Gross Anatomy
Four-chambered muscular organ (~300 g) in the middle mediastinum. Apex points left/inferiorly (5th ICS, MCL). Base faces right/superiorly.
| Chamber | Wall | Key Feature |
|---|---|---|
| Right atrium | Thin | SVC/IVC inflow; fossa ovalis |
| Right ventricle | 3-4 mm | Moderator band; crista supraventricularis |
| Left atrium | Thin | 4 pulmonary veins; most posterior chamber |
| Left ventricle | 8-12 mm | Thickest; papillary muscles |
Pericardium
- Fibrous pericardium — tough outer layer; attaches to diaphragm
- Visceral pericardium (epicardium) — directly on heart; contains coronary vessels
- Pericardial cavity — 15-50 mL serous fluid; reduces friction
Cardiac tamponade: Beck's triad = hypotension + JVD + muffled heart sounds. Pulsus paradoxus >10 mmHg. Treat with pericardiocentesis.
Heart Valves
| Valve | Type | Location | Sound |
|---|---|---|---|
| Tricuspid | AV; 3 leaflets | R atrium → R ventricle | S1 (closes) |
| Pulmonic | Semilunar; 3 cusps | RV → pulmonary trunk | S2 (closes) |
| Mitral | AV; 2 leaflets | L atrium → L ventricle | S1 (closes) |
| Aortic | Semilunar; 3 cusps | LV → aorta | S2 (closes) |
S1 = AV valves close (start of systole). S2 = semilunar valves close (start of diastole). S3 = ventricular filling = HF/volume overload. S4 = atrial kick against stiff ventricle = HTN, AS, HCM.
Auscultation sites clockwise "APT M": Aortic (R 2nd ICS), Pulmonic (L 2nd ICS), Tricuspid (L lower sternal border), Mitral (L 5th ICS MCL = apex).
Coronary Arteries
| Artery | Branches | Territory |
|---|---|---|
| LCA → LAD | Diagonal, septal perforators | Anterior LV wall, anterior 2/3 IVS, apex |
| LCA → LCx | Obtuse marginals | Lateral/posterior LV |
| RCA | SA nodal (60%), AV nodal (85%), marginals, PDA | RV, inferior LV, posterior IVS, SA/AV nodes |
Dominance: Right (~70%), Left (~10%), Co-dominant (~20%) — determined by which gives off PDA.
MI territories: LAD → anterior (V1-V4). RCA → inferior (II, III, aVF); RCA also → AV block. LCx → lateral (I, aVL, V5-V6). RCA occlusion → complete heart block (AV nodal ischemia).
Conduction System
| Structure | Rate | Notes |
|---|---|---|
| SA node | 60-100 bpm | Dominant pacemaker; R atrium near SVC; RCA 60% |
| AV node | 40-60 bpm | Floor of RA; delay impulse; RCA 85% |
| Bundle of His | — | Through fibrous skeleton; enters IVS |
| L+R bundle branches | — | LBBB = LV disease; RBBB = RV strain/congenital |
| Purkinje fibers | 20-40 bpm | Fastest conduction; subendocardial |
Wolff-Parkinson-White: accessory Bundle of Kent → delta wave + short PR + wide QRS. Risk of SVT. Treatment: ablation. Avoid digoxin/AV nodal agents.
Microscopic Anatomy
Cardiac Muscle Features
- Branching cells; single central nucleus
- Intercalated discs — gap junctions (electrical coupling, connexin 43) + desmosomes (mechanical coupling)
- T-tubules at Z lines (vs A-I junction in skeletal muscle)
- Abundant mitochondria (~30% cell volume) for aerobic demands
Frank-Starling law: increased preload (end-diastolic volume/stretch) → increased stroke volume. Length-dependent activation of actin-myosin crossbridges. Preload = LVEDV. Afterload = aortic pressure/systemic vascular resistance.
Embryology
Heart develops from splanchnic lateral plate mesoderm (cardiogenic region) at ~week 3; first organ to function (~day 22).
- Two heart tubes fuse → single primitive tube
- Cardiac looping (week 4): D-loop (rightward bending) → forms chambers
- Foramen ovale — R→L shunt in fetus; closes at birth (↑LA pressure)
- Ductus arteriosus — PA → aorta in fetus; closes with ↑O2 and ↓PGE2; indomethacin closes PDA; PGE1 keeps open
VSD = most common CHD overall. ToF = most common CYANOTIC CHD after infancy (boot-shaped heart). D-TGA = most common cause of cyanosis in first 24 hours. ASD = fixed split S2.
Clinical Pearls
| Condition | Key Features | Pearl |
|---|---|---|
| STEMI | ST elevation; troponin rise; complete occlusion | Door-to-balloon <90 min |
| NSTEMI | Troponin rise; no ST elevation | Partial occlusion; ACS protocol |
| HFrEF | EF <40%; dyspnea, edema, fatigue | ACEi/ARB + BB + MRA + diuretics |
| Aortic stenosis | Syncope, angina, dyspnea triad | Survival: angina 5yr, syncope 3yr, dyspnea 2yr |
| Endocarditis | Fever + new murmur + emboli | IV drug use → tricuspid; Duke criteria |
Cardiac biomarkers: Troponin I/T (rise 3-6h, peak 24h, normalize 7-10d) = most specific for MI. BNP/NT-proBNP = HF diagnosis and prognosis. CK-MB useful for re-infarction (normalizes in 48-72h).