23 Mediastinum

STUDYAIMS

At the end of your study, you should be able to:

Identifythe mediastinum

Identifythe major arteries and veins of the mediastinum

Identifythe trachea

Identifythe esophagus

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GUIDES

Thorax: Mediastinum

[Plate 210, Mediastinum: Cross Section]

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[Plate 224, Mediastinum: Right Lateral View]

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[Plate 225, Mediastinum: Left Lateral View]

General Description

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The mediastinum is the central compartment of the thoraxbetween the two pleural cavities.

Stretches from the thoracic inlet to the diaphragm and from the sternum to the bodies of the thoracic vertebrae posteriorly. Its contents include the heart, trachea, esophagus, great vessels of the heart, lymph nodes, nerves, and fat.

The mediastinum is divided into two parts:

The superior part extends from the thoracic inlet to a plane at the level of the sternal angle and the T4/5 intervertebral disc

The inferior mediastinum extends from this plane to the diaphragm. Superior mediastinum contains the

superior vena cava

arch of the aorta and its branches

trachea

phrenic nerves

thoracic duct

esophagus

vagus nerves

left recurrent laryngeal nerve

thymus

Inferior mediastinum is subdivided into

anterior mediastinum

middle mediastinum

posterior mediastinum

The anterior mediastinum contains fat and the remnants of the thymus gland

The middle mediastinum contains the heart surrounded bythe pericardium and the roots of the great vessels. The great vessels are:

ascending aorta

superior vena cava

pulmonarytrunk

The posterior mediastinum contains

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esophagus and the esophageal plexus descending aorta

thoracic duct tracheobronchial lymph nodes azygos and hemiazygos veins thoracic sympathetic trunks thoracic splanchnic nerves

Structures in the Mediastinum a. Thymus gland

plays a central role in the development of the immune system

lies posterior to the manubrium

receives blood from the internal thoracic and anterior intercostal arteries is graduallyreplace byadipose tissue after puberty

b.Heart and pericardial sac (Section 3-5: Thorax-Heart)

c.Superior vena cava

Formed bythe union of two brachiocephalic veins

Returns blood to the heart from all structures above the diaphragm except the heart and lungs

Descends verticallyand terminates in the right atrium

Lies to the right of the ascending aorta and to the left of the right phrenic nerve

Receives azygous veins before piercing fibrous pericardium d. Brachiocephalic veins

Are formed in the root of the neck posterior to the sternoclavicular joints byunion of the internal jugular and subclavian veins.

Right brachiocephalic vein

receives lymph from the right lymph duct.

is accompanied byright phrenic vein.

Left brachiocephalic vein

is twice as long as the right

runs obliquelydown and behind the manubrium

crosses the roots of the three major branches of the aorta

receives lymph from the thoracic duct

e. Aorta

Ascending

begins at the aortic orifice

ascends to the 2nd right sternocostal joint

Arch

Begins at the 2nd right sternocostal joint and arches superiorlyand to the left

Anterior to the right pulmonaryarteryand bifurcation of the trachea

Passes over the root of the right lung

Ends at the bodyof the T4 vertebra

Descending (thoracic)

begins at the bodyof T4 vertebra

descends on the left side of the bodies of T5-12 vertebrae, posterior to the root of the left lung and the pericardium

enters the abdomen through the aortic hiatus at the T12 vertebral body

has a number of branches:

bronchial (1-2)

pericardial (twigs)

superior phrenic (1 pair)

esophageal (2)

posterior intercostal (9 pairs)

subcostal (1 pair)

f. Trachea

Continues from the larynx

Contains cartilaginous semicircular rings

Is completed bymuscle posteriorly

Descends anterior to the esophagus, slightlyto the right of the midsagittal plane

Bifurcates into right and left main bronchi at level of T4/T5 (angle of Louis)

Right bronchi divides into upper and lower lobar bronchi before entering the right lung g. Esophagus

Is a fibromuscular tube from the pharynxto the stomach

Contains both circular and longitudinal muscles, both skeletal and smooth

upper 1/3-skeletal muscle

lower 2/3-smooth muscle

Passes through right crus of diaphragm at T10

Continues for 1-2 cm below diaphragm

Supplied byesophageal plexus of nerves derived from

right and left vagal nerves

sympathetic nerves

Surrounded bya number of lymph nodes:

inferior deep cervical nodes

posterior mediastinal nodes

intercostal nodes

paratracheal nodes

superior and inferior tracheobronchial nodes.

