Many theories have been proposed to describe the diastolic filling phase of the heart, which may be sorted out into two broad categories. The classical and most accepted theories assume filling to be passive: once the mitral valve is open, the pressure of the venous return is large enough to drive the blood flow from the atrium to the ventricle. The theories in the other category assume an active filling: the ventricle behaves as a suction pump.

The starting point for most of the theories refuting the passive filling is the discussion of the fact that during the diastole, atrial and ventricular pressures are nearly identical, and the small pressure difference between the two chambers is not large enough to explain the passive filling of the heart.

Despite many experimental evidence supports the active filling, the theory of passive filling still remains in the minds of most cardiologists. I don't know the reason behind it. If anyone can answer this, it will be very useful for my research.