When does the majority of pulmonary venous flow into the left atrium occur?

The majority of pulmonary venous flow into the left atrium occurs during diastole. This is the phase of the cardiac cycle when the ventricles are relaxed, and the heart is filling with blood. During diastole, the left atrium receives oxygenated blood from the pulmonary veins, and the pressure within the pulmonary veins is greater than that within the left atrium, facilitating this flow.

Diastolic filling is critical for ensuring that the left ventricle has adequate volume of blood to pump during systole, which occurs subsequently. During this time, the mitral valve is open, allowing the blood to flow freely into the left ventricle. This is a fundamental aspect of cardiac physiology and hemodynamics.

In contrast, during systole, the heart is contracting, which is when blood is being ejected from the ventricles. Thus, there is minimal or no flow from the pulmonary veins into the left atrium during this phase. Late systole could potentially have some influence on the pressure dynamics but does not represent the bulk of the pulmonary venous return.