Thorax · Heart & Mediastinum

The central compartment of the thoracic cavity, bounded by the pericardium. It contains the heart, the great vessels (ascending aorta, pulmonary trunk, superior vena cava, pulmonary veins), the phrenic nerves, and the pericardial sac. It lies between the sternum and the vertebral column, and is clinically important for cardiac access and imaging.

Fibroserous sac that surrounds the heart and roots of the great vessels. Provides mechanical protection, limits cardiac distension, and reduces friction.

• Fibrous pericardium: tough outer layer, attaches to central tendon of diaphragm and adventitia of great vessels at the sternal angle (T4/T5).
• Serous pericardium: double-layered (parietal lines fibrous pericardium; visceral = epicardium covers the heart).
• Pericardial cavity: potential space with 15–50 mL serous fluid.

Pericardial sinuses

• Transverse sinus: posterior to ascending aorta & pulmonary trunk, anterior to SVC & pulmonary veins. Surgical landmark to clamp great arteries.
• Oblique sinus: inverted U-shaped recess posterior to left atrium, bounded by reflections around pulmonary veins and IVC.

Obliquely placed in the middle mediastinum, mostly posterior to the sternum. The apex points inferolaterally to the left.

Sulci: coronary sulcus (atria/ventricles), anterior & posterior interventricular sulci (contain vessels and fat).

• Upper right: 3rd right costal cartilage
• Lower right: 6th right costal cartilage
• Upper left: 2nd left costal cartilage
• Apex: 5th left ICS, midclavicular line
• Right border: 3rd to 6th right costal cartilages (just right of sternum)
• Left border: 5th ICS to 2nd left costal cartilage
• Inferior border: 6th right cartilage → 5th left ICS
• Superior border: 2nd left → 3rd right cartilage

Right atrium

Receives venous blood from the body (except pulmonary veins).

• Auricle: pectinate muscles (rough myocardium).
• Sinus venarum: smooth-walled portion (from sinus venosus), receives SVC & IVC.
• Crista terminalis: vertical ridge separating rough from smooth; SA node lies in its upper part.
• Fossa ovalis: remnant of foramen ovale (interatrial septum).
• Right AV valve (tricuspid) → right ventricle.

Left atrium

Receives oxygenated blood from 4 pulmonary veins (right & left, superior & inferior).

• Left AV orifice → mitral (bicuspid) valve → left ventricle.

Right ventricle

• Inflow via tricuspid valve; outflow via pulmonary semilunar valve.
• Trabeculae carneae: muscular ridges.
• Papillary muscles attached to chordae tendineae.
• Infundibulum: smooth outflow tract to pulmonary valve.
• Moderator band (septomarginal trabecula): carries right bundle branch; connects septum to anterior papillary muscle.

Left ventricle

• Thicker walls, higher pressure.
• Inflow via mitral valve; outflow via aortic valve.
• Aortic vestibule → aortic semilunar valve.
• Two large papillary muscles (anterior & posterior).

• Epicardium: visceral pericardium (mesothelium + CT with fat and vessels).
• Myocardium: cardiac muscle cells (striated, central nuclei, smaller than skeletal). T-tubules at Z-lines.
• Endocardium: endothelium + thin CT, lines chambers.

Intercalated discs

Specialized junctions (gap junctions + desmosomes) that allow electrical & mechanical coupling; seen as dark transverse lines.

• AV valves: tricuspid (right), bicuspid/mitral (left).
• Semilunar: pulmonary (right), aortic (left) – each with 3 cusps.

Murmurs – high‑yield

Venous drainage

• Coronary sinus: main vein, drains to RA. Tributaries: great cardiac vein (anterior IVS), middle cardiac vein (posterior IVS).
• Anterior cardiac veins and venae cordis minimae (Thebesian) drain directly into chambers.

• SA node (pacemaker) – upper crista terminalis, supplied by RCA (SA nodal branch). Sympathetic ↑ rate, parasympathetic ↓.
• AV node – interatrial septum near coronary sinus; slows impulse.
• Bundle of His → right & left bundle branches (in IV septum) → Purkinje fibers.
• Moderator band carries right bundle branch.

Blood supply: SA node (RCA), AV node (RCA), bundle of His (LAD).

Cardiac plexus – sympathetic (T1–T5) and parasympathetic (vagus).

• Sympathetic: ↑ rate, contractility; pain (angina) follows sympathetic afferents to T1–T5.
• Parasympathetic: ↓ rate; afferent reflexes travel with vagus.

Domed musculotendinous sheet, innervated by phrenic nerves (C3–C5).

• Origins: septum transversum (central tendon), pleuroperitoneal membranes, dorsal esophageal mesentery (crura), body wall (periphery).

Major apertures