Thorax — Chest Wall, Pleura, Lungs, Heart and Great Vessels
Quick Review
Thorax is a core anatomy topic for INI CET — questions frequently involve the mediastinum, heart, lungs, and great vessels. Focus on surface anatomy of heart valves, border of pleura, segmentation of lungs, and coronary circulation. The heart’s position and borders are essential for clinical cardiology.
High-Yield Facts for INI CET:
- Heart position: Base at T5-T8, apex at 5th intercostal space midclavicular line (CMP)
- Cardiac apex beat: 5th left intercostal space midclavicular line
- Right border of heart: SVC, right atrium; Left border: Left ventricle, left atrial appendage; Superior border: Aorta, SVC, pulmonary trunk; Inferior border: Right ventricle, left ventricle
- Heart sounds: S1 (mitral, tricuspid — AV valve closure), S2 (aortic, pulmonary — semilunar valve closure)
- Left coronary artery: Supplies left ventricle, left atrium, most of interventricular septum; branches: LAD (supplies anterior wall, anterior 2/3 of septum), LCx (supplies lateral wall)
- Right coronary artery: Supplies right ventricle, inferior wall, SA node (60%), AV node (90%)
Exam tip: In chest X-ray interpretation, remember the left hemidiaphragm is usually higher than right (due to liver). The trachea is slightly to the right at the suprasternal notch. The heart’s transverse diameter should be <50% of the thoracic diameter.
The Thoracic Wall
Ribs and Costal Framework
- 12 pairs of ribs; first 7 true ribs (directly to sternum via costal cartilage); ribs 8-10 false ribs (join costal cartilage of rib 7); ribs 11-12 floating ribs (no anterior attachment)
- Typical rib anatomy: Head (2 facets for vertebral bodies), neck, tubercle (facet for transverse process), shaft, costal groove (carries neurovascular bundle)
- Intercostal spaces (11 on each side); contents: External intercostal (elevates ribs), internal intercostal (depresses ribs), innermost intercostal (same direction as internal); neurovascular bundle (vein, artery, nerve — VAN from superior to inferior in each space) runs in costal groove
Thoracic Vertebrae and Sternum
- T1-T12: 12 thoracic vertebrae; characteristic: Costal facets on vertebral bodies and transverse processes
- Sternum: Manubrium (has suprasternal notch, clavicular notches, 1st costal notches), body (ribs 2-7), xiphoid process
- Angle of Louis: Junction of manubrium and body; at level of T4-T5; used as a reference point for counting ribs (2nd rib at 2nd costal cartilage)
- Sternal angle (of Louis): Important landmark — level of aortic arch, bifurcation of trachea, left main bronchus crosses behind, T4/T5 IV disc
Intercostal Spaces and Vessels
- Posterior intercostal arteries: From thoracic aorta (direct branches); first two spaces from supreme intercostal artery (branch of costocervical trunk)
- Anterior intercostal arteries: From internal thoracic artery (for spaces 1-6); from musculophrenic artery for spaces 7-9
- Intercostal veins: Drain to azygos system; right → azygos; left upper spaces → hemiazygos; left lower spaces → accessory hemiazygos
- Intercostal nerves: Anterior rami of thoracic nerves (T1-T11); give off lateral cutaneous branches
Pleura and Lungs
Pleura
- Visceral pleura: Covers lung surfaces; sensitive to pain; follows fissures; adheres to lung surface
- Parietal pleura: Lines chest wall (costal), diaphragm (diaphragmatic), mediastinum (mediastinal), apex (cervical)
- Pleural cavity: Potential space between parietal and visceral pleura; contains thin fluid film; no air
Pleural Reflections
- Cervical pleura (dome over apex of lung — rises 2.5cm above clavicle)
- Costal pleura (follows ribs)
- Mediastinal pleura (midline structures)
- Diaphragmatic pleura (covers diaphragm)
Pleural Recesses
Costodiaphragmatic (between ribs and dome of diaphragm) and costomediastinal (between sternum and mediastinum) are where the lung can expand during inspiration; they contain no lung in quiet respiration.
