Mediastinum (anatomy)

The mediastinum is the region of the thoracic cavity bounded by:

• posteriorly: thoracic spine
• anteriorly: sternum and costal cartilages
• laterally: mediastinal pleura bounding the pleural cavities
• superiorly: thoracic inlet
• inferiorly: diaphragm

The mediastinum includes all of the structures within the thoracic cavity except the lungs and surrounding pleurae. This includes the heart, great vessels, trachea, oesophagus, thoracic duct, vagus and phrenic nerves. Some of these structures are in continuity with the neck superiorly and the abdomen inferiorly. Individual structures within each subdivision of the mediastinum are considered in the submenu. The region between them is filled with homogenous, elastic connective tissue; this permits the great deal of volume change and movement which is necessary for mediastinal structures e.g.:

• passage of oesophageal boluses inferiorly
• descent of trachea and principal bronchi with inspiration, the opposite movement for expiration
• increased volume of venous return to the right side of the heart with inspiration

The mediastinum is subdivided into various regions for the purposes of description; arbitrary boundaries have been set which have no absolute physiological or pathological correlates. Indeed, the size of each subcompartment can change with, for example, the phases of respiration:

• superior mediastinum
• anterior mediastinum
• middle mediastinum
• posterior mediastinum

Alternatively, the mediastinum can be divided up according to the position of structures relative to the lung roots.