4.3 The Heart & Great Vessels

Cardiac Chambers and Valves

The heart is a four-chambered muscular pump in the middle mediastinum, oriented with its apex directed anteriorly, inferiorly, and to the left (5th intercostal space, midclavicular line) and its base (posterior surface, mostly left atrium) directed posteriorly. Its wall has three layers: inner endocardium (endothelium and subendothelial connective tissue), thick myocardium (cardiac muscle, thickest in the left ventricle), and outer epicardium (visceral serous pericardium plus subepicardial fat containing the coronary vessels).

The right atrium receives systemic venous return from the superior vena cava (SVC) (upper, drains above the diaphragm) and inferior vena cava (IVC) (lower, drains below the diaphragm), plus the coronary sinus opening (draining the heart itself). The smooth-walled sinus venarum (posterior) is separated from the rough pectinate atrium by the crista terminalis; the fossa ovalis on the interatrial septum is the remnant of the fetal foramen ovale.

The right ventricle pumps deoxygenated blood through the pulmonary valve (three semilunar cusps: anterior, left, right) into the pulmonary trunk. The left atrium receives four pulmonary veins (right and left superior and inferior) carrying oxygenated blood from the lungs. The left ventricle ejects through the aortic valve (three semilunar cusps: right, left, posterior — with corresponding aortic sinuses of Valsalva giving rise to the coronary arteries) into the ascending aorta.

The atrioventricular valves prevent backflow during ventricular systole: the tricuspid valve (right, three cusps: anterior, posterior, septal) and the mitral (bicuspid) valve (left, two cusps: anterior/aortic and posterior). Both are anchored to papillary muscles via chordae tendineae, which prevent valve prolapse during contraction. The papillary muscles contract just before ventricular systole to tense the chordae.

Great Vessels

Coronary Circulation

The heart's blood supply arises from the right and left coronary arteries, which originate from the right (anterior) and left posterior aortic sinuses of the ascending aorta, respectively.

• Left coronary artery (LCA) — a short trunk that divides into the left anterior descending (LAD, anterior interventricular) artery, which supplies the anterior wall of the left ventricle, the anterior two-thirds of the interventricular septum (including the bundle branches of the conduction system), and the apex; and the circumflex artery, which wraps around the left atrioventricular groove to supply the lateral and posterior left ventricular wall. The LAD is the most commonly occluded coronary artery and the source of the largest anterior infarcts; a proximal LAD occlusion is sometimes called the "widow-maker."
• Right coronary artery (RCA) — runs in the right AV groove, supplies the right atrium, most of the right ventricle, the SA node (in roughly 55–60% of people), the AV node (in about 80–90%), and the posterior third of the interventricular septum via the posterior interventricular (descending) artery in a right-dominant circulation (~85% of individuals, determined by which coronary gives the posterior descending artery). When the LCA's circumflex gives the PDA, the circulation is left-dominant (~8%) or codominant (~7%).

Coronary venous drainage is dominated by the coronary sinus, a wide vein in the posterior AV groove that opens into the right atrium. Its tributaries include the great cardiac vein (anterior interventricular, accompanying the LAD), middle cardiac vein (posterior interventricular), small cardiac vein (right border), and oblique vein of the left atrium (remnant of the left SVC). The anterior cardiac veins drain the anterior right ventricle directly into the right atrium, bypassing the coronary sinus.

Conduction System and the Septal MI Sample Item

Cardiac conduction begins at the sinoatrial (SA) node (the pacemaker, firing ~60–100 times per minute) in the upper right atrium near the SVC opening; impulses travel through the atrial myocardium and internodal pathways to the atrioventricular (AV) node (lower right atrium, interatrial septum, near the coronary sinus opening — imposing a delay of about 0.1 s), then to the AV bundle (bundle of His) which pierces the interventricular septum and divides into right and left bundle branches running down either side of the septum to the Purkinje fibers in the subendocardium.

The PA-CAT Bulletin's sample item ties this to clinical anatomy: an acute myocardial infarction damaging the ventricular septum can injure the bundle branches traveling within it. An anterior septal MI (LAD occlusion) commonly produces a left bundle branch block (LBBB) because the LAD supplies the anterior two-thirds of the septum where the left bundle branch ramifies. A posterior/inferior septal MI from an RCA-dominant occlusion can produce a right bundle branch block or, if the infarct reaches the AV node or bundle of His, complete (third-degree) heart block — producing immediate electrical conduction changes (widened QRS, AV dissociation) on the ECG. Knowing which artery supplies which part of the septum lets you predict the conduction disturbance from the territory of the infarct — the exact reasoning the PA-CAT sample item tests.