cardiac plexus: sympathetic fibres from the cervical and upper thoracic ganglia and parasympathetic fibres from the vagus nerves.

Blood supply

Mainly the circumflex branch of the left coronary artery and its atrial branches; venous blood leaves through the small oblique atrial vein and other veins into the coronary sinus.