L23. Superior and Posterior Mediastinum

Question 1

Define the main (physical and functional) divisions of the thoracic cavity.

Answer 1

The R lung and R pleural cavity
The L lung and L pleural cavity
The Mediastinum containing the heart and all other thoracic structures

Question 2

What structures lie in the anterior mediastinum?

Answer 2

The thymus which regresses in late adolescence/early adulthood
At which point only fat and lymphatics

Question 3

From superficial to deep, list the major structures in the superior mediastinum (4)

Answer 3

1. Thymus
2. Great veins
3. The aortic arch, vagus and phrenic nerves
4. The prevertebral region containing the trachea, oesophagus, thoracic duct and left recurrent laryngeal nerve

Question 4

Describe the general trend for the location of arteries and veins and why this is so?

Answer 4

In the chest, the arteries tend to lie on the left side of the heart (left ventricle ejects into these) while the veins lie to the right (great veins empty into the right atrium)

Question 5

What are the major veins called and how are they arranged? (3)

Answer 5

They are initially arranged symmetrically coming from their areas
Internal Jugular Vein (IJV) drains the head and neck
SUBCLAVIAN veins drain the upper limbs

They run down the neck and cross the limbs and meet behind the sternoclavicular joints forming the BRACHEOCEPHALIC VEINS which join into the SVC behid the right 1st costal cartilage

Question 6

What is the difference between the left and right brachiocephalic vein routes?

Answer 6

The right bracheocephalic is shorter and more vertical while the left has to cross the body to get to the right atrium. The left is longer and more horizontal running across the manubrium and front of the traceha

Question 7

What is the azygous vein and where does it plug into?

Answer 7

Collects the blood from the thorax, it ascends against the posterior wall of the thorax until it arches up, forwards (above the lung route) and plugs into the SVC at the level of the 2nd costal cartilage

Question 8

At what level does the superior vena cava enter the right atrium?

Answer 8

It enters at the level behind the 3rd costal cartilage

Question 9

Describe the arch of the aorta

What branches stem off the ascending aorta?

Answer 9

The arch stems out of the left ventricle: upwards, backwards and to the left.

Off the ascending aorta are the coronary arteries and off the arch itself is the bracheocephalic artery, the left common carotid and the left subclavian arteries

Question 10

At what level does the ascending aorta become the aortic arch?

Answer 10

The manubriosternal junction

Question 11

At what level does the aortic arch become the descending aorta?

Answer 11

The level of T4-T5

Question 12

What is contained in the lung root?

Answer 12

Bronchus, pulmonary arteries and veins, hilar lymphatics

Question 13

What major structure lies within the concavity (under the curve) of the aortic arch?

Answer 13

The bifurcation of the pulmonary trunk

Question 14

What is the fibrous communication structure on the undersurface of the aortic arch? What is its significance?

Answer 14

The ligamentum arteriosum which was once a duct connecting the pulmonary trunk with the aortic arch such that any blood accidentally shunted into the pulmonary circulation was forced into the arch

Question 15

What is the trend of the branches of the arch of the aorta?

Answer 15

The branches off the aorta tend to start anteriorly and to the right.
They become progressively more posterior and to the left

Question 16

What are the braches off the aortic arch? (3)

Answer 16

The bracheocephalic artery, the left common carotid and the left subclavian artery

Question 17

What arteries form an asymmetrical V shape that clasps the trachea?

Answer 17

The bracheocephalic artery on the right hand side and the left common carotid on the left

Question 18

Describe the condition of having a retro-oesophageal right subclavian artery?

Answer 18

There is no bracheocephalic trunk coming off the aorta. Instead there is the right common carotid artery. The aorta then only ‘remembers’ to supply the right upper limb (right subclavian artery) after it has passed far down such that the right subclavian has to pass behind the oesophagus to supply the limb: pain swallowing

Question 19

What is always the most lateral structure of the mediastinum on either side?

Answer 19

The phrenic nerve

Question 20

What is the phrenic nerve formed by? And where do they come from? (Plexus)
What does the phrenic nerve supply?

Answer 20

The phrenic nerve is formed by the ventral rami of C3, 4 and 5 branching off the cervical plexus.
The phrenic supplies the diaphragm from the underside
It is a mixed nerve: sensory and motor supply (so has sensory signals from all structures it passes by)

Question 21

Describe the path down the neck of the phrenic nerve

Answer 21

Phrenic nerves run down the neck on the scalenus anterior muscle into the anterior mediastinum passing between the subclavian vein and artery
Passes down the mediastinum past the heart and into the diaphragm which it pierces at different points
It is always the most lateral landmark of the mediastinum

Question 22

Does the phrenic nerve pass anteriorly or posteriorly to the lung root?

Answer 22

Always passes anterior to the lung root

Question 23

Where and how does the right phrenic nerve pass through the diaphragm?

