Right atrium

The right atrium is the cardiac chamber that receives deoxygenated blood from the systemic circulation via the cranial and caudal vena cavae and the coronary sinus, and directs it into the right ventricle. It consists of an appendage (right auricle), a venous part (Sinus of venae cavae), and a vestibule (right atrioventricular orifice), and is separated from the left atrium by the interatrial septum.

Anatomy of the right atrium in dogs

General position and orientation

The right atrium is the most cranial and right-sided chamber of the canine heart. It lies dorsocranially to the right ventricle and forms much of the right cardiac border on thoracic radiographs. It receives systemic venous blood and directs it into the right ventricle.

External anatomy

Externally, the right atrium is less prominent than the right ventricle but contributes to the cranial portion of the right cardiac silhouette. The right auricle (auricula dextra) is a thin-walled, ear-shaped appendage projecting cranially and slightly leftward. It overlaps the root of the pulmonary trunk and increases atrial capacity.

Internal anatomy

The right atrium consists of two embryologically and structurally distinct parts:

1 - Sinus of venae cavae (Sinus venarum; Smooth part of right atrium) This forms the caudodorsal portion of the right atrium. It has a smooth endocardial surface and receives:

• Caudal vena cava (opening caudodorsally)

• Cranial vena cava (opening cranially)

• Right azygos vein that in some specises opens near the cranial vena cava

• Coronary sinus that opens ventromedially, near the atrioventricular junction.The coronary sinus returns venous blood from the myocardium and is guarded by a small valvular fold (valve of the coronary sinus).

2 - Atrium proper (pectinate part) This forms the cranioventral portion of the chamber and includes the auricle. The internal surface is characterized by prominent pectinate muscles (musculi pectinati), which are parallel muscular ridges improving atrial contraction efficiency.

Crista terminalis

A prominent muscular ridge, the crista terminalis, separates the smooth sinus venarum from the pectinate atrium proper. Internally, it runs from the opening of the cranial vena cava to that of the caudal vena cava. Externally, it corresponds to the sulcus terminalis.

Interatrial septum

The medial wall of the right atrium is formed by the interatrial septum. A shallow depression, the fossa ovalis, marks the remnant of the fetal foramen ovale. The surrounding raised margin is the limbus fossae ovalis. In normal dogs, the foramen ovale is functionally closed after birth.

Right atrioventricular orifice

The right atrium communicates with the right ventricle via the right atrioventricular (AV) orifice, guarded by the tricuspid valve. The atrial side of this opening is bordered by fibrous tissue of the cardiac skeleton.

Endocardium, myocardium, and epicardium

• Endocardium: thin endothelial lining, smooth in the sinus venarum, thicker over pectinate muscles

• Myocardium: thinner than ventricular myocardium; composed of atrial cardiac muscle fibers arranged for rapid conduction

• Epicardium: outer serous layer, continuous with the visceral pericardium

Conduction system relevance

The sinoatrial (SA) node is located in the right atrial wall near the junction of the cranial vena cava and the atrium, adjacent to the crista terminalis. This makes the right atrium the primary pacemaker region of the heart.

Vascular supply

Arterial supply is mainly from branches of the right coronary artery. Venous drainage occurs via small cardiac veins into the coronary sinus.

Innervation

The right atrium receives autonomic innervation from the cardiac plexus.