Lungs — External Features
- Apex: Rises above 1st rib into root of neck (2.5cm above clavicle)
- Base: Rests on diaphragm
- Borders: Anterior (ribs and sternum), posterior (vertebral column), medial (mediastinum)
- Fissures: Right lung — oblique (horizontal fissure divides right upper lobe) separating upper and middle+lower; left lung — oblique only; left lung has cardiac notch (impression from heart)
Lobes and Bronchopulmonary Segments
Right lung — 10 segments:
- Upper lobe: Apical (RB1), Posterior (RB2), Anterior (RB3)
- Middle lobe: Lateral (RB4), Medial (RB5)
- Lower lobe: Superior (RB6), Medial basal (RB7), Anterior basal (RB8), Lateral basal (RB9), Posterior basal (RB10)
Left lung — 8 segments:
- Upper lobe: Apical-posterior (LB1+2), Anterior (LB3), Superior (LB4), Inferior (LB5)
- Lower lobe: Superior (LB6), Anteromedial basal (LB7+8), Lateral basal (LB9), Posterior basal (LB10)
Hilum of Lung
Where structures enter/exit the lung; contains: principal bronchi, pulmonary arteries and veins, bronchial arteries, bronchial veins, lymphatics, nerves; surrounded by mediastinal pleura.
Blood Supply
- Pulmonary circulation (functional): Pulmonary trunk → right/left pulmonary arteries (deoxygenated) → lung → pulmonary veins → left atrium
- Bronchial circulation (nutritional): Bronchial arteries from thoracic aorta → lung tissue → bronchial veins → azygos/hemiazygos system
Innervation
- Vagal (parasympathetic) — bronchoconstriction, increased secretion
- Sympathetic (from T2-T5 via sympathetic chain) — bronchodilation
- Phrenic nerve (C3-C5) — carries sensory to central diaphragmatic pleura and peritoneum
Lymphatic Drainage
Deep (pulmonary) and superficial (subpleural) lymphatics → bronchopulmonary nodes → tracheobronchial nodes → right/left bronchomediastinal trunks → thoracic duct (left) and right lymphatic duct
Trachea and Bronchi
The trachea has C-shaped cartilage rings (incomplete posteriorly — smooth muscle called trachealis). Bifurcation occurs at T4-T5 (carina — keel-shaped). The right main bronchus is wider, shorter, and more vertical (aspiration more common into the right lung).
The Heart
Position and Surface Anatomy
The heart lies in the middle mediastinum; apex at 5th intercostal space midclavicular line; base posteriorly at T5-T8; anterior surface formed mainly by the right ventricle; posterior surface by the left atrium.
Pericardium
- Fibrous pericardium: Tough outer sac; attached to central tendon of diaphragm; fused with outer adventitia of great vessels; protects the heart
- Serous pericardium: Parietal (lines fibrous pericardium) + visceral (epicardium — covers heart surface, contains coronary vessels and fat)
- Pericardial reflections: Oblique sinus (posterior to heart — bounded by IVC, left atrium, right pulmonary veins); Transverse sinus (between great arteries anteriorly and atria posteriorly)
Chambers of the Heart
Right Atrium:
- Receives systemic venous blood from SVC (upper body), IVC (lower body), coronary sinus (heart itself)
- Crista terminalis separates smooth posterior part (from sinus venosus) from rough anterior part (from primitive atrium)
- Fossa ovalis: Remnant of foramen ovale in interatrial septum
Right Ventricle:
- Receives blood from right atrium through the tricuspid valve
- Tricuspid valve: 3 leaflets (anterior, posterior, septal); tethered to papillary muscles via chordae tendineae (prevent valve prolapse during systole)
- Outflow tract: Pulmonary valve (right ventricular outflow tract, infundibulum)
Left Atrium:
- Receives oxygenated blood from 4 pulmonary veins (2 right, 2 left)
- Posterior wall is smooth; appendage is anterior with pectinate muscles
