AN22.1-7 | Heart & Pericardium — Summary & Reflection

REFLECT

Place your fingers on your radial pulse (wrist, thumb side). Count the beats for 15 seconds and multiply by 4 — that's your resting heart rate.

Now think about the anatomy behind each beat:

1. The SA node fires an electrical impulse (you don't feel this — it happens automatically).
2. The impulse spreads through both atria → atrial contraction fills the ventricles.
3. The AV node delays the impulse by 0.1 seconds (this is the gap between the first and second heart sounds).
4. The impulse races down the Bundle of His → bundle branches → Purkinje fibres → both ventricles contract simultaneously, pushing blood into the aorta and pulmonary trunk.
5. The coronary arteries fill during diastole (between beats) — the heart feeds itself during its 'rest' period.

Now consider: if the LAD is 90% blocked, during which phase of the cardiac cycle will the patient first notice symptoms — and why? (Hint: think about when the coronary arteries fill and when the heart's oxygen demand is highest.)

KEY TAKEAWAYS

Key takeaways — your study checklist:

1. Pericardium — fibrous (tough, inelastic) + serous (parietal and visceral/epicardium). Pericardial cavity: 15–50 mL fluid. Transverse sinus (behind aorta/pulmonary trunk), oblique sinus (behind left atrium, dead-end).
2. Cardiac tamponade — fluid in pericardial cavity compresses the heart. Beck's triad: hypotension, distended neck veins, muffled heart sounds. Emergency: pericardiocentesis.
3. Four chambers — RA (receives SVC, IVC, coronary sinus; fossa ovalis), RV (moderator band, infundibulum), LA (4 pulmonary veins), LV (thickest wall, 3× RV).
4. Septal defects — ASD (fossa ovalis region, L→R shunt), VSD (most common congenital defect, usually membranous part).
5. Fibrous skeleton — four valve annuli fused. Functions: valve support, electrical insulation (atria from ventricles), muscle attachment.
6. Coronary arteries — first branches of aorta. LCA → LAD (anterior wall + septum) + LCx (lateral wall). RCA (inferior wall + SA/AV nodes). Right dominant in 85%.
7. IHD — atherosclerosis narrows coronary arteries. LAD occlusion = anterior MI (widow-maker). RCA occlusion = inferior MI (± heart block). LCx = lateral MI.
8. Coronary sinus — main venous drainage, runs in posterior AV groove, drains into right atrium. Tributaries: great, middle, small cardiac veins.
9. Conducting system — SA node (pacemaker, 60–100 bpm) → AV node (delay 0.1s) → Bundle of His (only pathway through fibrous skeleton) → bundle branches → Purkinje fibres → ventricular contraction.
10. Clinical correlations — tamponade (pericardium), ASD/VSD (septa), widow-maker (LAD), heart block (AV node via RCA).