Respiratory: Anatomy - Overview of the mediastinum Flashcards
Describe the divisions of the mediastinum
Superior mediastinum: from superior thoracic aperture to thoracic plane
Inferior mediastinum: divided into anterior (anterior to pericardium), middle (pericardium and its contents), and posterior (posterior to pericardium)
Describe the contents of the superior mediastinum from anterior to posterior
- Thymus
- Great vessels:
- Veins anterior to arteries
- Nerves - Cervical viscera and related nerves:
- Trachea anterior to oesophagus - Lymphatic trunks (including thoracic ducts)
At what landmark do the internal jugular and subclavian veins join to form the brachiocephalic veins?
Behind sternoclavicular joints
At what landmark do the R and L brachiocephalic veins drain into the SVC?
Inferior border of R 1st costal cartilage
At what landmark does the SVC join the RA?
3rd costal cartilage
Describe the relationship of the SVC to the trachea and ascending aorta
Anterolateral to trachea
Posterolateral to ascending aorta
The R phrenic nerve lies between which two superior mediastinal structures?
SVC and mediastinal pleura
What is the diameter of the ascending aorta?
2.5cm
What are the branches of the ascending aorta and where do they originate?
Coronary arteries from aortic sinuses
At what landmark does the arch of the aorta begin?
Behind 2nd R sternocostal joint
Describe the course of the aortic arch
Begins at level of right 2nd sternocostal joint
Ascends anteriorly to pulmonary artery and bifurcation of trachea
Reaches apex to left of trachea and oesophagua, passes over root of left lung
Descends posterior to root of left lung beside T4 vertebra
Becomes thoracic aorta behind left 2nd sternocostal joint at level of T4
Describe the course of the azgous vein
Similar to arch of aorta but to R of trachea
Descends over right lung root
Between what two structures does ligamentum arteriosum pass?
From left pulmonary artery to inferior arch of aorta
Describe the course of brachiocephalic trunk in the superior mediastinum
Arises behind manubrium and L brachiocephalic vein, and anterior to trachea
Ascends to reach right side of trachea and R SCJ
Divides into right common carotid and subclavian arteries
Describe the course of the left common carotid artery in the superior mediastinum
Arises posterior to manubrium, and posterior and to the left of the brachiocephalic trunk
Ascends anterior to left subclavian, and initially anterior than to the left of trachea
Enters the neck behind the L SCJ
Describe the course of the left subclavian artery in the superior mediastinum
Arises posterior to left common carotid
Ascends lateral to trachea and left common carotid
Does not give off any branches within the mediastinum
Enters neck behind the L SCJ
List the 10 nerves/plexuses of the superior mediastinum
Right and left vagus nerves
Right and left pulmonary plexuses
Right and left phrenic nerves
Right and left recurrent laryngeal nerves
Oesophageal plexus
Cardiac plexus
Describe the course of the right vagus nerve in the superior mediastinum
Enters behind R SCJ and R brachiocephalic vein, anterior to R subclavian artery
Gives off R recurrent laryngeal
Runs posteroinferiorly to right of trachea, then passes behind R brachiocephalic vein, SVC and R lung root
Divides to form R pulmonary plexus
Then exits as single trunk before contributing to the oesophageal plexus
Similarly exits oesophageal plexus as single trunk before contributing to the cardiac plexus
Enters abdominal cavity as anterior and posterior trunks via the oesophageal hiatus
Describe the course of the right recurrent laryngeal nerve in the superior mediastinum
Comes off vagus nerve and hooks around R subclavian artery
Ascends between oesophagus and trachea
Describe the course of the right phrenic nerve in the superior mediastinum
Enters superior mediastinum between R subclavian artery and R brachiocephalic vein
Passes laterally to R brachiocephalic vein, SVC and pericardium over RA
Passes anterior to R lung root and descends to R of IVC, entering abdomen via caval opening
Describe the course of the left vagus nerve in the superior mediastinum
Enters superior mediastinum between L common carotid and subclavian arteries
Diverges posteriorly from L phrenic nerve to the left of the aortic arch, separated laterally by L superior intercostal vein
Curves medially at inferior border of aortic arch and gives off L recurrent laryngeal nerve
Passes posterior to L lung root and divides to form L pulmonary plexus
Describe the course of the left recurrent laryngeal nerve in the superior mediastinum
Given off by L vagus nerve
Hooks around aortic arch, lateral to ligamentum arteriosum
Ascends between oesophagus and trachea
Describe the course of the left phrenic nerve in the superior mediastinum
Enters superior mediastinum between L common carotid and subclavian arteries, and origin of L brachicephalic vein
Crosses left surface of aortic arch anterior to L vagus and passes over L superior intercostal vein (vagus passes behind and medially)
Descends anterior to L lung root and runs along pericardium over LA/LV
Pierces diaphragm to L of pericardium
Where does the trachea begin?
Below cricoid cartilage at C6, as a continuation of the larynx
Describe the passage of the trachea in the superior mediastinum
Descends anterior to oesophagus and enters slightly to the right before bifurcating at T5
What is the level of the tracheal bifurcation (carina)? How does this change with respiration?
