4. Superior mediastinum

Question 1

Location of the mediastinum

Answer 1

Extends from the superior thoracic aperture (inlet) to the inferior thoracic aperture and between the sternum anteriorly and the thoracic vertebrae posteriorly

Question 2

What is the purpose of the mediastinum?

Answer 2

Acts as a conduit for structures (e.g. nerves, airways, GI tract) that pass through the thorax from one body region to another and for structures that connect thoracic organs to other body regions

Question 3

What vertebral levels do the IVC, oesophagus and aorta go through the diaphragm?

Answer 3

T8: IVC
T10: Oesophagus
T12: Descending aorta

Question 4

What are the different divisions of the mediastinum?

Answer 4

Superior (superior thoracic aperture to sternal angle)

Inferior (sternal angle to inferior thoracic aperture)

Question 5

What are the 3 divisions of the inferior mediastinum?

Answer 5

Anterior: anterior to heart in pericardial sac
Middle: pericardial sac and heart (largest)
Posterior: posterior to pericardial sac and diaphragm

Question 6

List the (8) contents of the superior mediastinum from anterior to posterior

Answer 6

Thymus
Phrenic Nerves 
Great veins
Main lymphatic trunks
Vagus nerves
Great arteries
Trachea and main bronchi
Upper oesophagus

Question 7

Describe the position of the Superior vena cava

Answer 7

Enters right atrium from above
Offset slightly to right of chest
Formed by asymmetric union of right and left brachiocephalic veins

Question 8

Describe the position of the Inferior vena cava

Answer 8

Enters right atrium from below, through central tendon of diaphragm (at T8)
Majority of IVC is in abdomen (little in thorax)

Question 9

Which veins join to form the left and right brachiocephalic veins?

Answer 9

Internal jugular veins from head and subclavian veins from upper limbs

Question 10

Describe the positioning of where the left brachiocephalic vein joins the right brachiocephalic vein

Answer 10

Left brachiocephalic vein crosses posterior to manubrium to join the right brachiocephalic vein to form the SVC.
Joining occurs just off to the right between the sterno-clavicular joint and the joint between the manubrium and 1st rib

Question 11

Role of the azygous vein

Answer 11

Drains the posterior wall of the thorax (e.g. posterior intercostal muscles) and abdomen
Drains into the superior vena cava

Question 12

What are the 3 components of the azygous system?

Answer 12

Azygous vein
Accessory hemi-azygous vein
Hemi-azygous vein

Question 13

What are the branches of the ascending aorta and aortic arch?

Answer 13

Ascending aorta:
Right and left coronary arteries 
Aortic arch:
Brachiocephalic trunk: divides into right common carotid and right subclavian arteries
Left common carotid artery 
Left subclavian artery

Question 14

Describe the relations of the aorta and great arteries to the airway

Answer 14

Aortic arch arises anterior to trachea
Arches over the left main bronchus at the lung root
Trachea lies behind and between brachiocephalic and left common carotid arteries
Oesophagus lies behind trachea

Question 15

What and where do common carotid arteries divide into?

Answer 15

Divide into external and internal carotids high in the neck

Question 16

Where is the pulmonary trunk and what is its function?

Answer 16

Emerges from right ventricle
Carries deoxygenated blood to lungs
Divides into left and right pulmonary arteries

Question 17

What connects the pulmonary trunk to the aortic arch? What is this a remnant of?

Answer 17

Ligamentum arteriosum connects PT to aortic arch.

Remnant of the ductus arteriosus (blood bypasses lungs in foetal life)

Question 18

Describe the ductus arteriosus

Answer 18

At junction between pulmonary trunk and aorta
Any blood on way to lungs in foetus can get diverted through the ductus arteriosus which connects straight back to aorta
After birth, ductus arteriosus closes to form ligamentum arteriosum

Question 19

Where are phrenic nerves formed?

Answer 19

Formed in the cervical plexus from C3, 4, 5

“C3,4,5 keeps the diaphragm alive”

Question 20

What are the phrenic nerves motor to?

Answer 20

Diaphragm

Question 21

What are the phrenic nerves sensory to?

Answer 21

Central tendon of the diaphragm
Mediastinal pleura
Pericardium
Peritoneum of central diaphragm

Question 22

The right phrenic nerve reaches the diaphragm lying on the surfaces of…

Answer 22

Right brachiocephalic vein
Superior vena cava
Right side of heart and pericardium (in front of lung root)

Question 23

Describe the relations of great arteries to main nerves

Answer 23

Vagus nerves lateral to common carotids
Left vagus passes anterior to aortic arch
Left phrenic crosses vagus to cross aortic arch more anteriorly

Question 24

Describe the relations of lung roots to main nerves

Answer 24

Vagus nerves go behind lung roots

Phrenic nerves go infront of lung roots

Question 25

Describe the positioning of left phrenic and vagus nerves

Answer 25

Cross arch of aorta
Left phrenic descends in front of lung root
Left vagus crosses behind lung root, gives off left recurrent laryngeal nerve: recurs around ligamentum arteriosum and aortic arch
Left vagus breaks up into many branches round oesophagus (so also passes through diaphragm at T10)

Question 26

Describe the positioning of the right vagus nerve

Answer 26

Lies on the trachea
Crosses behind the root lung
Recurrent laryngeal branch: recurs around right subclavian artery
Breaks up into branches on oesophagus