Middle Mediastinum

The middle mediastinum is the central subdivision of the inferior mediastinum and contains the heart, pericardium, and the roots of the great vessels. It is the most dynamic part of the mediastinum, accommodating the beating heart and the movement of the lungs and diaphragm during respiration. Structurally, it serves as the core compartment around which other mediastinal structures are organized.

Boundaries

The middle mediastinum is bordered by the following anatomical structures:

• Anterior: Anterior mediastinum (pericardium lies posterior to the sternum)
• Posterior: Posterior mediastinum
• Lateral: Mediastinal pleura of both lungs
• Superior: An imaginary line extending from the sternal angle (T4–T5 intervertebral disc) to the lower border of the fourth thoracic vertebra (separates it from the superior mediastinum)
• Inferior: Diaphragm

Contents of the Middle Mediastinum

The middle mediastinum contains vital cardiovascular structures along with associated nerves, vessels, and lymphatics. Its contents include:

• Heart enclosed within the pericardium
• Ascending aorta
• Pulmonary trunk and arteries
• Superior vena cava and its tributaries
• Inferior vena cava (terminal portion)
• Phrenic nerves
• Pericardiacophrenic vessels
• Tracheobronchial (inferior) lymph nodes

Pericardium

The pericardium is a double-layered fibroserous sac that encloses the heart and the roots of the great vessels. It consists of:

• Fibrous pericardium: The outer tough connective tissue layer that anchors the heart to the diaphragm and sternum
• Serous pericardium: The inner layer, further divided into:
  • Parietal layer – lines the fibrous pericardium
  • Visceral layer (epicardium) – adheres to the surface of the heart

The space between these two serous layers is the pericardial cavity, which contains a small amount of serous fluid that reduces friction as the heart contracts.

Heart

The heart is centrally positioned in the middle mediastinum, oriented obliquely from the right shoulder to the left hip. It consists of four chambers—right atrium, right ventricle, left atrium, and left ventricle—and is enclosed within the fibrous pericardium. It rests on the diaphragm and is bordered laterally by the lungs.

Key anatomical features of the heart in this space include:

• Right atrium receiving venous return via the superior and inferior vena cava
• Right ventricle forming most of the anterior surface
• Left ventricle forming the apex and much of the diaphragmatic surface
• Left atrium positioned posteriorly, receiving pulmonary veins

Great Vessels

Ascending Aorta

Emerges from the left ventricle and is enclosed within the pericardium. It gives rise to the coronary arteries and continues as the aortic arch as it enters the superior mediastinum.

Pulmonary Trunk

Originates from the right ventricle and splits into the right and left pulmonary arteries, which travel to the lungs. This bifurcation occurs within the middle mediastinum, near the level of the T5 vertebra.

Superior Vena Cava (SVC)

Formed by the union of the right and left brachiocephalic veins, the SVC descends vertically along the right side of the ascending aorta and drains into the right atrium.

Inferior Vena Cava (IVC)

The terminal portion of the IVC enters the middle mediastinum through the diaphragm (at the T8 vertebral level) and delivers venous blood from the lower body into the right atrium.

Phrenic Nerves

Both the right and left phrenic nerves pass through the middle mediastinum on their way to the diaphragm. They descend anterior to the root of the lung and lie between the fibrous pericardium and mediastinal pleura. Each phrenic nerve is accompanied by a pericardiacophrenic artery and vein.

They provide motor innervation to the diaphragm and sensory innervation to the pericardium, mediastinal pleura, and central diaphragm.

Pericardiacophrenic Vessels

The pericardiacophrenic arteries are branches of the internal thoracic arteries. These vessels descend with the phrenic nerves and supply the pericardium and diaphragm. The veins drain into the internal thoracic or brachiocephalic veins.

Lymph Nodes

The tracheobronchial lymph nodes, particularly the inferior tracheobronchial (carinal) nodes, are located near the bifurcation of the trachea and the main bronchi. These nodes form part of the lymphatic drainage of the lungs, heart, trachea, and esophagus, and are prominent structures within the middle mediastinum.

Topographical Relationships

• Anteriorly: Sternum, anterior mediastinum, ribs
• Posteriorly: Esophagus, descending thoracic aorta, vertebral bodies
• Laterally: Lungs and pleural cavities
• Superiorly: Superior mediastinum and continuation of great vessels
• Inferiorly: Diaphragm

The middle mediastinum is central to thoracic anatomy and lies at the crossroads of cardiovascular and pulmonary pathways. It is surrounded by mobile organs and is highly affected by movements of respiration and cardiac contraction.