MEDIASTINAL CONTENTS
I. GENERALITIES
A. Mediastinum - defined as median partition between pleural sacs
B. Subdivided
1. Superior mediastinum
a. Boundaries
i. below - horizontal plane of sternal angle – T4
ii. above - oblique plane of thoracic inlet
iii. laterally - mediastinal pleura
b. Contents: great vessels, thymus, trachea, esophagus, vagus n., phrenic n., thoracic duct
2. Inferior mediastinum - subdivided into anterior, middle and posterior
a. Boundaries similar in all
i. Superior: plane of sternal angle (T4-5)
ii. Inferior: diaphragm
iii. Laterally: pleura (mediastinal and parietal -costal portions)
3. Middle mediastinum
a. Boundaries
i. Anteriorly: pericardium
ii. Posteriorly: pericardium
b. Contents: heart, pericardium, phrenic nerve, stems of great vessels
4. Anterior mediastinum
a. Boundaries
i. Anteriorly: body of sternum and sternocostalis m.
ii. Posteriorly: pericardium
b. Contents: lymph nodes, fat, sternopericardial ligaments
5. Posterior mediastinum
a. Boundaries
i. Anteriorly: pericardium and diaphragm
ii. Posteriorly: vertebral bodies, ribs
b. Contents: aorta, esophagus, azygous system of veins, vagus n., thoracic duct, lymph nodes, sympathetic and parasympathetic trunk and branches
II. MEDIASTINAL CONTENTS, CIRCULATORY, RESPIRATORY AND G.I. SYSTEMS
A. Heart and stems of great vessels - discussed elsewhere
B. Esophagus (thoracic portion)
1. Courses through superior and posterior mediastinum
a. continuous superiorly with pharynx in neck
b. pierces diaphragm at T10 to join stomach
c. upper portion in thorax, slightly left of midline
d. at lower end passes through diaphragm to left of midline
e. pushed toward midline by aortic arch
2. Muscular tube
a. striated muscle in neck
b. smooth muscle in lower third
c. mixed in middle
3. Relationships
a. anterior: trachea, l. bronchus (crosses it), pericardium (l. atrium)
b. posteriorly: vertebral bodies, thoracic duct, azygous system, intercostal aa.
c. right: terminal azygous
d. left: aortic arch crosses
4. Constrictions
a. superior end in neck
b. where aorta and left bronchus compress it
c. near gastric end
5. Arteries - esophageal branches of thoracic aorta
a. anastomose with inferior thyroid aa. and left gastric aa.
6. Veins
a. submucosal plexus and surface plexus
b. drain to azygous system
i. may drain superiorly to inferior thyroid vv.
ii. may drain inferiorly to gastric vv. (portal-caval)
7. Innervation
a. striated (branchiomeric) by vagal (recurrent br.)
