The right atrium is one of the four chambers of the heart. It receives deoxygenated blood from the systemic circulation and delivers it to the right ventricle through the tricuspid valve.
Location and External Features
The right atrium forms the right border of the heart and is located anterior and to the right of the left atrium. It is a thin-walled chamber that receives blood from the superior vena cava, inferior vena cava, and coronary sinus.
The external appearance of the right atrium is characterized by:
- Auricle (Right Atrial Appendage): A triangular, ear-shaped muscular pouch that extends anteriorly and overlaps the ascending aorta. The auricle increases atrial capacity without increasing chamber diameter.
- Sulcus Terminalis: A shallow vertical groove on the external surface, running between the superior vena cava and the inferior vena cava. It marks the internal location of the crista terminalis.
Internal Anatomy
The interior of the right atrium is divided into two regions by the crista terminalis:
Sinus Venarum
The posterior portion of the right atrium, smooth-walled and derived from the embryologic sinus venosus. It receives the venous tributaries:
Superior Vena Cava: Opens into the superior posterior aspect of the right atrium. It has no valve and drains blood from the upper body, head, and upper limbs.
Inferior Vena Cava: Opens into the inferior posterior aspect. It has a rudimentary valve (eustachian valve) that directs blood from the inferior vena cava toward the foramen ovale during fetal life.
Coronary Sinus: Opens between the inferior vena cava orifice and the tricuspid valve. It drains most of the cardiac venous blood. The coronary sinus orifice is guarded by the thebesian valve.
Atrial Septum
The medial wall of the right atrium is formed by the interatrial septum. Key features include:
Fossa Ovalis: An oval depression in the lower part of the septum. This is the remnant of the foramen ovale, which allows blood to bypass the pulmonary circulation during fetal life.
Limbus of Fossa Ovalis: The prominent muscular rim surrounding the fossa ovalis.
Triangle of Koch: A clinically important region bounded by the tricuspid valve annulus, the coronary sinus orifice, and the tendon of Todaro. The atrioventricular node is located within this triangle.
Pectinate Muscles
The anterior portion of the right atrium (including the auricle) contains pectinate muscles — parallel muscular ridges that resemble the teeth of a comb. These muscles:
- Originate from the crista terminalis
- Extend into the auricle
- Increase the contractile capability of the atrial wall
Crista Terminalis
A prominent muscular ridge that separates the smooth posterior wall (sinus venarum) from the muscular anterior wall. It extends from the superior vena cava orifice to the inferior vena cava orifice.
Tributaries
The right atrium receives blood from:
- Superior vena cava
- Inferior vena cava
- Coronary sinus
- Anterior cardiac veins (directly from the anterior right ventricle)
- Thebesian veins (small veins that drain directly from the myocardium)
Dimensions and Capacity
The right atrium can hold approximately 100-120 mL of blood. Its dimensions vary with body size, but average measurements are:
- Vertical diameter: 5-6 cm
- Transverse diameter: 3-4 cm
- Wall thickness: 2-3 mm
Clinical Significance
Atrial Septal Defect (ASD)
A congenital defect in the interatrial septum, most commonly the ostium secundum type involving the fossa ovalis region. Blood shunts from left to right, causing right ventricular volume overload.
Patent Foramen Ovale (PFO)
A persistent opening of the foramen ovale, present in approximately 25% of the population. Usually asymptomatic but can allow paradoxical embolism where a clot crosses from right to left.
Atrial Fibrillation
Abnormal electrical activity originating in the atria, often triggered by the pulmonary vein ostia (in the left atrium) or from the crista terminalis region. Results in irregular heart rhythm and increased stroke risk.
Right Atrial Enlargement
Can result from tricuspid valve disease, pulmonary hypertension, or right ventricular failure. Seen on ECG as tall P waves (P pulmonale).