Semilunar valves

Where are the semilunar valves located?

The semilunar valves close the openings of the aorta and the pulmonary trunk. Their skeletotopy on the anterior chest wall:

• Aortic orifice — projected behind the left sternal border at the level of the 3rd intercostal space.
• Pulmonary trunk orifice — located more laterally than the aortic orifice, at the left sternal border at the level of the attachment of the cartilage of the 3rd rib.

Structure of the valve apparatus

The cusps of all valves attach to the fibrous ring, which bounds the valve orifice and serves as a supporting skeleton. The semilunar valves consist of three semilunar cusps (pockets).

Pulmonary valve (valva trunci pulmonalis): Includes three semilunar cusps:

• Anterior (valvula semilunaris anterior);
• Right (valvula semilunaris dextra);
• Left (valvula semilunaris sinistra).

At the middle of the free margin of each cusp is the nodule (nodulus valvulae semilunaris), and on either side of it are the lunules (lunulae valvulae semilunaris).

Aortic orifice and aortic sinuses: The aortic bulb (bulbus aortae) is the initial widened part of the aortic wall. It forms three bulges — the aortic sinuses (sinus aortae), corresponding to the positions of the three semilunar cusps:

• Right (sinus aortae dexter) — from which the right coronary artery (a. coronaria dextra) originates;
• Left (sinus aortae sinister) — from which the left coronary artery (a. coronaria sinistra) originates;
• Posterior (sinus aortae posterior) — without branches.

How do the semilunar valves work?

The function of the valves is related to the phases of the cardiac cycle and ventricular pressure.

• Opening: after reaching a pressure of 15 mmHg in the right ventricle and 80 mmHg in the left ventricle, the semilunar valves open, and blood ejection begins. When blood leaves the heart, the pockets are pressed against the walls of the aorta and pulmonary trunk, not impeding blood flow.
• Closure: at the onset of ventricular relaxation, the pressure within them drops below the pressure in the aorta and pulmonary trunk. A reverse blood flow occurs, which snaps the semilunar valves shut.

The impact of blood against the closed semilunar valve cusps and their vibration form the second heart sound (diastolic S2).

Clinical significance and auscultation

Auscultation points: S2, associated with the semilunar valves, is best heard at the base of the heart:

• Aorta — in the 2nd right intercostal space near the sternum.
• Pulmonary trunk — in the 2nd left intercostal space near the sternum.

Pathological changes and murmurs:

• Dicrotic pulse: at the beginning of diastole, part of the blood in the aorta moves backward toward the heart and strikes the closed semilunar valves. This hydraulic shock forms a secondary dicrotic wave following the primary one. Normally, it is recorded only instrumentally on a sphygmogram; it is rarely palpable, mainly occurring when peripheral vascular tone is reduced.
• Protodiastolic murmurs: occur immediately after S2 and are associated with blood regurgitation through the semilunar valves — aortic or pulmonary.
• Aortic regurgitation: produces a blowing, protodiastolic, high-frequency murmur in the 2nd right intercostal space and at Erb's point. A faint murmur is auscultated with the diaphragm of the stethoscope pressed firmly against the chest, with the patient sitting, leaning forward, and holding their breath in expiration.
• Pulmonary regurgitation: is usually a consequence of severe pulmonary hypertension leading to dilation of the valve's fibrous ring. Its specific name is the Graham Steel murmur; localization is the 2nd left intercostal space.

Class and exam questions

What else to study on this topic

• Aorta and aortic sinuses
• Pulmonary trunk
• Atrioventricular valves
• Phases of the cardiac cycle

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.