Vascular supply

arterial: esophageal branches of the thoracic aorta venous: azygos, hemiazygos and accessoryazygos veins h. Thoracic duct

Originates from the cisterna chili in the abdomen Contains valves

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Ascends through the aortic hiatus in the diaphragm

Lies anterior to the bodies of T6-12 vertebral bodies, between the thoracic aorta and the azygos vein

Conveys lymph from the limbs, pelvic and abdominal cavities, left side of the thorax, left upper limb and left side of the head and neck

Empties into the venous system at the junction of the left internal jugular and left subclavian veins i. Azygos venous system

Drains blood from the back and thoracoabdominal walls

Is highlyvariable

Is composed of an unpaired azygos vein and its main tributary, the hemiazygos vein.

Offers an alternate route for blood to reach the heart if the inferior vena cava is blocked

Azygos vein

arises from ascending lumbar and/or renal veins and subcostal veins

ascends on the right side of the bodies of T5-T12 vertebrae

arches over the root of the right lung to join the superior vena cava

receives blood from

hemiazygos vein

posterior intercostals veins

esophageal veins

bronchial veins

communications with the vertebral venous plexus

Hemiazygos vein

arises from the left subcostal and ascending lumbar veins

ascends on the left side of the vertebral column from T12-T9

crosses the T9 vertebrae posterior to the aorta, esophagus and thoracic duct to emptyinto the azygos vein

receives blood from

inferior three posterior intercostals

inferior esophageal veins

accessoryhemiazygos (occasionally)

Accessoryhemiazygos vein

begins at the 4th or 5th intercostal space on the left

descends on the left side of vertebrae T5-T8

crosses over T7 or T8 vertebrae to join the azygos

sometimes joins the hemiazygos

receives blood from

posterior intercostal veins from the 4th through 8th intercostal spaces

left superior intercostal vein (occasionally) j. Vagus nerves and recurrent laryngeal nerves

The right vagus nerve

enters the thoraxanterior to the right subclavian arteryand immediatelygives rise to the right recurrent laryngeal nerve, which loops around the right subclavian arteryand ascends into the neck

descends on the right side of the trachea

passes posterior to the right brachiocephalic vein, superior vena cava and root of the right lung

gives rise to branches of the right pulmonaryplexus

continues to the esophagus, where it contributes to the esophageal nerve plexus and continues as the anterior vagal trunk into the abdomen

The left vagus nerve

enters the mediastinum between the left common carotid and left subclavian arteries

descends with the left phrenic nerve to the aortic arch

gives off the left recurrent laryngeal nerve just below the arch, which loops around the arch and ascends into the neck

passes posterior to the root of the left lung where it contributes to the left pulmonaryplexus

continues as a single nerve to the esophagus where it contributes to the esophageal nerve plexus and continues as the posterior vagal trunk into the abdomen

k. Phrenic nerves

Supplymotor and sensoryfibers to the diaphragm

Enter the superior mediastinum between the subclavian arteryand brachiocephalic vein on either side

Pass anterior to the roots of the lungs, unlike the vagus nerve

The right phrenic nerve descends on the right side of the inferior vena cava to the diaphragm

The left phrenic nerve

crosses the arch of the aorta

descends anterior to the root of the left lung and along the pericardium over the left atrium and ventricle pierces the diaphragm to the left of the pericardium

1. Thoracic sympathetic trunks

Are continuous with the cervical and lumbar sympathetic trunks

Shift mediallyas theydescend, crossing from the heads of the ribs to the costovertebral joints to the sides of the vertebral bodies

Give off the paired thoracic splanchnic nerves

greater splanchnic: from T5-T9 vertebral levels

lesser splanchnic: from T10-T11 vertebral levels least splanchnic: from T12 vertebral level

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[Plate 226, Esophagus In Situ]

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[Plate 231, Arteries of Esophagus]

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[Plate 232, Veins of Esophagus]

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[Plate 234, Nerves of Esophagus]

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