Location of the Semilunar Valves
The semilunar valves close the aortic and pulmonary trunk orifices. Their skeletal projections (skeletope) on the anterior chest wall are:
- Aortic orifice — projects behind the left margin of the sternum at the level of the 3rd intercostal space.
- Pulmonary trunk orifice — located more to the left than the aortic orifice, at the left margin of the sternum at the level of attachment of the 3rd rib cartilage.
Structure of the Valve Apparatus
The cusps of all heart valves are anchored to a fibrous ring, which defines the valve orifice and serves as a supportive skeleton. The semilunar valves consist of three pocket-like semilunar cusps.
Pulmonary valve (valva trunci pulmonalis): Includes three semilunar cusps:
- Anterior cusp (valvula semilunaris anterior);
- Right cusp (valvula semilunaris dextra);
- Left cusp (valvula semilunaris sinistra).
At the middle of the free margin of each cusp is a nodule (nodulus valvulae semilunaris), and on either side of it are the lunules (lunulae valvulae semilunaris).
Aortic Orifice and Aortic Sinuses: Root of the aorta (bulbus aortae) is the initial expanded part of the aortic wall. It forms three dilations — aortic sinuses (sinus aortae), corresponding to the position of the three semilunar cusps:
- Right aortic sinus (sinus aortae dexter) — gives rise to the right coronary artery (a. coronaria dextra);
- Left aortic sinus (sinus aortae sinister) — gives rise to the left coronary artery (a. coronaria sinistra);
- Posterior aortic sinus (sinus aortae posterior) — gives off no branches.
Mechanism of Action
The function of the valves is coupled with the phases of the cardiac cycle and ventricular pressure.
- Opening: upon 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. As blood leaves the heart, the pockets are pressed against the walls of the aorta and pulmonary trunk without obstructing blood flow.
- Closure: at the onset of ventricular relaxation, the pressure within the ventricles drops below the pressure in the aorta and pulmonary trunk. A retrograde blood flow is generated, which snaps the semilunar valves shut.
Blood striking the closed semilunar valve cusps and their subsequent vibration produce the second heart sound (S2, diastolic).
Clinical Significance and Auscultation
Auscultation Points: The second heart sound, associated with the semilunar valves, is best heard at the base of the heart:
- Aorta — in the 2nd intercostal space to the right of the sternum.
- Pulmonary trunk — in the 2nd intercostal space to the left of the sternum.
Pathological Changes and Murmurs:
- Dicrotic pulse: at the beginning of diastole, a portion of blood in the aorta moves backward toward the heart and strikes the closed semilunar valves. This hydraulic impact creates a secondary dicrotic wave following the primary one. Normally, this is recorded only instrumentally on a sphygmogram and is rarely palpable, mostly seen when peripheral vascular tone is reduced.
- Protodiastolic murmurs: occur immediately after S2 and are associated with blood regurgitation through the semilunar valves — either the aortic or pulmonary valve.
- Aortic regurgitation (insufficiency): produces a blowing, high-pitched protodiastolic murmur in the 2nd right intercostal space and at Erb's point. A faint murmur is heard best with the diaphragm of the stethoscope pressed firmly against the chest, with the patient sitting upright, leaning forward, and holding their breath in expiration.
- Pulmonary regurgitation (insufficiency): typically results from severe pulmonary hypertension leading to dilation of the valve fibrous ring. It is specifically known as the Graham Steell murmur, localized to the second left intercostal space.