T5
Stretches in full inspiration and may reach T6
Describe the structure of the trachea
15-20 horseshoe-shaped hyaline cartilages
Gaps in rings at back with trachealis (smooth muscle)
High content of elastic fibres
Four anatomical features of the trachea and their functions
Cartilage maintains airway patency
Cilia clear debris
Goblet cells secrete mucus to trap particles
Secretions humidify air
Describe the lymphatic drainage of the trachea
Posteroinferior group of deep cervical lymph nodes
Pre and para tracheal lymph nodes
Describe the nerve supply of the trachea
Vagal and recurrent laryngeal nerves
Also receives sympathetic innervation
Describe the posterior, lateral and anterior relations of the trachea
Posterior: oesophagus, recurrent laryngeal nerves
Anterior: L brachiocephalic vein, thymus, manubrium
Lateral: carotid sheaths, lateral lobes of thyroid
How long are the cervical and thoracic portions of the trachea?
~5cm each
Describe the structure of the oesophagus
Fibromuscular tube extending from pharynx to stomach
What type of muscle lines the proximal vs distal oesophagus?
Proximal half: striated skeletal muscle
Distal half: smooth muscle
What type of epithelium lines the proximal vs distal oesophagus?
Proximal: stratified squamous
Distal: columnar
What is the total length of the oesophagus?
25cm
Describe the course of the oesophagus through the mediastinum
Begins at cricoid cartilage (C6)
Enters superior mediastinum anterior to vertebral bodies T1-4 and posterior to trachea
Inclines to left but pushed back to median by aortic arch
Compressed by L lung root, with thoracic duct lying to its left
Inclines to left again and crosses over aorta distally
Exits diaphragm via oesophageal hiatus (T10)
Ends at cardiac orifice of stomach (T11)
What forms the three oesophageal impressions?
- Aortic arch
- Left main bronchus
- Diaphragm
What nerve supplies the oesophagus?
Vagus nerve
Describe the blood supply and venous drainage of the upper, middle, and lower thirds of the oesphagus
Upper: interior thyroid artery, brachiocephalic veins
Middle: oesophageal branches of thoracic aorta, azygos vein
Lower: branches of L gastric artery, tributaries to L gastric vein -> portal vein
What it the pathophysiology of oesophageal varices in cirrhosis?
Cirrhosis leads to portal HTN
Portal HTN -> increased pressure through L gastric tributaries draining oesophageal veins -> oesophageal varices
Describe the borders of the posterior mediastinum
Anterior: pericardium and diaphragm
Posterior: T5-12 vertebrae
Superior: thoracic plane
Between parietal pleura of the two lungs
Five contents of the posterior mediastinum
- Thoracic aorta
- Thoracic duct and lymphatic trunks
- Posterior mediastinal lymph nodes
- Azygos and hemi-azygos veins
- Oesophagus and oesophageal nerve plexus
Explain the concept of the “planes” of aortic branches
Anterior: midline unpaired branches to gut and its derivatives (including oesophagus in the thorax)
Lateral: paired visceral branches to organs other than the gut and its derivatives (including bronchial in the thorax)
Posterolateral: paired parietal to body wall
What are the 6 branches of the thoracic aorta? How many of each are there? Which are exceptions to the concept of “planes” of aortic branches and why?
- Posterior intercostal (9) and subcostal arteries
- Oesophageal arteries (4-5)
- Bronchial arteries (1-2)
- Mediastinal branches*
- Pericardial branches*
- Superior phrenic arteries**
- exception: arise anteriorly but do not supply gut and its derivates
** exception: supplies part of body wall but arises anterolaterally (instead of posterolaterally)
What is the thoracic duct and what parts of the body does it drain?
Largest lymphatic channel in the body
Carries all lymph except that from the superior R quadrant (which drains to the R lymphatic duct)
Describe the course of the thoracic duct
Starts at cisterna chyli at the level of T12
Runs behind R crus and to R of aorta, ascends through aortic hiatus into mediastinum
Runs behind oesophagus
Crosses midline to L at T4-6
Enters venous system somewhere at confluence of L subclavian and internal jugular veins (origin of L brachiocephalic vein)
Which other three trunks does the thoracic duct receive in the mediastinum?
Subclavian
Jugular
Bronchomediastinal
At what vertebral level does the thoracic duct cross the mediastinum to the left?
Around T4-6
Which hiatus does the thoracic duct ascend through to enter the posterior mediastinum?
Aortic
What venous drainage is received by the azygos vs the hemiazygos veins?
Azygos: posterior intercostal veins (except the inferior three), vertebral venous plexuses, mediastinal veins, bronchial veins, oesophageal veins
Hemiazygos: inferior three posterior intercostal veins, inferior oesophageal veins, several small mediastinal veins
Describe the course of the hemiazygos vein
Arises from posterior IVC and/or renal vein, with additions on its L side from subcostal and ascending lumbar veins
Ascends on left side of vertebral column to level of T9 before crossing (posterior to aorta, thoracic duct and oesophagus) to join the azygos
At what level does the accessory hemiazygos arise and where does it descend to?
Medial 4th/5th intercostal space
Descends to level of T5-8
Describe the relations of the sympathetic trunks in the posterior mediastinum
In superior thorax: lie against head of ribs
In mid-thorax: lie against costovertebral joints
In inferior thorax: lie against sides of vertebral bodies
Four (or five) contents of anterior mediastinum
- Sternopericardial ligaments
- Fat
- Lymphatic vessels and (few) lymph nodes
- Branches of internal thoracic vessels
(In children/infants: 5. Inferior part of thymus)
How long is the thoracic duct?
45cm