Left Ventricle:
- Thickest wall (3x RV); receives blood from left atrium through mitral valve
- Mitral (bicuspid) valve: 2 leaflets (anterior aortic, posterior mural); anterior leaflet is larger and in continuity with the aortic valve
Cardiac Conduction System
- Sinoatrial (SA) node: Right atrium near SVC opening; primary pacemaker (60-100 bpm); supplied by SA nodal artery (branch of RCA in 60%, LCx in 40%)
- Atrioventricular (AV) node: Triangle of Koch (bounded by Tendon of Todaro posteriorly, coronary sinus opening inferiorly, septal tricuspid leaflet anterosuperiorly); supplied by AV nodal artery (branch of RCA in 90%)
- Bundle of His: Penetrates fibrous skeleton; divides into right bundle branch and left bundle branch (splits into anterior and posterior hemifascicles)
- Purkinje fibres: Terminal branches distribute through ventricular myocardium
Coronary Circulation
Left Coronary Artery (from left aortic sinus)
- LAD (left anterior descending): Runs in anterior interventricular groove → apex; supplies anterior 2/3 of interventricular septum (via septal branches), anterior wall of LV, anterior papillary muscle of LV (dual supply)
- LCx (left circumflex): Runs in left AV groove; supplies lateral wall of LV; obtuse marginal branches
Right Coronary Artery (from right aortic sinus)
Runs in right AV groove → posterior interventricular groove (posterior descending — PDA) in 80% (right dominant). Supplies: RA, RV, inferior wall of LV, SA node (60%), AV node (90%), posterior 1/3 of interventricular septum.
Coronary Dominance
Right dominant (80%) — RCA gives PDA; Left dominant (15%) — LCx gives PDA; Balanced (5%).
Valves and Auscultation
| Valve | Location |
|---|---|
| Aortic | Right 2nd intercostal space right sternal border |
| Pulmonary | Left 2nd intercostal space left sternal border |
| Tricuspid | Left lower sternal border (5th intercostal space) |
| Mitral | Left 5th intercostal space midclavicular line (cardiac apex) |
Heart Sounds
- S1: AV valve closure (mitral and tricuspid) — best heard at apex
- S2: Semilunar valve closure (aortic and pulmonary) — best heard at base; split by inspiration
- S3: Rapid passive filling of ventricle; normal in children, pathological in adults (ventricular failure)
- S4: Atrial contraction (atrial kick) against stiff ventricle; pathological — indicates ventricular non-compliance (LVH, HCM, IHD)
Key Murmurs
- Mitral stenosis: Low-pitched rumbling mid-diastolic murmur at apex; opening snap before murmur
- Mitral regurgitation: High-pitched blowing holosystolic murmur at apex radiating to axilla
- Aortic stenosis: Ejection systolic murmur at right 2nd intercostal space radiating to carotids
- Aortic regurgitation: Early diastolic decrescendo murmur at left sternal border; wide pulse pressure; water-hammer pulse
- Tricuspid regurgitation: Holosystolic murmur at left lower sternal border; accentuated by inspiration (Carvallo’s sign)
- VSD: Harsh holosystolic murmur at left 4th intercostal space; no radiation
Applied Anatomy
Pleural Effusion
Fluid in pleural space → dull to percussion, absent breath sounds; thoracentesis done at 9th intercostal space (mid-axillary line) just above the rib (to avoid neurovascular bundle).
Pneumothorax
Air in pleural space → no lung markings on X-ray, hyperresonant to percussion; tension pneumothorax — mediastinal shift away, tracheal deviation, hypotension.
Lung Cancer
May involve phrenic nerve (paralysis of diaphragm), superior vena cava (SVC syndrome — facial swelling, distended neck veins), recurrent laryngeal nerve (hoarseness), sympathetic chain (Horner’s syndrome — ptosis, miosis, anhidrosis).