Respiratory System 5: Mediastinum Flashcards
What is the mediastinum?
It is the central compartment of the thoracic cavity, sandwiched between the left and right pulmonary cavities, containing all thoracic viscera and structures except the lungs.
What are the boundaries of the mediastinum?
It is covered on each side by the mediastinal pleura.
What forms the superior boundary of the mediastinum?
The superior thoracic aperture.
What structure forms the inferior boundary of the mediastinum?
The diaphragm.
What structure forms the anterior boundary of the mediastinum?
The sternum.
What forms the posterior boundary of the mediastinum?
The vertebral bodies of T1 to T12.
What forms the lateral boundaries of the mediastinum?
The mediastinal pleura on the left and right sides.
How is the mediastinum positioned within the thoracic cavity?
It is centrally located, extending from the sternum anteriorly to the vertebral bodies posteriorly and bounded laterally by the lungs.
How is the mediastinum divided for descriptive purposes?
It is divided into superior and inferior parts.
What separates the superior and inferior mediastinum?
An imaginary transverse plane at the level of the sternal angle (also known as the transverse thoracic plane).
How is the inferior mediastinum further subdivided?
3 parts: Into anterior, middle, and posterior parts.
What lies in the anterior part of the inferior mediastinum?
The space between the sternum and the pericardium.
What is contained in the middle part of the inferior mediastinum?
The pericardium, the heart, and the roots of the great vessels.
What structures are found in the posterior part of the inferior mediastinum?
The space between the pericardium and the bodies of T5-T12 vertebral bodies.
What anatomical landmark is used to define the transverse thoracic plane?
The sternal angle.
Where is the superior mediastinum located?
It lies between the manubrium and the upper four thoracic vertebrae.
What are the structures found in the superior mediastinum from anterior to posterior?
- Thymus
- Great vessels (brachiocephalic veins and SVC) anterior to the arteries (arch of aorta and roots of its major branches)
- Vagus and phrenic nerves
- Inferior continuation of trachea and esophagus, including the left recurrent laryngeal nerve
- Thoracic duct and lymphatic trunks
What great veins are located in the superior mediastinum?
The brachiocephalic veins and the superior vena cava (SVC).
What major arteries are found in the superior mediastinum?
The arch of the aorta and its major branches.
Where is the anterior mediastinum located?
It lies between the sternum and the pericardium.
What type of tissue is commonly found in the anterior mediastinum?
A variable quantity of fat.
What is the role of the thymus in the anterior mediastinum during childhood?
The thymus lobes are located in the superior and anterior mediastinum, enlarging to reach their greatest size at puberty and regressing with age.
What structures are found in the anterior mediastinum?
Sternopericardial ligaments (extremely variable and often not detectable)
Some lymph nodes
Branches of the internal thoracic vessels.
Where is the middle mediastinum located?
It is bounded by the pericardium and contains everything deep to it, including the roots of the great vessels.
What major structures are contained in the middle mediastinum?
- The pericardium, the heart, and the ascending aorta
- The lower half of the superior vena cava (SVC), receiving the azygos vein (the arch)
- The pulmonary trunk and the left and right pulmonary arteries and veins
- The phrenic nerves (running anterior to the roots of the lungs).
Where is the posterior mediastinum located?
It is located posterior to the pericardium and posterior part of the diaphragm, containing the roots of the great vessels.
What major structures are contained in the posterior mediastinum?
- Descending aorta, azygos vein, hemiazygos, and accessory hemiazygos veins
- Oesophagus and the anterior and posterior vagal trunks
- Thoracic duct
- Thoracic sympathetic trunks (technically not included as they lie lateral to the vertebral bodies)
- Splanchnic nerves.
What is the thoracic aorta?
The thoracic aorta is the continuation of the arch of the aorta, giving rise to multiple branches.
Where does the thoracic aorta begin and how does it relate to the oesophagus?
It begins more on the left side of the body at the T4 vertebra and gradually approaches the median plane, displacing the oesophagus to the right.
What are the major branches of the thoracic aorta?
Posterior intercostal arteries (3rd–12th)
Bronchial branches (1–2 branches on the left)
Oesophageal branches (4–5 branches)
Superior phrenic arteries
What do the posterior intercostal arteries supply?
They supply the intercostal spaces between the ribs.
