Give the exact location of Pulmonary semilunar valve. When does it close?
Topic: Pulmonary semilunar valve
Answer
The pulmonary semilunar valve lies at the opening of the RIGHT VENTRICLE into the PULMONARY ARTERY.
It CLOSES at the beginning of ventricular DIASTOLE — when the ventricles relax and the pressure in them falls below that in the pulmonary artery — so preventing blood from flowing back into the ventricle.
One mark for the exact location and one for when it closes, so answer both halves.
THE REASON IT CLOSES follows from pressure, which is the rule governing every valve in the heart: a valve opens in the direction of flow and closes against it. During systole the ventricle pressure is HIGHER than the artery, so blood is driven out and the valve is open. The moment the ventricle relaxes, the pressure falls BELOW that in the artery and the blood would run back — which snaps the valve shut.
The closing of BOTH semilunar valves produces the second heart sound, "DUP", which marks the beginning of diastole.