b. smooth by parasympathetic vagal, vasculature by sympathetic
c. esophageal plexus
i. surrounds lower thoracic esophagus
ii. mostly vagal fibers - ant. and postganglionic plexus
iii. some GVA of sympathetic
C. Trachea and main stem bronchi - discussed with lungs
1. D-shaped tubes supported by cartilage
2. Trachea courses from larynx to bifurcation
a. bifurcation at T4-5
b. anteriorly: great vessels and thymus
c. posteriorly: esophagus
d. azygous vein to rt. of bifurcation
e. aortic arch to l. of bifurcation
3. Carina
4. Main bronchi
a. l. bronchus anterior to aorta
b. rt. bronchus inferior to azygous arch
5. Blood supply from bronchial aa. and inferior thyroid aa.
6. Innervation - pulmonary plexus and br. of vagus and sympathetic trunk
D. Aorta
1. Subdivisions
a. Ascending aorta - from left ventricle to aortic arch - mostly within pericardial sac
b. Aortic arch
i. above plane of sternal angle
ii. courses posteriorly and laterally
c. Descending portion (thoracic aorta)
i. continuation of arch on left side of vertebrae
ii. course downward toward midline
iii. reaches midline at T12 as it passes through diaphragm
iv. continuous with abdominal aorta
2. Major branches
a. coronary aa. from sinuses of ascending aorta
b. great arteries from arch
i. brachiocephalic (innominate)
(a) superiorly and to rt. of trachea
(b) divides into rt. subclavian and common carotid aa. behind SC rt. joint
ii. left common carotid
(a) superior to trachea
(b) anterior to left subclavian a. and trachea
iii. left subclavian
(a) superior to trachea and esophagu
(b) lies against left lung and pleura
c. branches of descending aorta
i. posterior intercostals – 9 pr. (ICS 3-11)
ii. subcostal
iii. pericardial
iv. esophageal (2)
v. bronchial
vi. superior phrenic
vii. mediastinal
E. Pulmonary trunk and arteries
1. Courses upward and to left
2. Divides into left and right aa. in concavity of aortic arch
3. Right pulmonary artery
a. posterior to ascending aorta and s. vena cava to root of lung
4. Left pulmonary artery
a. anterior to descending aorta to root of lung
b. ligamentum arteriosum connects proximal portion to aortic arch at "take off point" of L. Subclavian a.
F. Great veins of thorax
1. Generally in plane anterior to arteries
2. Inferior vena cava
a. pierces diaphragm at T8
b. immediately enters pericardium and heart
3. Left brachiocephalic vein
a. formed by confluence of internal jugular and subclavian vv.
b. courses obliquely to right in front of great aa.
c. joins right brachiocephalic v. behind ICS 1
4. Right brachiocephalic vein
a. formed similar to left
b. course vertically behind manubrium
c. joins left vein to form s. vena cava
5. Superior vena cava
a. formed behind ICS 1 by two brachiocephalics
b. course vertically to right atrium
c. receives azygous v.
6. Pulmonary veins
a. directly from root of lung to left atrium
G. Azygous (a.k.a. ‘azygos’ = “unpaired”) venous system
1. Generally drains intercostal veins, some lumbar segmental vv.
2. Azygous vein
a. formed by union of right ascending lumbar v. and right subcostal v.
b. ascends along right side of vertebral column to T4
i. superficial to intercostal aa.
ii. thoracic duct to the left
c. arches over bronchus to join s. vena cava
d. tributaries
i. r. posterior intercostal vv. - 5-11
ii. r. superior intercostal v. - drains ICS 2-4
iii. hemiazygous v.
iv. accessory hemiazygous v.
v. esophageal, mediastinal, pericardial, bronchial vv.
3. Hemiazygous vein
a. formed by union similar to azygous
b. ascends left of vertebral column
c. crosses over to join azygous at T8-9
d. posterior to aorta and esophagus
e. tributaries
i. left posterior intercostal vv. 7-11
ii. esophageal, mediastinal, pericardial vv.
4. Accessory hemiazygous vein
a. formed by posterior intercostals 4-8
b. left of vertebral column
c. crosses to join azygous at T8
H. Left superior intercostal vein
1. formed by upper posterior intercostal vv.
2. drains to left brachiocephalic vein
I. Thoracic duct
1. drains all of body below diaphragm and left side of thorax
2. arises from union of
a. r. lumbar trunk
b. l. lumbar trunk
c. intestinal trunk
3. cisterna chyli - dilation of origin of thoracic duct
4. ascends through aortic hiatus of diaphragm
5. ascends between azygous vein and aorta to T4-5 plane
6. crosses to left at T4-5 to ascend beside esophagus into neck
7. joins confluence of internal jugular and subclavian vv.