Into which part of the mediastinum does the oesophagus descend?
The oesophagus descends into the posterior mediastinum from the superior mediastinum.
What are the three parts of the oesophagus?
Cervical
Thoracic
Abdominal
What is the path of the oesophagus through the body?
It travels from the neck into the thoracic cavity, passes through the oesophageal hiatus, enters the abdominal cavity, and connects with the stomach.
Where are the cervical and upper thoracic parts of the oesophagus located?
They are located directly posterior to the trachea up to the tracheal bifurcation.
Describe the positioning of the oesophagus relative to the thoracic aorta.
Initially, the oesophagus descends to the right of the arch and the thoracic aorta. Just before passing through the oesophageal hiatus, it crosses to run anterior to the thoracic aorta.
Where does the oesophagus receive its nerve supply from?
The oesophagus receives its nerve supply from the oesophageal plexus, which contains both sympathetic and parasympathetic fibres.
Which nerves form the oesophageal plexus?
The left and right vagus nerves give off branches that form the oesophageal plexus.
On which walls of the oesophagus is the oesophageal plexus found?
It is found on both the anterior and posterior walls of the oesophagus.
What do the anterior and posterior vagal trunks form as continuations of?
They form as continuations of the oesophageal plexus and cross the oesophageal hiatus into the abdomen, joining with the gastric plexus of the stomach.
How do postganglionic sympathetic fibres reach the oesophageal plexus?
They reach via the cardiopulmonary splanchnic (cardiac) nerves and potentially the greater splanchnic nerve.
From which thoracic spinal cord segments do preganglionic sympathetic fibres originate?
They originate from the 2nd to 6th thoracic spinal cord segments.
What are the effects of sympathetic innervation on the oesophagus?
Decreases peristalsis
Decreases gland secretions
What are the effects of parasympathetic innervation on the oesophagus?
Increases peristalsis
Increases gland secretions
Is sympathetic innervation more or less extensive than parasympathetic innervation?
Sympathetic innervation is much less extensive than parasympathetic innervation.
What causes oesophageal constrictions?
The oesophagus is compressed by neighboring structures, appearing as impressions or “constrictions” in its thoracic part.
How are oesophageal constrictions typically observed?
They appear as narrowings of the lumen in a radiograph when barium is swallowed.
What are the three main structures causing constrictions in the oesophagus?
Arch of the aorta
Left main bronchus
Diaphragm
What is required to navigate past oesophageal constrictions during a procedure?
Careful maneuvering of the gastroscope is required.
What is the thoracic duct?
It is the largest lymphatic channel in the body.
Where is the thoracic duct located?
It lies on the anterior aspect of the bodies of the inferior 7 thoracic vertebrae within the posterior mediastinum.
What regions does the thoracic duct drain?
It drains lymph from all body regions except the right superior quadrant.
The thoracic duct drains lymph of all body regions except the
right superior quadrant.
Where does the thoracic duct originate?
It originates from the cisterna chyli in the abdomen.
Through which structure does the thoracic duct ascend into the thorax?
It ascends through the aortic hiatus in the diaphragm.
What structures surround the thoracic duct in the posterior mediastinum
Left: Thoracic aorta
Right: Azygos vein
Anteriorly: Oesophagus
Posteriorly: Vertebral bodies
Where does the thoracic duct terminate?
At the left venous angle.
At what vertebral level does the thoracic duct cross to the left?
At the level of the T4/T5 intervertebral disc.
What is another name for the sympathetic ganglia along the sympathetic trunks?
Paravertebral ganglia.
What do the splanchnic nerves convey?
Sympathetic fibers to thoracic (cardiopulmonary nerves) and abdominopelvic viscera (greater, lesser, and least splanchnic nerves).
Where do the greater, lesser, and least splanchnic nerves synapse?
At the prevertebral ganglia in the abdominal cavity.
Where do the preganglionic fibers of the splanchnic nerves originate?
From the cell bodies in the lateral horns of the gray matter of spinal cord segments T5-L2/3.
What spinal levels do the greater splanchnic nerves arise from?
T5-T9/10 levels.
What spinal levels do the lesser splanchnic nerves arise from?
T10 and T11 levels.
What spinal level does the least splanchnic nerve arise from?
T12 level.
