Introduction
The heart is a highly organized pump that relies on unidirectional blood flow to maintain efficient circulation. This directionality is enforced by the cardiac valves, which open and close in response to pressure gradients during the cardiac cycle. Broadly, these valves are divided into two categories: atrioventricular (AV) valves and semilunar valves. The AV valves (mitral and tricuspid) regulate flow between the atria and ventricles, while the semilunar valves (aortic and pulmonary) control ejection of blood from the ventricles into the great arteries. Understanding their anatomy, structure, and function is crucial for clinicians, surgeons, and imagers, as valve pathology underlies a significant proportion of cardiovascular disease.
1. Atrioventricular (AV) Valves
Overview
AV valves are located between the atria and ventricles. They prevent retrograde flow from ventricles to atria during ventricular systole. Key features of AV valves include:
- Leaflets (cusps) — thin, pliable flaps of connective tissue lined by endocardium.
- Chordae tendineae — fibrous cords attaching the valve leaflets to papillary muscles.
- Papillary muscles — ventricular projections that contract to prevent leaflet prolapse during systole.
- Annulus — fibrous ring providing structural support to the valve.
The two AV valves are the mitral valve on the left side and the tricuspid valve on the right side.
1.1 Mitral Valve (Left AV Valve)
Anatomy
- Location: Between the left atrium and left ventricle.
- Cusps: Two leaflets — anterior and posterior.
- Chordae tendineae: Attach each leaflet to two papillary muscles (anterolateral and posteromedial).
- Annulus: Fibrous, slightly D-shaped structure that integrates with the left fibrous trigone and aortic root.
- Orifice: Normally 4–6 cm² in adults.
Function
During diastole, the mitral valve opens to allow passive and active filling of the left ventricle. During systole, ventricular contraction closes the leaflets, preventing backflow into the left atrium.
Clinical Significance
- Mitral regurgitation (MR): Incompetent closure leads to backflow into the left atrium.
- Mitral stenosis (MS): Narrowed orifice restricts left ventricular filling.
- Valve repair/replacement: Knowledge of leaflet anatomy, chordae, and papillary muscles is essential for surgical planning.
1.2 Tricuspid Valve (Right AV Valve)
Anatomy
- Location: Between the right atrium and right ventricle.
- Cusps: Three leaflets — anterior, posterior, and septal.
- Chordae tendineae: Connect leaflets to papillary muscles.
- Annulus: Fibrous ring around the tricuspid orifice.
- Orifice: Larger than mitral valve (~7–9 cm²), reflecting lower right heart pressures.
Function
The tricuspid valve allows unidirectional flow from the right atrium to the right ventricle during diastole and prevents regurgitation during systole.
Clinical Significance
- Tricuspid regurgitation (TR): Common in right ventricular dilation or pulmonary hypertension.
- Tricuspid stenosis (TS): Rare, usually rheumatic; impedes right ventricular filling.
- Surgical relevance: Knowledge of papillary muscles and chordal attachments is critical for valve repair.
2. Semilunar Valves
Overview
Semilunar valves are located at the outflow tracts of the ventricles. They consist of three cusps, lack chordae tendineae, and rely on pressure gradients and the shape of the cusps for function. The two semilunar valves are the aortic valve and the pulmonary valve.
2.1 Aortic Valve
Anatomy
- Location: Between the left ventricle and the ascending aorta.
- Cusps: Three semilunar cusps — right coronary cusp (RCC), left coronary cusp (LCC), and non-coronary cusp (NCC).
- Sinuses of Valsalva: Outpouchings behind each cusp that facilitate coronary artery flow.
- Annulus: Fibrous ring that merges with the left ventricular outflow tract and aortic root.
Function
During systole, increased left ventricular pressure forces the cusps open, allowing blood ejection into the aorta. During diastole, aortic pressure closes the cusps, preventing backflow into the left ventricle. The sinuses of Valsalva also promote coronary perfusion.
Clinical Significance
- Aortic stenosis (AS): Narrowed valve increases afterload on the left ventricle.
- Aortic regurgitation (AR): Incompetent valve allows retrograde flow into the LV.
- Valve replacement: Anatomical knowledge of cusps and sinuses is crucial for surgical or transcatheter valve implantation.
2.2 Pulmonary Valve
Anatomy
- Location: Between the right ventricle and pulmonary trunk.
- Cusps: Three semilunar cusps — anterior, left, and right.
- Sinuses: Small sinuses behind each cusp.
- Annulus: Fibrous ring surrounding the valve orifice.
Function
During systole, the pulmonary valve opens to allow blood ejection into the pulmonary artery. During diastole, it closes to prevent pulmonary backflow into the right ventricle.
Clinical Significance
- Pulmonary stenosis (PS): Obstructed outflow leads to RV hypertrophy.
- Pulmonary regurgitation (PR): Common post-surgical complication in congenital heart disease (e.g., after tetralogy of Fallot repair).
- Surgical relevance: Knowledge of cusp and sinus anatomy guides pulmonary valve repair or replacement.
3. Comparative Features of AV vs. Semilunar Valves
| Feature | AV Valves | Semilunar Valves |
|---|---|---|
| Location | Between atria and ventricles | At ventricular outflow tracts |
| Leaflets/Cusps | Mitral (2), Tricuspid (3) | Aortic (3), Pulmonary (3) |
| Chordae tendineae | Present | Absent |
| Papillary muscles | Present | Absent |
| Function | Prevent atrial backflow during systole | Prevent ventricular backflow during diastole |
| Pressure tolerance | Low-pressure valves | High-pressure valves (aortic) |
| Common pathologies | Stenosis, regurgitation | Stenosis, regurgitation, congenital malformations |
4. Functional Dynamics
The cardiac valves open and close passively in response to pressure gradients:
- Diastole: AV valves open, semilunar valves closed → ventricular filling.
- Systole: AV valves closed, semilunar valves open → ventricular ejection.
Efficient valve function minimizes turbulence, prevents regurgitation, and optimizes hemodynamics.
5. Clinical Correlations
- Echocardiography: First-line tool for visualizing valve morphology, function, and flow dynamics.
- MRI and CT: Offer 3D visualization for complex valvular lesions or surgical planning.
- Valve repair/replacement: Knowledge of leaflet anatomy, chordal attachments, and annular size is critical.
- Congenital anomalies: Bicuspid aortic valve, Ebstein anomaly (tricuspid malformation), pulmonary stenosis — understanding normal anatomy aids recognition and management.
6. Embryological Considerations
- AV valves: Formed from endocardial cushions; chordae and papillary muscles differentiate from ventricular myocardium.
- Semilunar valves: Derived from truncal and bulbar ridges; proper cusp formation is essential for competence.
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