Subvalvular apparatus: The papillary muscles and the chordae tendineae, known as the subvalvular apparatus, hold the valves closed so that they do not prolapse.

Papillary muscles, together with the chordae tendineae, make up the subvalvular apparatus. 

Blood flows through an atrioventricular (AV) valve Blood flows through an atrioventricular (AV) valve when blood pressure in the atria becomes high during atrial systole and blood pressure in the ventricles becomes low enough during ventricular diastole, creating a blood pressure gradient.

Papillary muscles, finger-like projections from the wall of the ventricles, connect the chordae tendineae (heartstrings) to the cusps of the atrioventricular valves. This connection prevents the valve from prolapsing under pressure.