Atrium

Structure

The right atrium receives deoxygenated blood through the superior and inferior venae cavae. The left atrium receives oxygenated blood from the four pulmonary veins. A thin wall called the interatrial septum separates them.

Function

During diastole the atria fill passively. A brief atrial contraction at the end of diastole tops off the ventricles with roughly 20 percent of their final volume before ventricular contraction begins.

Conduction

The sinoatrial node, located in the wall of the right atrium near the superior vena cava, initiates the heartbeat. Its impulse spreads across both atria before reaching the atrioventricular node.