Chapter 18 Heart Valves

Composition & General Role of Heart Valves

  • Made of dense irregular connective tissue (strong, irregularly-arranged collagen fibers)

  • External surface covered by simple squamous epithelium → endocardium

  • Core purpose: enforce unidirectional blood flow through heart → “vein → atrium → ventricle → artery”

    • Prevent reverse flow even under large pressure shifts during ventricular contraction

  • Valves also permit ventricles to build pressure before ejecting blood into arteries

Four-Chamber Layout (Spatial Orientation Reminder)

  • Right side: right atrium (RA) above right ventricle (RV)

  • Left side: left atrium (LA) above left ventricle (LV)

  • Visualization tip: picture heart as two side-by-side pumps sharing a wall (interventricular septum)

The Four Valves & Their Multiple Names

  1. Right AV valve

    • Structural name: Tricuspid (3 cusps/leaflets)

    • Positional name: Right atrioventricular (Right AV)

  2. Left AV valve

    • Structural names: Bicuspid (2 cusps) or Mitral (resembles a bishop’s miter hat)

    • Positional name: Left atrioventricular (Left AV)

  3. Right semilunar valve

    • AKA Pulmonary valve

    • Between RV & pulmonary trunk

  4. Left semilunar valve

    • AKA Aortic valve

    • Between LV & aorta

Cardiac Skeleton & Ring Bodies

  • Dense irregular connective-tissue framework → “fibrous skeleton” or “cardiac skeleton”

  • Each valve cusp’s fixed edge anchored to a ring body, itself fused to the cardiac skeleton

  • Roles:

    • Physically positions/steadies valves; prevents “wandering” during beating

    • Electrical insulator (connective tissue cannot carry action potentials) ⇒ isolates atrial myocytes from ventricular myocytes

    • Contains the AV node → only path by which atrial action potentials reach ventricles

Detailed Anatomy of AV Valves

  • Components

    • Ring body (fixed edge)

    • Cusps (3 right, 2 left) — thin, floppy connective-tissue sheets

    • Chordae tendineae (“heart strings”) — collagenous cords tethering free edge of cusps to ventricular wall

    • Papillary muscles — specialized ventricular myocytes that contract simultaneously with rest of ventricle

  • Functional insight

    • Chordae + papillary muscles do not open/close AV valves; instead they steady closed cusps under high pressure, preventing prolapse/inversion

Pressure-Operated Valve Mechanics (Equations)

  • Opening & closing is purely pressure-driven; cusps themselves are passive

  • Atrioventricular valves

    • P{atria} > P{ventricle} \Rightarrow \text{AV valve opens}

    • P{ventricle} > P{atria} \Rightarrow \text{AV valve closes (chordae + papillary tense)}

  • Semilunar valves

    • P{artery} > P{ventricle} \Rightarrow \text{Semilunar valve closed (cusps fill like cups)}

    • P{ventricle} > P{artery} \Rightarrow \text{Semilunar valve opens, blood ejected}

Semilunar Valve Structure & Analogy

  • Three crescent ("semi-lunar") cup-shaped cusps; imagine bringing fingertips together to form a small cup

  • Cup fills with arterial blood when arterial pressure exceeds ventricular → cusps billow together forming a tight seal

  • Aortic & pulmonary valves look identical; only difference = circuit served

Pressure Numbers (Context)

  • Resting peak arterial pressures

    • Systemic (aorta): \approx 120\,\text{mmHg}

    • Pulmonary trunk: \approx 25\,\text{mmHg}

  • Ventricles must exceed these values momentarily to open respective semilunar valves

Pathologies of Heart Valves

  1. Incompetent (Leaky) Valve

    • Cusps fail to seal → backflow (regurgitation/insufficiency/prolapse)

    • Ventricle must repump leaked blood ⇒ ↑ cardiac workload

  2. Stenotic Valve

    • Cusps cannot open fully → narrowed orifice ⇒ ↑ resistance, ventricles generate abnormally high pressures to maintain normal stroke volume

  3. Dual pathology possible (stenotic + incompetent)

  • Causes

    • Infection (e.g., bacterial endocarditis; prophylactic antibiotics for major dental work)

    • Post-inflammatory scarring (e.g., rheumatic fever in early 20th cent.)

    • Congenital malformations (e.g., bicuspid aortic valve instead of tricuspid)

    • Degenerative calcification with aging

Replacement / Repair Modalities

  • Traditional open-heart surgery (heart-lung machine) → removal & sewing of mechanical or bioprosthetic (pig) valve

  • Minimally invasive catheter approach (e.g., TAVR – transcatheter aortic valve replacement) for aortic valve; new valve deployed inside old one via femoral artery catheter

Mortality Statistics (Mayo data cited)

  • Deaths attributable to valvular disease

    • Aortic valve ≈ 61\%

    • Mitral (left AV) ≈ 15\%

    • Remaining 24\% → pulmonary + tricuspid valves

  • Higher left-side mortality correlates with higher systemic pressures

Mnemonics & Metaphors

  • Mitral like a bishop’s mitre hat (two pointed cusps)

  • Chordae tendineae = “heart strings”

  • Semilunar cusps = fingers forming a cup/crescent moon

Integrated Flow Snapshot

  1. Vein \rightarrow Atrium (low pressure)

  2. P{atria}!>!P{ventricle} \rightarrow AV valve open, blood enters ventricle

  3. Ventricular depolarization → ventricles & papillary muscles contract

  4. P{ventricle}!>!P{atria} \rightarrow AV valve snaps shut (steady by chordae)

  5. Continued contraction raises P{ventricle} above P{artery}

  6. Semilunar valve opens → blood ejected to artery

  7. Ventricles relax, P{artery}!>!P{ventricle} → semilunar cusps fill & close

  8. Cycle repeats; AV node ensures atrial depolarization precedes ventricular