8. tributaries
a. from intercostal nodes
b. posterior mediastinal trunks
c. vessels in neck
J. Mediastinal relations of lungs
1. fixed lungs show impressions of mediastinal structures
III. NERVES IN MEDIASTINUM
A. Vagus nerve(s)
1. Carry
a. preganglionic parasympathetic fibers for thoracic and abdominal viscera
b. GVA
c. SVE to striated mm. of upper esophagus
2. Course of right vagus
a. enters thorax posterolateral to brachiocephalic v., anterior to rt. subclavian
i. R. recurrent br. hooks under subclavian a. to ascend near trachea and esophagus
b. descends to lateral side of trachea
c. posterior to root of lung
d. on posterior esophagus to form plexus
e. passes through diaphragm with esophagus as post. vagal trunk
3. Course of left vagus
a. enters thorax between carotid and subclavian arteries
b. descends to cross left side of aortic arch
i. recurrent br. recurves around ligamentum arteriosum to ascend in neck between trachea and esophagus
c. behind root of lung
d. onto anterior esophagus - forms plexus
e. through diaphragm with esophagus as ant. vagal trunk
4. Branches of vagus in thorax
a. left recurrent laryngeal
b. br. to pulmonary plexus
c. br. to esophagus
d. br. to cardiac plexus
B. Phrenic nerves (somatic)
1. Carry
a. motor fibers to all of diaphragm (C3-5)
b. sensory to parietal pleura and peritoneum, pericardium and central diaphragm
2. Course of right phrenic
a. descends along right side great veins
i. right brachiocephalic v.
ii. sup. vena cava
iii. right atrium
iv. inf. vena cava
b. anterior to root of lung
3. Course of left phrenic
a. along left side of subclavian a.
b. crosses left side of aortic arch
c. anterior to root of lung
d. along left side of pericardial sac
C. Sympathetic trunk
1. Continuous with cervical and lumbar trunks
2. Vertical orientation - medial inclination at lower end
a. against neck of ribs in upper thorax
b. against heads of ribs midthorax
c. on vertebral bodies in lower thorax
d. curves anteriorly in neck
e. pierces diaphragm
3. Ganglia on trunk vary - 11-12 with fusion of segmental ganglia common at upper end
a. stellate ganglion = fused upper thoracic + lower cervical
4. Branches of trunk
a. white rami communicantes – preganglionic (T1-L2)
b. gray rami communicantes – postganglionic – all levels
c. direct thoracic visceral branches - postganglionic
d. splanchnic nerves - to prevertebral plexus
i. greater splanchnic - ganglia T5-9
ii. lesser splanchnic - ganglia T10-11
iii. least splanchnic - ganglion T12
IV. THYMUS
A. Glandular tissue - part of immune system
B. Varies in size
1. Prominent in children - greatest relative size at 2 yrs. of age
2. Maximum size near puberty
3. Involutes in later life - glandular tissue replaced by fat
C. Develops as endodermal outpocketing of 3rd branchial arch
D. Structure and location
1. two flattened lobes near median plane
2. in superior mediastinum
a. anterior to great veins
b. posterior to sternum and parietal pleura
E. Blood supply - general mediastinal branches
F. Innervation - br. from vagus and sympathetic trunk
G. Lymphatics > anterior mediastinal nodes
H. Function – develop and maintenance of immune system
V. LYMPHATICS IN THORACIC CAVITY
A. Thoracic wall - discussed with intercostal spaces
B. Lungs and heart
C. Mediastinum
1. Anterior mediastinal nodes
a. near great vessels in superior mediastinum
b. afferents: thymus, pericardium, pleura, right heart
c. efferents: join retrosternal and tracheobronchial to form bronchomediastinal trunk
2. Posterior mediastinal nodes
a. near esophagus and aorta
b. afferents: mediastinal viscera
c. efferents: thoracic duct
D. Thoracic duct receives efferents from mediastinal and posterior intercostal nodes - drains to confluence of vv.
E. Right lymphatic duct - formed by confluence of lymph trunks from right head, arm,
bronchomediastinal. Drains to confluence of veins on right
F. Bronchomediastinal trunks drain separately to confluence of vv.