Answer 23

RIGHT: passes through with the IVC at the level of T8 (very closely adjacent to it) through the caval orifice of the central tendon - not exactly but enough to say so

LEFT: passes on its own at the level of the apex of the heart lateral

Question 24

Describe the path of the vagus nerve

Answer 24

Intimately associated with the aortic arch
A large cranial nerve (CX) emerges from the cranial cavity and runs through the neck posterolateral to the common carotid artery (inside the carotid sheath)
Descends into the thorax where right and left have different paths to the anterior oesphagous

Question 25

What is the carotid sheath and what does it contain?

Answer 25

Between the common carotid artery and IJV is a sheath containing a neurovascular bundle suppying the head and neck

Question 26

Where is the vagus nerve aiming to go? Where and how does the vagus nerve pass through the diaphragm?

Answer 26

Wants to get to the midline: the anterior surface of the oesophagus to go through the diaphragm with it

Question 27

What does the vagus nerve supply and how?

Answer 27

Parasympathetic fibres that contribute to the pulmonary plexus and the cardiac plexus

Question 28

Describe the similarities and differences between the left and right vagus nerves
Where do the two sides meet?

Answer 28

Right tucks next to the trachea, behind the lung root then onto the anterior surface of the oesophagus forming the oesphageal plexus

Left wants to tuck to the trachea but the aortic arch is in the way: so runs lateral to the arch, behind the lung root then onto the anterior surface of the oesophagus

They meet and ramify to form the oesophagel plexus on the anterior surface of the oesphagus

Question 29

What are the right and left recurrent laryngeal nerves and where do they pass through?

Answer 29

The vagus nerves ‘forget’ to supply the larynx and so send recurrent laryngeal nerves back up the thorax

Right: Remebers at the level of the right subclavian artery and hoks under the right subclavian artery and ascends in the the tracheal-oesophageal groove

Left: given off (later than the right) on the lateral aspect of the arch and hooks around the ligamentum arteriosum, under the arch of the aorta and tucks into the tracheal-oesophageal groove into the larynx

Question 30

Where does the oesephagus begin and where does it run through?

Answer 30

Begins in the neck at the level of C6 and descends through the superior mediastinum then through the posterior mediastinum to pass through the diaphragm at the level of T10

Question 31

Does the oesophagus lie in front or behind the trachea?

At what level and through where does the oesphagus pierce the diaphragm?

Answer 31

It lies behind the trachea

Pierces the muscular part of the diaphragm through the oesphageal hiatus at the level of T10, just left of the midline.

Question 32

What structures run through the diaphragm with the oesphagus?
How deeply does it run in the body and how does this change towards its termination?
How does the relationship between the oesphagus and descending aorta change in the thorax?

Answer 32

The vagus nerve runs through the diaphragm with it
Sits just on top of the vertebral column (but it moves slightly forwards towards termination)

They swap: oesophagus moves forwards and to the left from back midline while the aorta from the left moves to the midline)

Question 33

Where are the major sites of narrowing in the oesophagus? Are they all intrinsic to the oesophagus?

Answer 33

The beginning (superior)
The end (inferior)
And the middle where the aortic arch impinges on it

Question 34

What is the function of the thoracic duct? Where does the thoracic duct begin and what with?

Answer 34

Collects lymphatics from 3/4 of the body and drains it into the venous system.
It begins at the Aortic Hiatus at T12 with the CISTERNA CHYLI (lymphatic sac collection between the crura) and ascends on the back of the oesphagus

Question 35

What two structures does the thoracic duct intervene between?

Answer 35

The oesophagus and the vertebral column

Question 36

Describe the ascending path of the thoracic duct

Answer 36

Up through the aortic hiatus, through the posterior mediastinum (between the oesophagus and vertebral column) and as it ascends moves from midline towards the left and arches in the superior mediastinum to the left and empties into the junction of the IJV and subclavian vein

Question 37

Describe the difference between lymph drainage of the upper right quadrant and the rest of the body

Answer 37

The upper right quadrant drains through the right thoracic duct which is less prominent (and sometimes not present) and empties into the IJV and subclavian vein on the right

The upper left quadrant joins in on draining into the thoracic duct on its way down to the venous drainage

Question 38

What are the major components within the posterior mediastinum? [5]

Answer 38

1. Descending thoracic aorta and its branches
2. Oesophagus and oesophageal plexus
3. Thoracic Duct
4. Azygous System of veins
5. Sympathetic trunks

Question 39

Describe the path down the thorax of the descending aorta and its branches

Answer 39

Starts to the left, approaches the midline and eventually passes behind the diaphragm between the crura at the level of T12: and gives off multiple branches on its way down

Question 40

What are the major branches of the descending aorta?

Answer 40

bronchial arteries, intercostal arteries, pericardial branches, oesophageal branches

Question 41

Describe the right side of the azygous system of veins. What is the significance of the right side?

Answer 41

The right side sits back on the posterior thoracic wall receiving posterior thoracic veins, arches forwards over the lung root and empties into the SVC. This right side is common to almost all individuals.

The left side has a lot of variability

Question 42

Where do the sympathetic trunks lie?

Answer 42

Lateral to the vertebral column on each side (roughly corresponding to the neck of the ribs)