Transcript 18- Mediastinum

Mediastinum
The thoracic cavity is divided into median partition called mediastinum and
laterally placed pleurae and lungs.
On each side 1st rib
It is a movable partition that extends
superiorly to the thoracic inlet and the
root of the neck and inferiorly to the
diaphragm ( thoracic outlet) .
It extends anteriorly to the sternum and
posteriorly to the 12th thoracic vertebrae.
7 to 10th costal cartilages
Lateral: 11 & 12 ribs.
It is divided into superior & inferior
mediastina by an imaginary plane
passing from the sternal angle
anteriorly to the lower border of the body
of the 4th thoracic vertebra posteriorly.
The inferior mediastinum is subdivided
into the middle mediastinum which
consists of the pericardium and heart.
The anterior mediastinum is the space between the pericardium and the sternum.
The posterior mediastinum lies between the pericardium and the vertebral column.
The superior mediastinum contains: Thymus, large veins & arteries, trachea,
esophagus, thoracic duct and sympathetic trunks. It is bounded anteriorly by
manubrium sterni and posteriorly by the first 4 thoracic vertebrae.
The inferior mediastinum contains: Thymus, heart, phrenic nerves, esophagus,
thoracic duct, descending aorta and sympathetic trunks. It is bounded in front by the
body of the sternum and behind by the lower 8 thoracic vertebrae.
Large Veins
1- Brachiocephalic Veins:
The right vein is formed at the root of the neck by the union of the right subclavian and
the right internal jugular veins.
The left vein passes obliquely downward and to the right behind the manubirum
sterni and in front of the large branches of the aortic arch. It joins the right vein to
form the superior vena cava.
Superior Vena Cava
It is formed by the union of the 2 brachiocephalic veins. It passes downward to end in
the right atrium of the heart. The vena azygos joins the posterior aspect of the SVC
just before it enters the pericardium.
Inferior Vena Cava:
It pierces the central tendon of the diaphragm opposite the 8th thoracic vertebra and
enters the lowest part of the right atrium.
Azygos Veins
They consist of main azygos vein, the inferior hemiazygos vein and superior
hemiazygos vein. They drain blood from the posterior parts of the intercostal spaces,
the posterior abdominal wall, the pericardium, the diaphragm, the bronchi, the
esophagus.
Azygos Vein
It is formed by the union of the right ascending lumbar vein and the right subcostal
vein. It ascends through the aortic opening ( 12th thoracic V. )in the diaphragm on the
right side of the aorta to the level of the 5th thoracic vertebra. It then arches forward
above the root of the right lung to empty into the posterior surface of the SVC.
It has numerous tributaries including the 8th lower right intercostal veins, the right
superior intercostal vein, the superior & inferior hemiazygos veins and numerous
mediastinal veins.
Inferior hemiazygos vein:
It is formed by the union of the left ascending lumbar vein and the left subcostal vein.
It ascends through the left crus of the diaphragm and at the level of the 8th thoracic
vertebra turns to the right and joins the azygos vein. It receives tributaries from the
lower left intercostal veins and mediastinal veins.
Superior hemiazygos Vein:
It is formed by the union of the 4th to 8th intercostal veins. It joins the azygos vein at
the level of the 7th thoracic vertebra.
Pulmonary Veins
Two pulmonary veins leave each lung carrying oxygenated blood to left atrium of heart.
Large arteries - Aorta
1- Ascending aorta
It begins at the base of the left ventricle and runs upward and forward to come to lie
behind the right half of the sternum at the level of the sternal angle, where it
becomes continuous with the arch of the aorta. It lies within the fibrous pericardium
and is enclosed with the pulmonary trunk in a sheath of serous pericardium. At its
root it possesses 3 bulges (sinuses ) one behind each aortic valve cusp.
Branches: The right coronary artery arises from the anterior aortic sinus.
The left coronary artery arises from the left posterior aortic sinus.
Arch of the aorta
It lies behind the manubrium sterni and arches upward, backward and to the left in front
of the trachea. It then passes downward to the left of the trachea and at the level of the
sternal angle becomes continuous with the descending aorta.
