Heart — Chambers, Valves & Surface Anatomy

The heart is a four-chambered muscular pump lying obliquely in the middle mediastinum, enclosed in the pericardium. For NEET PG, the heavy-scoring zones are the internal architecture of each chamber (fossa ovalis, crista terminalis, septomarginal trabecula), the valve apparatus (cusps, chordae, papillary muscles), and the surface markings that separate where a valve sits from where it is auscultated.

Position, Orientation & Coverings

The heart occupies the middle mediastinum. Roughly one-third lies to the right and two-thirds to the left of the median plane. It is set obliquely, so the right side is largely anterior and the left side largely posterior.

It has an apex (directed down, forward, to the left — in the left 5th intercostal space, 9 cm/3.5 inches from midline, just medial to the midclavicular line) and a base (directed up, backward, to the right — formed mainly by the left atrium).

The fibrous pericardium fuses with the central tendon of the diaphragm below and the adventitia of great vessels above. The serous pericardium has parietal and visceral (epicardium) layers; reflections create the transverse sinus (behind aorta/pulmonary trunk, in front of atria — used to clamp arterial outflow during bypass) and the oblique sinus (a cul-de-sac behind the left atrium, bounded by pulmonary veins and IVC).

High-yield: The transverse pericardial sinus lies between the arterial (aorta + pulmonary trunk) outflow in front and the venous (atria) inflow behind. A finger placed here allows a ligature around the great arteries.

Surfaces & Borders

The heart is described as having an apex, a base, and surfaces/borders. Memorise which chamber forms what — examiners love this.

Feature	Formed mainly by
Apex	Left ventricle
Base (posterior surface)	Left atrium (+ small part right atrium)
Sternocostal (anterior) surface	Right ventricle (most), part of right atrium & left ventricle
Diaphragmatic (inferior) surface	Left ventricle (2/3) + right ventricle (1/3)
Right border	Right atrium
Left border (obtuse margin)	Left ventricle (+ auricle)
Inferior border (acute margin)	Right ventricle

High-yield: The anterior surface is mostly right ventricle, and the base is the left atrium. A penetrating anterior chest wound is most likely to injure the right ventricle.

The cardiac borders for surface marking are drawn between four points:

1. Upper border — 2nd left costal cartilage (lower border) to 3rd right costal cartilage (upper border).
2. Right border — 3rd right costal cartilage to 6th right costal cartilage (slightly convex, lateral to sternum).
3. Inferior border — 6th right costal cartilage to the apex (left 5th space, midclavicular line).
4. Left border — apex to the 2nd left costal cartilage.

The Four Chambers — Internal Anatomy

Right Atrium

The right atrium has two parts demarcated by the crista terminalis (a vertical muscular ridge inside) corresponding externally to the sulcus terminalis:

• Sinus venarum (smooth posterior part) — derived from the embryonic sinus venosus; receives SVC (above), IVC (below) and coronary sinus.
• Atrium proper (rough anterior part + auricle) — has musculi pectinati (pectinate muscles), derived from the primitive atrium.

Key landmarks:

• Fossa ovalis — oval depression on the interatrial septum, a remnant of the foramen ovale (closed septum primum). Its prominent upper margin is the limbus fossae ovalis (annulus ovalis).
• Opening of IVC guarded by the rudimentary valve of the inferior vena cava (Eustachian valve).
• Opening of coronary sinus guarded by the valve of the coronary sinus (Thebesian valve).
• Sinoatrial (SA) node lies at the upper end of the crista terminalis, near the SVC opening (subepicardial).
• AV node lies in the triangle of Koch — bounded by the tendon of Todaro, the septal cusp of the tricuspid valve, and the orifice of the coronary sinus.

High-yield: The SA node sits at the junction of SVC and right atrium (upper end of crista terminalis). The AV node lies in the triangle of Koch. The crista terminalis is the developmental boundary between sinus venarum (smooth) and atrium proper (rough).

Right Ventricle

Triangular in cross-section, with walls 3–4 mm thick (thinner than LV). Two parts:

• Inflow (sinus) — rough, trabeculated, contains the tricuspid valve.
• Outflow (conus arteriosus / infundibulum) — smooth-walled funnel leading to the pulmonary trunk, separated from the inflow by the supraventricular crest (crista supraventricularis).

