Pulmonary Valve

The pulmonary valve is one of the two semilunar valves of the heart. It regulates blood flow from the right ventricle into the pulmonary trunk, ensuring that blood flows in only one direction—toward the lungs. Structurally, it consists of three semilunar cusps and functions passively by responding to pressure differences between the right ventricle and pulmonary artery during the cardiac cycle.

Location

The pulmonary valve is situated at the superior aspect of the right ventricle, specifically at the outflow tract known as the conus arteriosus (or infundibulum). It marks the junction between the right ventricular chamber and the pulmonary trunk. It is positioned anterior and to the left of the aortic valve, and it is the most superior of all four heart valves in anatomical position.

Structure

The pulmonary valve is composed of three crescent-shaped (semilunar) cusps that are thin, pocket-like flaps of connective tissue covered by endothelium.

Cusps

• Anterior cusp
• Right cusp
• Left cusp

Each cusp is attached to the wall of the pulmonary trunk and features a free edge that curves upward. In the center of each free edge is a fibrous thickening called the nodule, which ensures complete closure. On either side of the nodule is a lunule, a thinner area that assists in sealing the cusps together when closed.

Pulmonary Sinuses

Behind each cusp is a shallow pocket formed by the dilation of the wall of the pulmonary trunk. These pockets are called pulmonary sinuses and help prevent the valve cusps from sticking to the vessel wall during closure. The sinuses fill with blood during diastole, pushing the cusps toward the center to close the valve.

Function

The pulmonary valve allows blood to exit the right ventricle during systole (ventricular contraction) and prevents its return during diastole (ventricular relaxation).

• During systole: Pressure in the right ventricle rises, pushing the cusps aside and allowing blood to flow into the pulmonary trunk
• During diastole: As pressure in the pulmonary trunk exceeds that in the right ventricle, the cusps fill with blood and come together to close the valve, preventing regurgitation

Microscopic Anatomy

The cusps of the pulmonary valve, like other heart valves, are composed of three histological layers:

• Fibrosa: Collagen-rich core that provides structural support
• Spongiosa: Central layer containing loose connective tissue that acts as a shock absorber
• Ventricularis: Elastic fiber-rich layer facing the ventricular lumen

The valve is avascular and relies on diffusion for nutrient exchange. It is covered by endothelium continuous with the endocardium.

Topographic Relationships

• Anterior: Sternum and left costal cartilages (2nd–3rd)
• Posterior: Ascending aorta (posterior and to the right)
• Inferior: Right ventricle
• Superior: Pulmonary trunk
• Left: Left auricle and left atrium

Surface Landmark

The pulmonary valve is best auscultated in the left second intercostal space at the parasternal line. This position corresponds to the superior left region of the sternum, where pulmonary valve sounds are transmitted most clearly.

Summary

• Valve type: Semilunar
• Number of cusps: Three (anterior, right, left)
• Function: Prevents backflow into the right ventricle
• Opens during: Ventricular systole
• Closes during: Ventricular diastole
• Best heard: Left second intercostal space