In anatomy, the<unk> s are valves in the<unk>  that maintain the unidirectional flow of blood by opening and closing depending on the difference in pressure on each side. The mechanical equivalent of the<unk> s would be the reed valves.
There are four valves of the<unk>  (not counting the valve of the coronary sinus and valve of the inferior vena cava):
The two atrioventricular (AV) valves ensure blood flows from the atria to the ventricles, and not the other way.
The two semilunar (SL) valves are present in the arteries leaving the<unk> , and they prevent blood flowing back from the arteries into the ventricles.
The sound of the<unk> s shutting causes the<unk>  sounds.
Heart valve
Cusps of heart valves
Pulmonary circulation
Coronary circulation
Pulmonary valve
Systemic circulation
Blood vessel
Airway