Competencies
- AN23.3: Describe and demonstrate the origin, course, relations, tributaries, and termination of superior vena cava, azygos, hemiazygos, and accessory hemiazygos veins.
- AN23.4: Mention the extent, branches, and relations of arch of aorta and descending thoracic aorta.
Great Vessels of Thoracic Cavity
The thoracic cavity contains the major vessels connected with the heart, including the superior vena cava, aorta, and pulmonary trunk.
These vessels transport blood between the heart, lungs, and the rest of the body. The venae cavae return systemic venous blood to the heart, the pulmonary vessels carry blood between the heart and lungs, and the aorta distributes oxygenated blood from the heart to the systemic circulation.
Superior Vena Cava
The superior vena cava (SVC) is a large vein that returns venous blood from the upper half of the body to the right atrium.
- Length: Approximately 7 cm
- Width: Approximately 1.25 cm
- Location: Superior and middle mediastinum
Formation
The superior vena cava is formed by the union of the right and left brachiocephalic veins behind the lower border of the right first costal cartilage.
Course
The superior vena cava descends almost vertically behind the right first and second intercostal spaces.
It enters the pericardium at the level of the second right costal cartilage. Consequently, its lower part lies within the fibrous pericardium.
Termination
- The superior vena cava opens into the upper part of the right atrium behind the right third costal cartilage.
- The superior vena cava has no valves. The brachiocephalic veins and azygos vein are also valveless. The superior vena cava attains approximately its adult dimensions by about 10 years of age.
Relations
The upper part of the superior vena cava lies in the superior mediastinum, whereas the lower part lies in the middle mediastinum and is enclosed by the pericardium.
Anterior relations
- Anterior chest wall corresponding to the first and second intercostal spaces and adjacent costal cartilages
- Anterior margin of the right lung and pleura
- Pericardium over its lower part
- Right internal thoracic vessels
Posterior relations
- Trachea and right vagus nerve posteromedially
- Right lung and pleura posterolaterally
- Right pulmonary hilum behind its lower part
Right lateral relations
- Right phrenic nerve
- Right pleura and lung
Left lateral relations
- Ascending aorta
- Brachiocephalic trunk
Tributaries
- Right and left brachiocephalic veins: These are the formative tributaries of the superior vena cava.
- Azygos vein: It arches over the root of the right lung and joins the superior vena cava near the level of the right second costal cartilage.
- Mediastinal and pericardial veins: Several small veins from the mediastinum and pericardium drain directly into the superior vena cava.
Clinical Integration
Obstruction of the superior vena cava
The superior vena cava (SVC) may be compressed or obstructed by conditions such as bronchogenic carcinoma or enlarged mediastinal lymph nodes.
The pattern of collateral venous drainage depends on the level of obstruction in relation to the entry of the azygos vein.
- Obstruction above the entry of the azygos vein:
Venous blood from the upper part of the body is diverted through collateral channels into the azygos vein. The azygos vein then drains into the SVC below the level of obstruction, allowing blood to reach the right atrium. - Obstruction below the entry of the azygos vein:
Blood from the upper part of the body is diverted through collateral pathways to the inferior vena cava. The thoracoepigastric venous channel provides an important communication between the superior and inferior caval systems.
The azygos vein enters the superior vena cava shortly before the SVC enters the pericardium.
Patients with SVC obstruction may present with facial swelling, cough, chest discomfort, and dysphagia. Common clinical signs include:
- Distension of the veins of the neck and chest wall
- Facial edema
- Edema of the upper limbs
- Prominent dilated superficial veins over the chest wall
Brachiocephalic Veins
There are two brachiocephalic veins, right and left.
- Formation: Each brachiocephalic vein is formed by the union of the corresponding internal jugular vein and subclavian vein behind the sternoclavicular joint.
- Termination: The right and left brachiocephalic veins unite to form the superior vena cava.
- Extent: Each brachiocephalic vein extends from the corresponding sternoclavicular joint to the lower border of the right first costal cartilage, where the two veins join to form the superior vena cava.
