Cardiovascular Anatomy, Physiology, and Electrophysiology
Anatomy and circulation
Four chambers and four valves move blood in one direction: superior and inferior vena cava to right atrium, tricuspid valve to right ventricle, pulmonary valve to the lungs, pulmonary veins to left atrium, mitral valve to left ventricle, aortic valve to the aorta. The right heart handles deoxygenated blood; the left heart pumps oxygenated blood systemically. Coronary supply: the right coronary artery feeds the right atrium, right ventricle, and inferior wall and often the SA or AV node; the left main divides into the LAD (anterior wall and septum) and circumflex (lateral wall). These territories explain which leads show ST changes.
Conduction system
SA node (intrinsic 60-100 bpm) fires first and dominates, traveling through internodal tracts to the AV node (40-60 bpm), which delays the impulse about 0.1 second so the atria can empty. Then bundle of His, right and left bundle branches, and Purkinje fibers (20-40 bpm). The fastest pacemaker controls the heart.
Cellular electrophysiology
Resting cells are negative inside (about -90 mV). Depolarization is sodium influx (fast fibers) or calcium influx (slow SA/AV nodal fibers); repolarization is potassium efflux, with the sodium-potassium pump restoring gradients. On the tracing: P wave = atrial depolarization; QRS = ventricular depolarization (atrial repolarization is buried inside it); T wave = ventricular repolarization. The PR segment reflects AV nodal delay; an isoelectric ST segment reflects the plateau phase.
Physiology and electrolytes
Cardiac output = heart rate × stroke volume. Sympathetic stimulation raises rate, contractility, and conduction velocity; vagal tone slows the SA and AV nodes. Hyperkalemia peaks T waves, hypokalemia flattens T waves and produces U waves, and hypocalcemia prolongs the QT interval.
Traps
Calling the T wave "depolarization," naming the AV node as the primary pacemaker, and confusing pulmonary artery with pulmonary vein are classic distractors.
Rule of thumb: 60-100, 40-60, 20-40 - SA, AV, Purkinje.