During which phase of the heart cycle is blood received in the atria?

Blood is received in the atria during diastole, which is the phase of the heart cycle when the heart muscles relax. During this time, the atrioventricular (AV) valves (the tricuspid and mitral valves) are open, allowing blood to flow from the atria into the ventricles. The relaxed state of the heart enables the atria to fill with blood returning from the body through the superior and inferior vena cavae and from the lungs through the pulmonary veins.

In contrast, during systole, the heart muscle contracts, leading to the pumping of blood out of the ventricles and into the arteries. Therefore, the atria do not receive blood during systole; instead, they are filled during the preceding diastolic phase. The isovolumetric contraction occurs at the beginning of systole when the ventricles contract but all heart valves are closed, which means there is no blood entering the atria during this brief transitional phase either. Thus, diastole is specifically characterized by the filling of the atria with blood.