1.10.3 Semilunar Valve Definition

Semilunar Valve Definition is a description of the pair of cardiac valves, the aortic and pulmonary valves, that separate the ventricles from their corresponding great arteries and regulate the forward flow of blood out of the heart during ventricular ejection. Each semilunar valve is composed of three crescent-shaped, or "half-moon," cusps that open under the force of rising ventricular pressure and close passively once ventricular pressure falls below arterial pressure, preventing the backward flow of ejected blood into the ventricles.

The Two Semilunar Valves

Each ventricle is guarded by one semilunar valve at its outflow tract.

Aortic Valve

The aortic valve lies between the left ventricle and the aorta and consists of three cusps, generally described as the right, left, and posterior (or non-coronary) cusps, two of which give rise to the coronary arteries just above their attachment.

Pulmonary Valve

The pulmonary valve lies between the right ventricle and the pulmonary artery and likewise consists of three cusps, arranged to direct deoxygenated blood toward the lungs.

Structural Features

Semilunar valves differ structurally from the atrioventricular valves in ways that reflect their distinct mechanical role.

Cusp Shape

Each cusp is shaped like a shallow pocket, so that when the valve closes, blood attempting to flow backward fills these pockets and pushes the cusps together, reinforcing closure.

Absence of Chordae Tendineae

Unlike the atrioventricular valves, semilunar valves have no chordae tendineae or papillary muscles, since the pocket-like cusp geometry and the direction of arterial back-pressure are themselves sufficient to keep the valve securely closed without additional tethering.

Functional Cycle

Semilunar valve motion is governed entirely by the pressure gradient between the ventricle and its downstream artery.

Opening Phase

During ventricular ejection, once intraventricular pressure exceeds arterial pressure, the cusps are pushed open, allowing blood to be expelled into the aorta or pulmonary artery.

Closing Phase

As ventricular contraction wanes and ventricular pressure falls below arterial pressure, the cusps snap closed, producing the second heart sound (S2) and marking the onset of isovolumetric relaxation.

Diagrammatic Summary

Clinical Relevance

Degenerative or congenital abnormalities of the semilunar valves, such as aortic stenosis, aortic regurgitation, or pulmonary valve dysfunction, alter the normal pressure–flow relationships of ventricular ejection and can significantly impair cardiac output. The passive, pressure-driven mechanics of the semilunar valves make them essential reference points for understanding both normal ventricular ejection and the pathophysiology of outflow tract valve disease.