Theme 1: Lecture 2 - The Superior Mediastinum Flashcards
Where is the mediastinum located?
Between the left and right pleural cavities
What are the three names for the junction between the manubrium and sternal body?
- Level T4/5
- Sternal angle
- Transverse thoracic plane
What are the divisions of the mediastinum?
Superior and inferior
What is the inferior mediastinum further divided into?
Anterior, middle and posterior
What is the superior boundary of the superior mediastinum?
Superior thoracic aperture (manubrium, 1st rib and costal cartilage, T1 vertebra)
What is the inferior boundary of the superior mediastinum?
Transverse thoracic plane or T4/5 or sternal angle
What is the anterior boundary of the superior mediastinum?
Manubrium
What is the posterior boundary of the superior mediastinum?
T1-T4/5 vertebral bodies
What are the lateral boundaries of the superior mediastinum?
Mediastinal pleura`
What is contained in the superior mediastinum?
- Arteries (arch of aorta and branches)
- Veins (brachiocephalic veins, superior vena cava)
- Phrenic and Vagus nerves
- Oesophagus
- Trachea
- Thoracic duct
- Thymus gland (part)
Where is the thymus gland located?
Immediately deep to the sternum and superficial to the heart. It is in both the superior and anterior mediastinum
What is the thymus gland?
- It’s a lymphoid organ
- T and B cells are produced in the bone marrow, T cells mature in the thymus (once they’ve matured they migrate to lymph nodes throughout the body)
- Lymph drains through the parasternal, brachiocephalic and tracheobronchial nodes
- Atrophies after puberty and is replaces by fat
Which arteries supply the thymus gland?
Internal thoracic arteries (these branch off the subclavian arteries and travel down either side of the sternum on the inside of the thoracic wall)
What are the arteries in the superior mediastinum?
The arch of the aorta and its 3 branches:
- Brachiocephalic trunk (this branches into the right common carotid and subclavian arteries)
- Left common carotid artery
- Left subclavian artery
What arteries branch off the ascending aorta?
Left and right coronary arteries
What arteries branch off the descending aorta?
- Posterior intercostal arteries (anastomose with anterior intercostal arteries coming from the internal thoracic arteries)
- Left bronchial arteries
- Lumbar arteries
What veins are contained in the superior mediastinum?
- Superior vena cava
- brachiocephalic veins
Which veins form the superior vena cava?
The union of the left and right brachiocephalic veins
Which veins form the brachiocephalic veins?
The union of the internal jugular vein and subclavian vein
What is the ligamentum arteriosum?
- A ligamentous structure that connects the pulmonary trunk to the aorta
- An embryological remnant of the Ductus Arteriosus
What is the Ductus Arteriosus?
- An open duct connecting the pulmonary trunk and the arch of aorta
- Important in foetal circulation allowing blood to bypass the immature lungs
- Closes at birth
Describe the Trachea
- Tube with C-shaped cartilage rings
- Trachealis muscle runs posteriorly
- Runs from larynx to bronchi
Describe the oesophagus
- Muscular tube
- Runs from pharynx to stomach
Describe the thoracic duct
- Transports lymph to the venous system
- Starts beneath the diaphragm, travels all the way through the thorax between the vertebral bodies and oesophagus before draining into the venous system at the venous angle
- Receives multiple branches throughout the thorax
- Numerous branches associated with thoracic (descending) aorta
Where do the phrenic nerves originate from?
Anterior rami of spinal nerves C3, C4 and C5
Where do the phrenic nerves travel?
- Enter mediastinum between brachiocephalic veins an subclavian arteries
- Pass anterior to the main bronchi
What do the phrenic nerves do?
- Provide motor supply to the diaphragm
- Provide sensory supply to the diaphragm (central region) and parietal pleura (diaphragmatic and mediastinal)
How many phrenic nerves are there?
2
Where do the vagus nerve originate from?
The brainstem (AKA cranial nerves 10)
Where do the vagus nerves travel?
- They enter the mediastinum just medial to the phrenic nerves
- They continue into the posterior mediastinum coursing posterior to the main bronchi
What do the vagus nerve give rise to?
The recurrent laryngeal branch to supply the larynx
What do the vagus nerves do?
Provide parasympathetic supply via the pulmonary, cardiac and oesophageal plexi
What does the anterior mediastinum contain?
- Thymus gland in children which atrophies in the adult and is replaced by fatty tissue
- Pericardial-sternal ligaments
- Lymph nodes (parasternal) and vessels
- Internal thoracic artery
What is the superior border of the anterior mediastinum?
Level T4/5
What is the inferior border of the anterior mediastinum?
Diaphragm
What is the anterior border of the anterior mediastinum?
Sternum
What is the posterior border of the anterior mediastinum?
Pericardial sac
How does the heart develop?
- Angiogenic clusters develop in the mesoderm and these canalise (become hollow) to form early blood vessels
- Two large blood vessels known as heart tubes form in the cardiogenic area
- The paired heart tubes grow and fuse
What day do the paired heart tubes grow and fuse?
21 (this is also when the heart starts to beat, this is important as the embryo can no longer satisfy its needs by diffusion alone)
What are the regions that the newly formed heart tube are divided into?
- Sinus venosus
- Primordial atrium
- Primordial ventricle
- Truncus arteriosus
What is the sinus venosus?
The primitive inflow vessels into the heart tube of an embryo
What is the truncus arteriosus?
The primitive inflow vessels into the heart tube of the embryo
On which day does the heart tube become too long for the pericardial cavity and start to fold?
Day 23
What does folding of the heart result in for the heart’s orientation?
- Ventricles and outflow tracks positioned anteriorly
- atria and veins posteriorly
What are the two features of the foetal heart that allow blood to bypass the lungs?
- Foramen ovale
- Ductus arteriosus
What is the foramen ovale?
Opening between the left and right atria in the foetus and allows oxygenated blood to bypass the lungs
What is the fossa ovalis?
The embryological remnants of the foramen ovale which closes after birth
Why can blood travel from the right to left atrium through the foramen ovale?
Due to the higher pressure in the right than the left atrium
