Learning Objective: Describe the anatomical borders, contents, and clinical significance of the middle mediastinum, including its organs, vessels, nerves, and lymphatic structures.
The mediastinum is the central compartment of the thoracic cavity, situated between the two pleural sacs. It houses most thoracic organs and serves as a conduit for structures traveling between the neck, thorax, and abdomen.
An imaginary line from the sternal angle (junction of manubrium and body of sternum) to the T4 vertebral body divides the mediastinum into two parts:
| Division |
Description |
| Superior mediastinum |
Extends upward to the thoracic inlet |
| Inferior mediastinum |
Extends downward to the diaphragm, subdivided into anterior, middle, and posterior parts |
This section explores the middle mediastinum — its borders, contents, and clinical relevance.
Borders of the Middle Mediastinum
| Border |
Structure / Landmark |
| Anterior |
Anterior margin of the pericardium |
| Posterior |
Posterior border of the pericardium |
| Lateral |
Mediastinal pleura of both lungs |
| Superior |
Imaginary line from the sternal angle to T4 vertebra |
| Inferior |
Superior surface of the diaphragm |
🩻 Note: The middle mediastinum is the largest subdivision of the inferior mediastinum.
Contents of the Middle Mediastinum
The middle mediastinum contains several vital organs, vessels, nerves, and lymphatics essential to cardiopulmonary function.
Organs
| Organ |
Function / Description |
| Heart |
Enclosed within the fibrous pericardium, responsible for pumping blood through the systemic and pulmonary circuits |
| Pericardium |
Double-walled sac that protects and anchors the heart |
| Tracheal bifurcation |
Divides into the right and left main bronchi at the level of the sternal angle |
Vessels
| Vessel |
Course / Function |
| Ascending aorta |
Originates from the aortic orifice of the left ventricle; gives rise to right and left coronary arteries before becoming the aortic arch |
| Pulmonary trunk |
Short, wide vessel emerging from the right ventricle; bifurcates into right and left pulmonary arteries |
| Superior vena cava (SVC) |
Returns deoxygenated blood from the upper body to the right atrium; formed by the right and left brachiocephalic veins |
Key Insight:
Only the origins of these great vessels lie within the middle mediastinum; they continue superiorly into the superior mediastinum.
Nerves
| Nerve / Plexus |
Function / Description |
| Cardiac plexus |
Autonomic network at base of the heart; composed of sympathetic fibers (T1–T4) and parasympathetic fibers (vagus nerve) |
| Phrenic nerves (C3–C5) |
Provide motor innervation to the diaphragm and sensory innervation to the pericardium and mediastinal pleura; pass anterior to the lung roots on each side |
Lymphatics
| Lymph Node Group |
Location / Significance |
| Tracheobronchial lymph nodes |
Surround tracheal bifurcation and main bronchi; drain lungs and bronchi; enlarged in lung infections or malignancies |
Clinical Correlations
| Condition |
Description / Clinical Note |
| Pericardial effusion |
Fluid accumulation between the pericardial layers can compress the heart (cardiac tamponade). |
| Phrenic nerve injury |
Causes diaphragmatic paralysis on the affected side. |
| Enlarged tracheobronchial nodes |
Seen in lung carcinoma or tuberculosis, it can cause bronchial compression. |
| Aortic root dilation |
It may affect coronary artery origins, leading to myocardial ischemia. |
Summary Table
| Aspect |
Middle Mediastinum |
| Boundaries |
Between the anterior and posterior mediastina, it extends from the sternal angle to the diaphragm |
| Major contents |
Heart, pericardium, ascending aorta, pulmonary trunk, SVC, phrenic nerves, cardiac plexus, tracheobronchial nodes |
| Clinical importance |
Site of cardiac and pericardial pathologies; contains key cardiovascular structures |