Branches:
1- The brachiocephalic artery: It arises from the convex surface of the aortic arch. It
passes upward and to the right of the trachea and divides into the right subclavian
and right common carotid arteries behind the right sternoclavicular joint.
2- The left common carotid artery: It arises from the convex surface of the aortic arch
on the left side of the brachiocephalic artery.
It runs upward and to the left of the trachea and enters the neck behind the left
sternoclavicular joint.
3- The left subclavian artery: It arises from the arch behind the left common
carotid artery. It runs upward along the left side of the trachea and the
esophagus to enter the root of the neck. It arches over the apex of the left
lung.
Descending Thoracic Aorta
It lies in the posterior mediastinum and
begins as a continuation of the arch of the
aorta on the left side of the lower border
of the body of the 4th thoracic vertebra.
It runs downward till become forward and
medially to the anterior surface of the
vertebral column.
At the level of the 12th thoracic vertebra, it
passes behind the diaphragm through the
aortic opening in the midline and becomes
continuous with the abdominal aorta.
Branches:
1- Posterior intercostal arteries:
They are given off to the lower 9
intercostal spaces on each side.
2- Subcostal arteries:
they are given off on each side and run
along the lower border of the 12th rib
to enter the abdominal wall.
3- Pericardial
4- Esophageal
5- Bronchial arteries:
They are small branches
Pulmonary Trunk
It conveys deoxygenated blood from the right ventricle to the lungs. It leaves the upper
part of the right ventricle and runs upward, backward and to the left.
It is about 2 inch ( 5 cm ) long and terminates in the concavity of the aortic arch by
dividing into right & left pulmonary arteries. Together with the ascending aorta, it is
enclosed in the fibrous pericardium and a sheath of serous pericardium.
Branches:
1- Right pulmonary artery: It runs to the right behind the ascending aorta and SVC
to enter the root of the right lung.
2- Left pulmonary artery:
It runs to the left in front of the
descending aorta to enter the root of
the left lung.
Ligamentum arteriosum
It is a fibrous band that connects the bifurcation of the pulmonary trunk to the lower
concave surface of the aortic arch.
It is the remains of the dictus arteriosus which in the fetus conducts blood from the
pulmonary trunk to the aorta.
After birth the ductus closes. If it remains patent, aortic blood will enter the pulmonary
circulation producing pulmonary hypertension and hypertrophy of the right ventricle.
Surgical ligation of the ductus is necessary.
The left recurrent laryngeal nerve hooks around the lower border of this ligament.
Thymus gland
It is a flattened, bilobed structure lying
between the sternum and the
pericardium in the anterior
mediastinum.
In the newborn infant, it reach its
largest size relative to the size of the
body at which time it may extend up
through the superior mediastinum in
front of the great vessels into the root
of the neck.
It continues to grow until puberty .
After this, it undergoes involution.
It has a pink, lobulated appearance
and is the site for development of T
thymic ) lymphocytes.
Blood supply:
From the inferior thyroid and internal
thoracic arteries.
(
Thoracic part of the Sympathetic Trunk
It is the most laterally placed structure in the
mediastinum and runs downward on the heads of
the ribs. It has 11 to 12 ganglia.
It leaves the thorax on the side of the body of the
12th thoracic vertebra by passing behind the
medial arcuate ligament.
The 1st ganglion is often fused with the inferior
cervical ganglion to form the stellate ganglion.
Branches:
1- Gray rami communicantes go to all the
thoracic spinal nerves. The postganglionic
fibers
are distributed to the blood vessels,
sweat
glands and arrector pilli muscles
of the skin.
2- The first 5 ganglia give postganglionic
fibers to the heart, aorta, lungs and
esophagus.
3- The lower 8 ganglia give preganglionic
fibers to form the splanchnic nerves to
supply the abdominal viscera. They enter
the
abdomen by piercing the crura of the
diaphragm.
