Semilunar Valves: Anatomy, Function, and Clinical Relevance

The semilunar valves—pulmonary and aortic—prevent backflow from the great arteries into the ventricles during diastole.

Structure

• Three crescent-shaped cusps (leaflets) each, composed of endothelialized fibrous tissue (fibrosa, ventricularis, spongiosa).
• No chordae tendineae or papillary muscles (unlike AV valves).
• Cusps meet at commissures; central coaptation line.

Location and Flow

• Pulmonary valve: Between right ventricle and pulmonary trunk; guards pulmonary circulation.
• Aortic valve: Between left ventricle and aorta; guards systemic circulation.

Function

• Systole: Ventricular pressure > arterial pressure → cusps open passively; low resistance, non-turbulent flow.
• Diastole: Arterial pressure > ventricular pressure → cusps close; elastic recoil of sinuses of Valsalva aids closure (no backflow).

Clinical Correlates

• Stenosis: Narrowed orifice → pressure overload, hypertrophy (AS: LVH; PS: RVH).
• Regurgitation: Incomplete closure → volume overload, dilation (AR: LV dilation; PR: RV dilation).
• Bicuspid aortic valve (BAV): 1–2% population; predisposes to early AS/AR, aortopathy.
• Infective endocarditis: High shear stress on cusps; vegetation on low-pressure side.

Echocardiography (TTE/TEE) is primary imaging; catheterization for hemodynamics.