Important structures:

• Trabeculae carneae — irregular muscular ridges.
• Septomarginal trabecula (moderator band) — crosses from the interventricular septum to the anterior papillary muscle; it carries the right branch of the AV bundle (right bundle branch), ensuring coordinated contraction.
• Three papillary muscles (anterior — largest, posterior, septal) with chordae tendineae to the tricuspid cusps.

High-yield: The moderator band (septomarginal trabecula) conducts the right bundle branch to the anterior papillary muscle. It is a feature unique to the right ventricle.

Left Atrium

Forms most of the base of the heart. Smooth-walled except in the auricle (pectinate muscles). Receives four pulmonary veins posteriorly (no valves). Interatrial septum shows the valve of the foramen ovale on its left side. The thicker, more muscular wall reflects the higher pulmonary venous pressure.

High-yield: Left atrial enlargement (e.g. mitral stenosis) compresses the oesophagus (dysphagia) and left recurrent laryngeal nerve (hoarseness — Ortner syndrome), and elevates the left main bronchus (widened carinal angle on X-ray).

Left Ventricle

Conical, with the thickest walls (8–12 mm, ~3× the RV) because it pumps against systemic resistance. Features:

• Trabeculae carneae — finer and more numerous than in the RV.
• Two large papillary muscles (anterior and posterior) with chordae to the two mitral cusps.
• Aortic vestibule — smooth-walled outflow tract below the aortic valve.

The Interventricular Septum

Composed of two parts:

Part	Size	Composition	Clinical note
Muscular	Larger, inferior	Thick myocardium	Bulges toward the RV
Membranous	Small, superior/posterior	Fibrous	Commonest site of VSD

The membranous septum is partly atrioventricular (separating RA from LV) because the tricuspid annulus is set slightly more apically than the mitral. The AV bundle of His pierces the membranous septum — hence septal surgery risks complete heart block.

High-yield: The membranous part of the interventricular septum is the most common site of a ventricular septal defect.

Heart Valves — Cusps & Apparatus

Valve	Number of cusps	Cusps	Papillary muscles/chordae?
Tricuspid (right AV)	3	Anterior, posterior, septal	Yes — 3 papillary muscles
Pulmonary (semilunar)	3	Anterior, right, left	No
Mitral / Bicuspid (left AV)	2	Anterior (aortic), posterior (mural)	Yes — 2 papillary muscles
Aortic (semilunar)	3	Right, left, posterior (non-coronary)	No

The two atrioventricular valves (tricuspid, mitral) have chordae tendineae anchoring cusps to papillary muscles, preventing prolapse during systole. The two semilunar valves (aortic, pulmonary) have no chordae; behind each aortic cusp is a sinus of Valsalva — the right coronary artery arises from the right (anterior) aortic sinus and the left coronary artery from the left (posterior) aortic sinus; the posterior (non-coronary) sinus gives off nothing.

High-yield: The anterior cusp of the mitral valve separates the inflow (mitral) from the outflow (aortic vestibule) of the left ventricle — fibrous continuity between mitral and aortic valves. The right and left coronary arteries arise from the corresponding aortic sinuses; the non-coronary (posterior) sinus has none.

The fibrous skeleton — four fibrous rings (annuli fibrosi) around the valve orifices, the right and left fibrous trigones, and the membranous septum — provides attachment for valve cusps and atrial/ventricular myocardium, and electrically insulates atria from ventricles (so the AV bundle is the only normal conduction pathway).

Surface Projections vs Auscultation Areas

This is one of the most repeated single-best-answer topics. The anatomical position of a valve and the site where it is best heard differ, because sound is carried in the direction of blood flow.

Valve	Anatomical surface projection	Best auscultation area
Pulmonary	Upper border, left 3rd costal cartilage / left of sternum	Left 2nd intercostal space (parasternal)
Aortic	Behind sternum, level of left 3rd intercostal space	Right 2nd intercostal space (parasternal)
Mitral	Behind sternum, left 4th costal cartilage	Apex — left 5th ICS, midclavicular line
Tricuspid	Behind sternum (midline), 4th/5th costal cartilage	Left lower sternal border, 4th–5th ICS

High-yield: All four valves project behind or just to the left of the sternum in a near-vertical line, but the auscultation areas are downstream. Memorise: aortic valve sits on the LEFT of sternum but is heard on the RIGHT 2nd space; pulmonary sits highest.

