Heart valves open and close in response to pressure changes as the chambers contract and relax. Two types of valves:
atrioventricular valves (tricuspid, bicuspid)
semilunar valves (pulmonary, aortic)
Atrioventricular valves are forced OPEN as pressure increases in the atria. Blood returning to atria forces heart places pressure on atrioventricular valves! As atria fills, A flaps hang loose into ventricles. Atria contract, forcing blood flow into ventricles. AV valves are then forced closed by ventrical pressure.
Semilunar Valves: (Pulmonary and aortic) Forced open as pressure grows in ventricles! Blood is pressing against them... so they open. Pressure from blood in the arteries makes it close.