Chapter 18: The Heart

Heart Basics and Heart Walls

The heart functions as two pumps side by side, each serving a distinct circuit: the pulmonary circuit and the systemic circuit. Its structure and location are optimized for efficient blood flow throughout the body.

• Pulmonary circuit: Right side of the heart pumps blood to the lungs for gas exchange (drops off CO2, picks up O2).

• Systemic circuit: Left side pumps oxygen-rich blood to the body.

• Atria: Thin-walled receiving chambers.

• Ventricles: Thick-walled pumping chambers; left ventricle wall is ~3x thicker due to higher resistance in systemic circuit.

• Location: Mediastinum, resting on the diaphragm; apex points to the left hip.

Pericardium and Heart Wall Layers

• Fibrous pericardium: Tough, outer layer; protects, anchors, prevents overfilling.

• Serous pericardium: Parietal layer lines fibrous pericardium; visceral layer (epicardium) covers heart surface.

• Pericardial cavity: Fluid-filled space reduces friction.

• Epicardium: Outermost layer; visceral serous pericardium.

• Myocardium: Middle layer; cardiac muscle; contains fibrous cardiac skeleton.

• Endocardium: Innermost layer; smooth lining continuous with vessel endothelium.

Cardiac skeleton: Dense collagen/elastic network; anchors muscle fibers, supports valves, limits spread of action potentials.

Clinical link: Pericarditis (inflammation), cardiac tamponade (fluid compresses heart).

Chambers and Landmarks

The heart contains four chambers, each with unique features and functions.

• Interatrial septum: Contains fossa ovalis (remnant of fetal foramen ovale).

• Interventricular septum: Separates ventricles.

• Trabeculae carneae: Muscle ridges inside ventricles.

• Papillary muscles & chordae tendineae: Anchor AV valve cusps.

• Coronary sulcus: Groove at atria/ventricles junction.

• Anterior/posterior interventricular sulci: Surface grooves marking septum.

Flow of Blood Through the Heart

Blood follows a specific pathway through the heart, passing through valves and chambers in sequence.

1. Body (systemic veins) → SVC/IVC/coronary sinus → Right atrium

2. Right atrium → Tricuspid valve → Right ventricle

3. Right ventricle → Pulmonary semilunar valve → Pulmonary trunk → Pulmonary arteries → Lungs

4. Lungs → Pulmonary veins (4) → Left atrium

5. Left atrium → Mitral (bicuspid) valve → Left ventricle

6. Left ventricle → Aortic semilunar valve → Aorta → Systemic arteries → Capillaries → Body

Arteries: Carry blood away from the heart (regardless of oxygenation). Veins: Carry blood toward the heart. Pulmonary arteries: Carry O2-poor blood.

Valves and Pressure Changes

Heart valves ensure unidirectional blood flow, opening and closing in response to pressure differences.

• AV Valves: Tricuspid (right, 3 cusps), Mitral/bicuspid (left, 2 cusps)

• Open when atrial pressure > ventricular pressure; close when ventricles contract.

• Chordae tendineae and papillary muscles prevent valve eversion.

• Semilunar (SL) Valves: Pulmonary and Aortic (3 crescent cusps each)

• Open when ventricular pressure > arterial pressure; close when ventricles relax.

• No valves between great veins and atria; backflow prevented by inertia and contraction.

• Valve disorders: Incompetent (backflow, murmur), Stenosis (narrow, high-pitched murmur).

Coronary Circulation

The heart receives its own blood supply via the coronary arteries and veins.

• Coronary arteries: Arise from the base of the aorta; encircle heart in coronary sulcus.

• Blood delivered mainly during ventricular relaxation.

• Heart receives ~1/20 of body's blood supply.

• Cardiac veins: Great, middle, small cardiac veins merge into coronary sinus → right atrium.

• Some anterior cardiac veins drain directly into right atrium.

Clinical link: Angina pectoris (temporary O2 lack), myocardial infarction (MI, prolonged blockage, scar tissue).

Cardiac Muscle vs. Skeletal Muscle

Cardiac muscle has unique structural and functional features compared to skeletal muscle.

Intercalated discs: Desmosomes (hold cells together) + gap junctions (allow ions/APs to pass).

Conduction System and the Action Potential

The heart's conduction system enables autorhythmicity, coordinating contraction through specialized pacemaker cells.