Tributaries
The tributaries of the brachiocephalic veins include:
- Formative tributaries
- Internal jugular vein
- Subclavian vein
- Vertebral vein
- Inferior thyroid vein
- Internal thoracic vein
- First posterior intercostal vein
- Left superior intercostal vein, which drains into the left brachiocephalic vein only.
Table 16.1: Difference between the right and left brachiocephalic veins
| Feature | Right brachiocephalic vein | Left brachiocephalic vein |
|---|---|---|
| Length | About 2.5 cm; shorter | About 6 cm; longer |
| Course | Nearly vertical | Oblique |
| Superior intercostal vein | Does not receive the right superior intercostal vein | Receives the left superior intercostal vein |
Aorta
The aorta is the largest artery in the body. It arises from the left ventricle and distributes oxygenated blood to the systemic circulation. Structurally, it is an elastic artery.
Part of aorta
The aorta is divided into four parts:
- Ascending aorta
- Arch of the aorta
- Descending thoracic aorta
- Abdominal aorta
Ascending Aorta
The ascending aorta is the first part of the aorta and begins at the aortic orifice of the left ventricle.
- Origin: It arises from the left ventricle at approximately the level of the lower border of the left third costal cartilage.
- Length: About 5 cm
- Diameter: About 3 cm
- Course: It passes upward, forward, and to the right behind the sternum.
- Termination: At the level of the upper border of the right second sternocostal joint, it continues as the arch of the aorta.
Aortic sinuses (sinuses of the Valsalva)
The root of the ascending aorta shows three localized dilatations immediately above the cusps of the aortic valve. These are called the aortic sinuses or sinuses of Valsalva.
The three aortic sinuses are:
- Right coronary sinus: Gives origin to the right coronary artery.
- Left coronary sinus: Gives origin to the left coronary artery.
- Non-coronary sinus: Does not give rise to a coronary artery.
Relations
The ascending aorta is intrapericardial and is enclosed within the pericardial sac.
Anterior relations
- Sternum
- Right lung and pleura in the upper part
- Infundibulum of the right ventricle and pulmonary trunk in the lower part
- Pericardium
Posterior relations
- Transverse pericardial sinus
- Right pulmonary artery
- Right main bronchus
Right side
- Right atrium
- Superior vena cava
Left side
- Pulmonary trunk
- Left atrium
Branches
The ascending aorta gives rise to two branches:
- Right coronary artery
- Left coronary artery
Clinical Integration
- An ascending aortic aneurysm is a localized abnormal dilatation of the ascending aorta resulting from weakening of its wall.
- As the aneurysm enlarges, it may compress neighboring mediastinal structures. It may also rupture into the pericardial cavity, producing hemopericardium and potentially cardiac tamponade.
Arch of Aorta
The arch of the aorta is a large elastic artery situated in the superior mediastinum, behind the manubrium sterni.
Origin
The arch begins as a continuation of the ascending aorta at the level of the upper border of the right second sternocostal joint, corresponding approximately to the plane of the sternal angle.
Termination
The arch continues as the descending thoracic aorta at the level of the lower border of the fourth thoracic vertebra.
The beginning and termination of the aortic arch lie approximately in the same transverse plane passing through the sternal angle.
Course
The arch passes upward, backward, and to the left, initially lying anterior to the trachea. It then arches over the root of the left lung and passes posterior to the left main bronchus before continuing as the descending thoracic aorta.
The arch is convex superiorly and also curves anteriorly and to the left.
Relations
As the arch crosses the superior mediastinum obliquely from right to left, its relations are described as anterior and left, posterior and right, superior, and inferior.
Anterior and to the left
- Left lung and pleura
- Left phrenic nerve
- Left vagus nerve
- Lower cervical cardiac branch of the left vagus nerve
- Superior cervical cardiac branch of the left sympathetic trunk
- Left superior intercostal vein, lying between the left phrenic and vagus nerves
Ligamentum arteriosum
The ligamentum arteriosum, a remnant of the fetal ductus arteriosus, connects the left pulmonary artery to the inferior aspect of the aortic arch. During fetal life, the ductus arteriosus allows blood from the pulmonary trunk to bypass the non-functioning lungs and enter the aorta.
