Left atrium

The left atrium (atrium sinistrum) is the caudodorsal chamber of the heart that receives oxygenated blood from the pulmonary circulation and delivers it to the left ventricle.

Location and external features

The left atrium forms most of the caudal border (basis cordis) of the heart. It lies dorsal to the left ventricle and laterocaudal to the right atrium.

The left auricle (auricula sinistra) is a narrow, elongated, ear-shaped muscular pouch that extends cranially and overlaps the root of the pulmonary trunk. The auricle increases atrial capacity and contains pectinate muscles.

Internal morphology

The left atrium is composed of:

• A main atrial body (sinus venarum pulmonalium)

• The left auricle

a) Atrial body

The internal surface of the atrial body is mostly smooth, reflecting its embryological derivation from incorporation of the pulmonary veins. The smooth region receives the pulmonary veins and forms the majority of the chamber.

Dogs typically have multiple pulmonary veins (commonly 4–6) entering the dorsal wall of the left atrium. These veins do not have valves at their atrial openings. The orifices are round to oval and are located dorsally and somewhat laterally.

The medial wall of left atrium is formed by the interatrial septum with the valve of foramen ovale that in dogs is only seen as remnant of where this valve closed and sealed, presenting as an irregularity of the smooth septal surface.

b) Left auricle

The auricle contains prominent musculi pectinati (pectinate muscles), which are parallel muscular ridges that increase contractile force. These are confined mainly to the auricle and do not extend extensively into the atrial body, in contrast to the right atrium.

Atrioventricular orifice

The left atrioventricular orifice is located ventrally and slightly cranially in the atrium. It communicates with the left ventricle and is guarded by the left atrioventricular valve [Bicsupid valve; Mitral valve]

The atrial myocardium around the AV orifice contributes to the fibrous cardiac skeleton (anulus fibrosus).

Wall structure

The wall of the left atrium consists of:

• Endocardium (simple squamous endothelium overlying connective tissue)

• Myocardium (cardiac muscle)

• Epicardium (visceral pericardium)

The atrial myocardium is thinner than that of the left ventricle but slightly thicker than the right atrium due to higher pulmonary venous return pressures. The muscle fibers are arranged in circular and longitudinal bundles, facilitating effective contraction during atrial systole.

Vascularization and innervation

Arterial supply: Primarily from branches of the left coronary artery (especially the circumflex branch).

Venous drainage: Through cardiac veins into the coronary sinus, which empties into the right atrium.

Innervation: Autonomic innervation via the cardiac plexus

The left atrium has three functional phases:

• Reservoir phase: collects blood during ventricular systole.

• Conduit phase: passively transfers blood to the left ventricle during early diastole.

• Booster pump phase: actively contracts in late diastole to augment ventricular filling.