Mnemonic for auscultation areas (top→bottom, right→left): "APT M" = Aortic (R 2nd) → Pulmonary (L 2nd) → Tricuspid (L lower sternum) → Mitral (apex). Also "All Patients Take Medicine."

Flow of a single point in the conduction system: SA node → atrial myocardium + internodal tracts → AV node (triangle of Koch) → bundle of His (pierces membranous septum) → right & left bundle branches → Purkinje fibres → ventricular myocardium.

Coronary & Catheterisation Anatomy (frequently linked)

• RCA — right aortic sinus → AV groove → gives the SA nodal branch (~60%), right marginal, and (in right dominance, ~85%) the posterior interventricular (PDA) artery and AV nodal branch.
• LCA — left aortic sinus → divides into LAD (anterior IV groove; supplies anterior 2/3 of IV septum, apex) and circumflex (left AV groove).

Right heart catheterisation route (Swan-Ganz): a catheter is passed via the femoral or internal jugular/subclavian vein → right atrium → through tricuspid valve → right ventricle → through pulmonary valve → pulmonary trunk → pulmonary artery → "wedged" in a branch (pulmonary capillary wedge pressure).

High-yield: The PCWP approximates left atrial pressure. The catheter passes through two valves (tricuspid then pulmonary) en route to the pulmonary artery. Right dominance (PDA from RCA) is present in ~85% of people.

Clinical Correlations & Differentials

• Mitral stenosis → left atrial enlargement → dysphagia, hoarseness (Ortner), AF, pulmonary venous hypertension. Loud S1, opening snap, mid-diastolic rumble at apex.
• VSD — commonest at the membranous septum; pansystolic murmur at the left lower sternal border.
• Probe-patent foramen ovale — incomplete fusion of septum primum and secundum; flap valve of fossa ovalis. Present in ~25% of adults.
• Tricuspid regurgitation — pansystolic murmur at lower left sternal edge, increasing on inspiration (Carvallo sign), with giant 'v' waves in JVP.
• Cardiac tamponade — pericardial fluid; Beck's triad (hypotension, raised JVP, muffled heart sounds).

High-yield: Differentiating right vs left ventricle on a specimen: RV has the moderator band and coarse trabeculae, a thin wall (3–4 mm), and a crescentic lumen; LV has a thick wall (8–12 mm), fine trabeculae and a circular lumen.

Recently asked / exam angle

• "Crista terminalis separates which two parts of the right atrium?" → smooth sinus venarum from rough atrium proper.
• "Moderator band carries which structure?" → right bundle branch.
• "Commonest site of VSD?" → membranous interventricular septum.
• "Aortic valve is auscultated in which space?" → right 2nd intercostal space (despite lying to the left of the sternum).
• "Triangle of Koch boundaries?" → tendon of Todaro, septal cusp of tricuspid, coronary sinus orifice → marks the AV node.
• "Which chamber forms the base of the heart?" → left atrium.
• "Non-coronary sinus of the aorta is which?" → posterior.
• "Number of cusps of the mitral valve?" → two (anterior/aortic and posterior/mural).
• Image-based: identifying the fossa ovalis, limbus, Eustachian/Thebesian valves on a right-atrial photograph.
• Swan-Ganz catheter — number of valves crossed and what PCWP measures.

Rapid revision

1. Apex = left ventricle, left 5th ICS, 9 cm from midline; base = left atrium.
2. Anterior surface is mainly the right ventricle — most vulnerable in anterior stab wounds.
3. Crista terminalis divides sinus venarum (smooth) from atrium proper (rough).
4. SA node = upper crista terminalis near SVC; AV node = triangle of Koch.
5. Fossa ovalis = remnant of foramen ovale; limbus is its raised margin.
6. Moderator band (septomarginal trabecula) carries the right bundle branch — RV only.
7. Membranous IV septum = commonest site of VSD; bundle of His pierces it.
8. Mitral = 2 cusps; tricuspid, aortic, pulmonary = 3 cusps each.
9. Only AV valves (mitral, tricuspid) have chordae/papillary muscles; semilunar valves do not.
10. RCA from right sinus, LCA from left sinus; non-coronary = posterior sinus.
11. Auscultation = APTM: Aortic R2, Pulmonary L2, Tricuspid lower sternum, Mitral apex.
12. Swan-Ganz crosses tricuspid then pulmonary valve; PCWP ≈ left atrial pressure; right-dominant circulation in ~85%.