Mediastinum

The mediastinum is the central compartment of the thoracic cavity, located between the two pleural sacs that enclose the lungs. It serves as a vital transit region that houses several of the body’s most critical organs, vessels, and nerves. The mediastinum plays a central role in cardiovascular, respiratory, nervous, and lymphatic systems.

Location

The mediastinum is situated in the middle of the thoracic cavity. It extends vertically from the thoracic inlet (superior thoracic aperture) above to the diaphragm below and horizontally from the sternum anteriorly to the thoracic vertebrae posteriorly. On either side, it is bordered by the mediastinal pleura of the lungs.

Divisions of the Mediastinum

The mediastinum is traditionally divided into four compartments for descriptive and clinical purposes: the superior mediastinum and three parts of the inferior mediastinum—anterior, middle, and posterior.

1. Superior Mediastinum

Located above the pericardium, the superior mediastinum extends from the superior thoracic aperture to the horizontal plane passing through the sternal angle and the intervertebral disc between T4 and T5. It contains major structures including:

• Thymus
• Aortic arch and its branches: brachiocephalic trunk, left common carotid artery, left subclavian artery
• Brachiocephalic veins and superior vena cava
• Trachea and esophagus
• Thoracic duct
• Vagus nerve (CN X) and phrenic nerves
• Left recurrent laryngeal nerve (branch of vagus)

2. Anterior Mediastinum

This is the smallest division of the inferior mediastinum, located between the sternum and the anterior pericardium. In adults, it primarily contains:

• Loose connective tissue and fat
• Lymphatic vessels and nodes
• Branches of the internal thoracic vessels
• Occasionally, remnants of the thymus gland

3. Middle Mediastinum

This is the central part of the inferior mediastinum and contains the pericardium and its contents. Key structures include:

• Heart and pericardium
• Ascending aorta
• Pulmonary trunk and pulmonary arteries
• Superior vena cava and pulmonary veins
• Phrenic nerves
• Main bronchi
• Tracheobronchial lymph nodes

4. Posterior Mediastinum

This lies behind the pericardium and anterior to the vertebral column (T5–T12). It contains a number of critical longitudinal structures, including:

• Thoracic (descending) aorta
• Esophagus
• Thoracic duct
• Azygos and hemiazygos veins
• Sympathetic trunks
• Splanchnic nerves
• Posterior mediastinal lymph nodes

Boundaries

• Anterior: Sternum and costal cartilages
• Posterior: Vertebral bodies of T1–T12
• Superior: Thoracic inlet
• Inferior: Diaphragm
• Lateral: Mediastinal pleura of lungs

Contents Overview

The mediastinum contains a variety of organs and structures, grouped by systems:

Cardiovascular

• Heart and great vessels (aorta, superior/inferior vena cava, pulmonary trunk)
• Brachiocephalic veins
• Thoracic aorta

Respiratory

• Trachea (superior mediastinum)
• Main bronchi (middle mediastinum)

Digestive

• Esophagus (superior and posterior mediastinum)

Lymphatic

• Thoracic duct
• Lymph nodes (e.g., tracheobronchial, posterior mediastinal)

Nervous

• Vagus nerve and recurrent laryngeal nerves
• Phrenic nerves
• Sympathetic trunks and splanchnic nerves

Development

Embryologically, the mediastinum forms as the thoracic cavity develops and partitions into pleural, pericardial, and peritoneal cavities. The thymus, which initially occupies a large portion of the anterior mediastinum in infants, gradually involutes after puberty and becomes a small fatty remnant in adults.

Functional Significance

The mediastinum acts as a central conduit through which major organs and structures pass between the head/neck, thorax, and abdomen. It supports essential physiological processes, including respiration (via the trachea and bronchi), circulation (via the heart and vessels), and digestion (via the esophagus).

Relations to Other Structures

• The pleural cavities (and lungs) lie on either side of the mediastinum.
• The diaphragm forms its inferior boundary, separating it from the abdominal cavity.
• The superior mediastinum connects upward into the root of the neck, allowing continuity of vascular, neural, and respiratory pathways.

Clinical Relevance

Although your platform focuses on anatomical learning, it's helpful to mention that the mediastinum is a region of clinical importance due to the variety of vital structures within it. Pathologies may include:

• Mediastinal masses – such as thymomas, lymphomas, or cysts
• Mediastinitis – inflammation, often due to esophageal perforation or infection
• Aortic aneurysm or dissection
• Tracheal or esophageal compression by tumors