Answer
When the ventricles are full, the ventricular walls contract. This increases the pressure in the ventricle. The pressure forces the cusps of the atrioventricular valve upward and closes the valve. At the same time, the papillary muscles contract and tighten the chordae tendineae. This prevents the cusps of the valves everting (folding back into the atrium) which would cause the blood under pressure to flow back into the atrium.
Work Step by Step
When a ventricle is full, it contracts ; the increased pressure forces the atrioventricular valve cusps outward. When they meet, they close the opening between atrium and ventricle. The ventricular pressure would evert the valvular cusps, but the papillary muscles contract when the ventricular myocardium contracts. The papillary muscle contraction tightens the chordae tendineae; this prevents the eversion of the valvular cusps and the leaking of blood back into the atrium.
