Semilunar Valves: Gatekeepers of Ventricular Ejection

The semilunar valves—aortic and pulmonic—are critical one-way gates that open during ventricular systole and close during diastole.

Function During the Cardiac Cycle

1. Systole (ventricular contraction): Pressure in ventricles > pressure in aorta/pulmonary artery → valves open → blood ejected.
2. Early diastole: Ventricular pressure falls below arterial pressure → valves snap shut → prevents backflow (regurgitation).
3. Dicrotic notch: The brief pressure rise on the arterial waveform caused by valve closure.

Anatomy

• Three semilunar cusps each (right/left/posterior for aortic; anterior/left/right for pulmonic).
• No chordae tendineae or papillary muscles (unlike AV valves).
• Cusps are thinner, more fibrous—designed for high-pressure, high-velocity flow.

Clinical Relevance

• Aortic stenosis/regurgitation: Murmurs, LV hypertrophy, heart failure.
• Pulmonic stenosis/regurgitation: Often congenital; right heart effects.
• Prosthetic valves: Mechanical (lifelong anticoagulation) vs. bioprosthetic (limited durability).

These valves ensure forward flow—every beat, every minute, for a lifetime.