The respiratory system - the mediastinum

Question 1

What is the mediastinum?

Answer 1

It is the central region of the thoracic cavity, located between the two laterally placed pleural cavities

Question 2

What is the mediastinum divided into?

Answer 2

Superior and inferior parts

The inferior mediastinum is divided into the anterior, middle and posterior mediastinum

Question 3

What divides the mediastinum?

Answer 3

Anatomically, the mediastinum is divided into two parts by an imaginary line that runs from the sternal angle (the angle formed by the junction of the sternal body and manubrium) to the T4 vertebrae:
• Superior mediastinum – extends upwards, terminating at the superior thoracic aperture.
• Inferior mediastinum – extends downwards, terminating at the diaphragm. It is further subdivided into the anterior mediastinum, middle mediastinum and posterior mediastinum.

Question 4

Describe the contents of the superior mediastinum

Answer 4

The superior mediastinum contains neural, vascular and respiratory structures passing from the adjacent regions of the neck and abdomen (via the inferior mediastinum).

Question 5

What great vessels of the heart lie in the superior mediastinum?

Answer 5

The three major branches of the aortic arch arise within the superior mediastinum:
Brachiocephalic artery – supplying the right side of the head & neck and the right upper limb.
Left Common carotid artery – to the left side of the head & neck.
Left Subclavian artery – to the left upper limb.

Question 6

What are the contents of the middle mediastinum?

Answer 6

The middle mediastinum is the largest subdivision of the inferior mediastinum. It contains several important organs, vessels, nerves and lymphatic structures.

The middle mediastinum contains the heart, and its protective sheath, the pericardium. It also contains the tracheal bifurcation and the left and right main bronchi.

Question 7

What vessels are found in the middle mediastinum?

Answer 7

The middle mediastinum is associated with the origins of the great vessels that run to and from the heart:

Ascending aorta – the first part of the aorta, which arises from the aortic orifice. It moves upwards, exiting the fibrous pericardium and entering the superior mediastinum – where it then becomes the aortic arch.
The ascending aorta gives rise to two branches; the left and right coronary arteries.

Pulmonary trunk – gives rise to the left and right pulmonary arteries. The trunk itself is relatively short and wide, allowing a large volume of blood to pass through it.

Superior vena cava – returns deoxygenated blood from the upper half of the body. It is formed by the right and left brachiocephalic veins.

Question 8

What are the borders of the inferior mediastinum?

Answer 8

• Lateral borders: Mediastinal pleura (part of the parietal pleural membrane).
• Anterior border: Body of the sternum and the transversus thoracis muscles.
• Posterior border: Pericardium.
• Roof: Continuous with the superior mediastinum at the level of the sternal angle.
• Floor: Diaphragm.

Question 9

What are the contents of the inferior mediastinum

Answer 9

• The anterior mediastinum contains no major structures. It accommodates loose connective tissue (including the sternopericardial ligaments, which tether the pericardium to the sternum), fat, some lymphatic vessels, lymph nodes and branches of the internal thoracic vessels.
• In infants and children, the thymus extends inferiorly into the anterior mediastinum. However the thymus recedes during puberty and is mostly replaced by adipose tissue in the adult.