1. SA node: Right atrial wall; ~75 bpm; pacemaker.

2. AV node: Inferior interatrial septum; 0.1 s delay; ~50 bpm.

3. AV bundle (bundle of His): Only electrical link between atria and ventricles.

4. Bundle branches: Right and left.

5. Purkinje fibers: Subendocardial network; ventricles contract from apex upward.

• Pacemaker cell action potential:

  • Pacemaker potential: K+ channels close, slow Na+ channels open → drifts toward threshold

  • Depolarization: Ca2+ channels open → Ca2+ influx

  • Repolarization: Ca2+ channels close, K+ channels open → K+ efflux

• Contractile cell action potential:

  • Depolarization: Fast Na+ channels open → Na+ influx

  • Plateau: Slow Ca2+ channels open, Ca2+ enters; K+ efflux balances

  • Repolarization: Ca2+ channels close, K+ channels open; Ca2+ pumped back into SR/out of cell

Cardiac AP lasts 200+ ms (skeletal muscle: 1–2 ms); long refractory period prevents tetanus.

Calcium-induced calcium release: Ca2+ from ECF triggers Ca2+ release from SR; Ca2+ binds troponin.

Extrinsic control: ANS modulates rate and force via sympathetic (↑ HR/force) and parasympathetic (↓ HR) pathways.

Clinical link: Fibrillation (uncoordinated quivering), heart block (defective AV node), ectopic focus (abnormal pacemaker).

The ECG (Electrocardiogram)

An ECG records the heart's electrical activity, reflecting the sequence of depolarization and repolarization.

Electrical events precede mechanical events (e.g., P wave → atrial contraction).

The Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including systole (contraction) and diastole (relaxation).

1. Ventricular filling (mid-late diastole): AV valves open, SL closed; >80% passive filling, atrial systole adds ~20%; ends at EDV.

2. Isovolumetric contraction: QRS → ventricles contract; AV valves close, SL valves still closed; all valves closed, volume constant.

3. Ventricular ejection: Ventricular pressure > arterial pressure; SL valves open, blood ejected (aortic ~120 mmHg, pulmonary ~24 mmHg).

4. Isovolumetric relaxation: T wave → ventricles relax; SL valves close (dicrotic notch); all valves closed; volume left = ESV; AV valves open again.

Heart sounds: "Lub" (S1): AV valves close at start of ventricular systole. "Dup" (S2): SL valves close at start of ventricular diastole. Murmurs: Abnormal sounds, usually valve problems.

Cardiac Output, Preload, Afterload

Cardiac output (CO) is the volume of blood pumped by each ventricle per minute. Stroke volume (SV) and heart rate (HR) are key determinants.

• CO formula:

• SV formula:

• Cardiac reserve: Max CO – resting CO (4–5x in non-athletes, up to 7x in athletes)

Regulation of heart rate (chronotropic factors):

• Sympathetic (SNS): Norepinephrine → ↑ HR, ↑ contractility

• Parasympathetic (vagus): Acetylcholine → ↓ HR

• Atrial (Bainbridge) reflex: ↑ venous return stretches atria → ↑ SNS output → ↑ HR

• Hormones: Epinephrine/norepinephrine (adrenal medulla), thyroxine

• Ions: Hyperkalemia (heart block, arrest), hypokalemia (arrhythmias), hypercalcemia (↑ HR/contractility), hypocalcemia (depresses heart)

• Other: Age (fetus fastest), sex (females faster), exercise (athletes lower resting HR), ↑ body temp → ↑ HR

Tachycardia: >100 bpm; Bradycardia: <60 bpm (normal in athletes). Congestive heart failure: CO too low; left-sided → pulmonary congestion/edema; right-sided → peripheral edema.

Quick Self-Quiz

1. List the 3 layers of the heart wall, outside → in: Epicardium, myocardium, endocardium

2. Which structure is the pacemaker, and its rate? SA node, ~75 bpm

3. Why does the AV node delay the impulse (0.1 s)? Lets the atria finish contracting and fill the ventricles first

4. What does the P wave represent? QRS? T? Atrial depolarization; ventricular depolarization; ventricular repolarization

5. Which ion causes the plateau in contractile cells? Ca2+ entering through slow channels (balanced by K+ leaving)

6. When are all four valves closed? Isovolumetric contraction and isovolumetric relaxation

7. What makes "lub" and "dup"? Lub = AV valves closing; Dup = SL valves closing

8. CO formula? Resting value? ; ~5.25 L/min (75 × 70 mL)

9. Preload vs. afterload? Preload = stretch before contraction (EDV/venous return). Afterload = arterial back pressure the ventricle must beat

10. Which coronary artery supplies the anterior septum and anterior ventricles? Left anterior interventricular (LAD) artery, branch of left coronary artery

Additional info: The notes have been expanded with academic context, definitions, and formulas for clarity and completeness.