Human Anatomy
Mediastinum
Mediastinum anatomy for MBBS: subdivisions, contents, aortic arch, oesophagus and thoracic duct with clinical relations, mapped to NMC codes AN23.1 to AN23.7.
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Mediastinum
Mediastinum anatomy for MBBS: subdivisions, contents, aortic arch, oesophagus and thoracic duct with clinical relations, mapped to NMC codes AN23.1 to AN23.7.
This chapter maps the mediastinum, the central partition of the thorax that holds the heart and the great vessels, nerves and viscera passing between the neck and abdomen. It covers the subdivisions and their contents and the clinical relations of the aortic arch, recurrent laryngeal nerve, oesophagus and thoracic duct.
High-yield: Mediastinum
- The mediastinum is the central compartment of the thorax between the two pleural sacs.
- It is divided by the sternal angle plane into the superior mediastinum above and the inferior mediastinum below.
- The inferior mediastinum is subdivided into anterior, middle and posterior parts.
- The middle mediastinum contains the heart and pericardium, the roots of the great vessels and the phrenic nerves.
- The superior mediastinum contains the arch of the aorta, the brachiocephalic veins, the trachea, oesophagus and thoracic duct.
- The thymus lies in the anterior mediastinum and is large in the child but atrophies in the adult.
- The arch of the aorta gives the brachiocephalic trunk, the left common carotid and the left subclavian arteries.
- The left recurrent laryngeal nerve hooks under the arch of the aorta, so aneurysms there can cause hoarseness.
- The thoracic duct ascends in the posterior mediastinum, crossing from right to left at about the T5 level.
- The oesophagus lies in the posterior mediastinum and has constrictions that are common sites of stricture and foreign-body lodgement.
- The azygos vein arches over the root of the right lung to drain into the superior vena cava.
- The phrenic nerves pass in front of the lung roots, while the vagus nerves pass behind them.
- The vagus nerves form the oesophageal plexus and continue as anterior and posterior vagal trunks.
- The descending thoracic aorta lies in the posterior mediastinum and gives posterior intercostal and visceral branches.
Subdivisions and key contents
- **Superior mediastinum:** Arch of aorta, brachiocephalic veins, trachea, oesophagus, thoracic duct, thymus remnant, phrenic and vagus nerves.
- **Anterior mediastinum:** Thymus (in the child), fat, lymph nodes and small vessels; behind the body of the sternum.
- **Middle mediastinum:** Heart and pericardium, roots of the great vessels, phrenic nerves, main bronchi.
- **Posterior mediastinum:** Descending thoracic aorta, oesophagus, thoracic duct, azygos system, vagus nerves, sympathetic chains.
NMC competencies in this chapter
- **AN23.1:** Subdivisions and boundaries of the mediastinum
- **AN23.2:** Contents of the superior mediastinum
- **AN23.3:** Contents of the anterior and middle mediastinum
- **AN23.4:** Contents of the posterior mediastinum
- **AN23.5:** Arch of the aorta and its branches
- **AN23.6:** Oesophagus, thoracic duct and azygos system
- **AN23.7:** Applied anatomy of the mediastinum
Frequently Asked Questions
How is the mediastinum divided?
A plane at the sternal angle separates the superior mediastinum above from the inferior mediastinum below. The inferior part is further divided into anterior, middle and posterior compartments.
Why can an aortic arch aneurysm cause hoarseness?
The left recurrent laryngeal nerve hooks under the arch of the aorta before running up to the larynx. An aneurysm of the arch can stretch or compress the nerve, paralysing a vocal cord and causing hoarseness.
What lies in the posterior mediastinum?
The posterior mediastinum contains the descending thoracic aorta, the oesophagus, the thoracic duct, the azygos and hemiazygos veins, the vagus nerves and the sympathetic chains.
Where does the thymus lie?
The thymus lies mainly in the anterior mediastinum behind the manubrium and body of the sternum. It is large in childhood and gradually atrophies and is replaced by fat in adult life.
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