The left atrium (LA) can be thought of as having three key haemodynamic functions. During left ventricular systole and its subsequent isovolumic relaxation phase, the LA acts as a reservoir for oxygenated blood returning to the heart from the lungs. During early diastole, the LA then acts as a passive conduit for this blood to enter the left ventricle. In late diastole, atrial contraction actively boosts left ventricular filling (unless the patient is in atrial fibrillation).