Mediastinum: In-Depth Notes

The mediastinum is the space between the right and left pleural sacs in the thoracic cavity, containing various vital structures.

Boundaries:

• Superior: Superior thoracic aperture (opening at the top of the thoracic cavity)

• Inferior: Diaphragm

• Anterior: Sternum

• Posterior: Thoracic vertebrae

• Lateral: Lungs & pleurae (membranes covering lungs)

Divisions of the Mediastinum
Divided by a horizontal plane from the sternal angle to the lower border of T4:

1. Superior Mediastinum: Above the plane

2. Inferior Mediastinum: Below the plane, further divided into:

   • Middle Mediastinum: Contains the heart

   • Anterior Mediastinum: In front of the heart

   • Posterior Mediastinum: Behind the heart

Superior Mediastinum Structures
Boundaries:

• Superior: Superior thoracic aperture

• Inferior: Horizontal plane at T4/T5

Key Structures (superficial to deep):

1. Thymus Gland: Two lobes connected by fibrous tissue. Lies posterior to the manubrium, anterior to the fibrous pericardium (outer layer of the heart's covering).

   • Blood supply: anterior intercostal & mediastinal branches of internal thoracic arteries

   • Venous drainage: into left brachiocephalic, internal thoracic, and inferior thyroid veins

2. Veins: Right & left brachiocephalic veins, superior vena cava

3. Arteries (branches of the aortic arch):

   • Brachiocephalic artery (supplies right side)

   • Left common carotid artery

   • Left subclavian artery (supplies left upper limb)

4. Nerves:

   • Phrenic nerves (motor control to diaphragm)

   • Vagus nerve (parasympathetic control for thoracic/abdominal organs)

   • Recurrent laryngeal nerves (branch of vagus; controls vocal cords)

5. Trachea & Esophagus: Trachea bifurcates (splits) at level of T4/T5

6. Thoracic Duct & Lymph Nodes (largest lymphatic vessel and immune structures)

Contents of the Inferior Mediastinum
Three compartments:

1. Anterior Mediastinum: Between sternum and pericardium; contains connective tissue, lymph nodes, and remnant thymus.

2. Middle Mediastinum: Houses the heart, pericardium, roots of great vessels (e.g., ascending aorta, pulmonary trunk, SVC), main bronchi, and phrenic nerves.

3. Posterior Mediastinum: Contains descending thoracic aorta, azygos veins (drain thoracic wall), vagus/splanchnic nerves (autonomic nerves), sympathetic chain, esophagus, thoracic duct, and posterior mediastinal lymph nodes.

Arrangement of the Pericardium

1. Fibrous Pericardium: Dense connective tissue around the heart; attached to diaphragm via pericardiacophrenic ligament.

2. Serous Pericardium: Has parietal and visceral layers; the visceral layer (epicardium) covers the heart directly. Contains pericardial fluid (lubricates heart movements).

Posterior Mediastinum Contents:

• Oesophagus

• Vagus Nerve

• Thoracic Duct

• Azygos Vein

• Descending Aorta

• Sympathetic Trunks

• Lymph Nodes

Arterial Supply & Venous Drainage of Thoracic Wall

1. Arterial Supply:

   • Anterior Intercostal Arteries: from internal thoracic artery (upper ribs) & musculophrenic artery (lower ribs)

   • Posterior Intercostal Arteries: from descending thoracic aorta

2. Venous Drainage:

   • Anterior intercostal veins → internal thoracic vein → brachiocephalic vein

   • Posterior intercostal veins → azygos system (right) or brachiocephalic vein (left)

Blood Supply to the Lungs

1. Pulmonary Blood Supply:

   • Deoxygenated blood: right ventricle → pulmonary arteries → lungs

   • Oxygenated blood: lungs → pulmonary veins → left atrium

2. Systemic Blood Supply:

   • Bronchial arteries from aorta supply lung tissue

   • Bronchial veins drain into azy