Atrium

The atria (singular: atrium) are the two upper chambers of the heart. The right atrium receives deoxygenated blood returning from the body through the superior and inferior vena cavae, and passes it to the right ventricle. The left atrium receives oxygenated blood returning from the lungs through the pulmonary veins, and passes it to the left ventricle.

The atria are thinner-walled and generate lower pressures than the ventricles, as they only need to move blood the short distance into the adjacent ventricle rather than pumping it around the circulation. The two atria are separated by the interatrial septum; a hole in this wall (atrial septal defect) is one of the more common congenital heart defects.

The atria are the site of several important arrhythmias. Atrial fibrillation occurs when chaotic electrical activity in the atria (particularly around the pulmonary vein openings in the left atrium) produces an irregular, often rapid ventricular response. Atrial flutter involves a more organised circular electrical circuit in the right atrium. Both conditions are treated with rate or rhythm control strategies and, where there is stroke risk, anticoagulation.

In the context of cardiac arrest, atrial arrhythmias are rarely the direct cause (ventricular arrhythmias are responsible for the vast majority of arrests), but underlying atrial disease and the haemodynamic effects of poorly controlled atrial fibrillation can contribute to structural heart disease that increases overall risk.