During which phase does the left atrium fill with blood?

The left atrium fills with blood primarily during the ventricular diastole phase. This is the phase of the cardiac cycle when the ventricles relax and the pressure within them decreases. As the ventricles relax, the atrioventricular (AV) valves (specifically, the mitral valve) open, allowing blood to flow from the left atrium into the left ventricle.

During this time, the left atrium is receiving blood from the pulmonary veins, which brings oxygenated blood from the lungs. This process of filling continues as the left atrium expands. The filling of the left atrium is crucial for maintaining an adequate stroke volume and ensuring that the heart pumps enough blood to meet the body's needs.

In contrast, atrial systole refers to the contraction of the atria, which helps finish filling the ventricles but does not primarily account for the initial filling of the left atrium. Also, isovolumetric relaxation happens after ventricular systole when all valves are closed and there is no change in volume in the ventricles, and it does not involve the filling of the atria.