The greater splanchnic nerve arises from ganglia
5-9, the lesser nerve arises from ganglia10and 11,
the lowest nerve arises from ganglion 12.
Esophagus
It is a tubular structure about 10 inch ( 25 ) long. It is continuous above with the
laryngeal part of the pharynx opposite the 6th cervical vertebra. It passes through the
diaphragm at the level of the 10th thoracic vertebra to join the stomach.
In the neck, it lies in front of the vertebral column – laterally, it is related to the lobes
of the thyroid gland – Anteriorly, it is related to trachea and recurrent laryngeal
nerves.
In the thorax, it passes downward and to the left through the superior then in the
posterior mediastinum.
At the level of the sternal angle, the aortic arch pushes the esophagus over to the
midline.
The relations from above downward are as follows:
Anteriorly: The trachea and the left recurrent laryngeal nerve; the principal
bronchus which constricts it and the pericardium which separates the esophagus from
the left atrium.
Posteriorly: The bodies of the thoracic vertebrae; the thoracic duct; the azygos
veins; the right posterior intercostal arteries and the descending thoracic aorta at its
lower part.
Right side: The mediastinal pleura and the terminal part of the azygos vein.
Left side: The mediastinal pleura ; left subclavian artery ; the aortic arch ; the
thoracic duct .
inferiorly
to the level of the roots of the lungs, the vagus nerves leave the pulmonary plexus
and join with sympathetic nerves to form the esophageal plexus.
The left vagus lies anterior to the esophagus and the right vagus lies posterior.
At the opening in the diaphragm, it is
accompanied by the 2 vagi; branches of
the left gastric blood vessels and
lymphatic vessels.
Fibers from the right crus of the
diaphragm pass around it in the form of
sling.
In the abdomen, it descends for about
0.5 inch ( 1.3 cm ) and enters the
stomach. It is related to left lobe of the
liver anteriorly and to the left crus of
the diaphragm posteriorly.
Lymph drainage of the esophagus
The lymph vessels of the upper third
drain into the deep cervical nodes.
The middle third into the superior and
posterior mediastinal nodes.
The lower third into nodes along the left
gastric blood vessels and the celiac node
Nerve Supply
It is supplied by parasympathetic and
sympathetic fibers via vagus and
sympathetic trunk.
In its lower part, it is surrounded by the
esophageal nerve plexus.
Blood supply
The upper third is supplied by the inferior
thyroid artery.
The middle third by branches from the
descending thoracic aorta.
The lower third by branches from the left
gastric artery.
The veins from the upper third drain into
the inferior thyroid vein.
The middle third into the azygos veins.
The lower third into left gastric vein and
a tributaries of the portal vein.
Its upper end with pharynx
Aortic arch & left bronchus
The swallowed foreign bodies can lodge through these constrictions. So passage of
The esophagoscope is difficult. Those constrictions are the common sites of
carcinoma.
Thoracic Duct
It begins below in the abdomen as a dilated sac, The cisterna chyli. It ascends
through the aortic opening in the diaphragm, on the right side of the descending
aorta. It gradually crosses the median plane behind the esophagus and reaches the
left border of the esophagus.
At the level of the lower border of the body of the 4th thoracic vertebra ( sternal angle
).
It then runs upward along the left edge of the esophagus to enter the root of the neck.
Here, it bends laterally behind the carotid sheath and in front of the vertebral
vessels. It turns downward in front of the left phrenic nerve and crosses the
At the root of the neck, it receives
the left jugular ; subcalvian ; and
bronchomediastinal lymph trunks.
It thus conveys to the blood, all
lymph from the lower limbs, pelvic
cavity, abdominal cavity left side of the
thorax and left side of the head, neck
and left arm
Right lymphatic Duct
It is about 0.5 inch ( 1.3 cm ) long and opens into the beginning of the right
brachiocephalic vein.
It is formed by the union between the right jugular ; subclavian and
bronchomediastinal trunks, which drain the right side of the head & neck ;
the upper right limb and the right side of the thorax, respectively. These trunks
may open into the great veins at the root of the neck.