Posterior and to the right
- Trachea
- Esophagus
- Left recurrent laryngeal nerve
- Thoracic duct
- Fourth thoracic vertebra
Superior
- Left brachiocephalic vein
- Brachiocephalic trunk
- Left common carotid artery
- Left subclavian artery
- Thyroidea ima artery, when present
Inferior
- Bifurcation of the pulmonary trunk
- Left main bronchus
- Ligamentum arteriosum
- Superficial cardiac plexus
- Left recurrent laryngeal nerve
Branches
The usual branches of the aortic arch are:
- Brachiocephalic trunk
- Left common carotid artery
- Left subclavian artery
An occasional additional branch is the thyroidea ima artery.
The brachiocephalic trunk divides into the right common carotid and right subclavian arteries.
Clinical Integration
Aortic knuckle: On a posteroanterior chest radiograph, the aortic arch forms a rounded projection at the upper part of the left mediastinal border. This radiographic prominence is called the aortic knuckle or aortic knob.
Coarctation of the aorta: Coarctation of the aorta is a congenital narrowing of the aorta, usually near the site of attachment of the ligamentum arteriosum. The narrowed segment obstructs systemic blood flow.
It may be classified as:
- Preductal coarctation: Narrowing proximal to the ductus arteriosus or ligamentum arteriosum.
- Postductal coarctation: Narrowing distal to the ductus arteriosus or ligamentum arteriosum.
Collateral circulation may develop between the anterior intercostal arteries, derived from the internal thoracic arteries, and the posterior intercostal arteries arising from the thoracic aorta. Enlargement of the posterior intercostal arteries may erode the inferior margins of the ribs, producing characteristic rib notching on radiographs.
Patent ductus arteriosus
During fetal life, the ductus arteriosus provides a vascular communication between the pulmonary trunk, near the origin of the left pulmonary artery, and the aorta. This allows most of the right ventricular output to bypass the fetal lungs. After birth, the ductus normally undergoes functional closure soon after the onset of respiration, followed by anatomical obliteration over the following weeks. Its fibrous remnant forms the ligamentum arteriosum. Failure of the ductus arteriosus to close results in patent ductus arteriosus (PDA), producing persistent communication between the pulmonary and systemic circulations.
Aortic arch aneurysm
An aortic arch aneurysm is a localized abnormal dilatation of the aortic arch. Enlargement may compress adjacent mediastinal structures and produce pressure-related symptoms.
Descending Thoracic Aorta
The descending thoracic aorta is the continuation of the arch of the aorta. It begins at the level of the lower border of the fourth thoracic vertebra (T4) and descends through the posterior mediastinum.
Termination
It passes through the aortic hiatus of the diaphragm at the level of the lower border of T12 and continues as the abdominal aorta.
Relations
Anterior
- Root of the left lung
- Pericardium and heart
- Esophagus
- Diaphragm
Posterior
- Vertebral column
- Hemiazygos vein
- Accessory hemiazygos vein
To the right
- Esophagus in the upper part
- Azygos vein
- Thoracic duct
- Right lung and pleura
To the left
- Left lung and pleura
Branches
The descending thoracic aorta gives rise to the following branches:
- Nine pairs of posterior intercostal arteries, supplying the third to eleventh intercostal spaces
- A pair of subcostal arteries
- Bronchial arteries, usually including two left bronchial arteries
- Esophageal branches
- Pericardial branches
- Mediastinal branches
- Superior phrenic arteries, which supply the diaphragm
Pulmonary Trunk
The pulmonary trunk carries deoxygenated blood from the right ventricle to the lungs.
- Origin: It arises from the infundibulum of the right ventricle at the level of the sternal end of the left third costal cartilage.
- Length: Approximately 5 cm.
- Termination: It divides into the right and left pulmonary arteries beneath the concavity of the aortic arch, approximately at the level of the sternal angle.
Branches
- Right pulmonary artery: Carries deoxygenated blood to the right lung.
- Left pulmonary artery: Carries deoxygenated blood to the left lung.
Clinical Integration
Acute occlusion of the pulmonary trunk
- Sudden obstruction of the pulmonary trunk may occur when a thromboembolus reaches the pulmonary circulation, commonly from the large systemic veins. A massive obstruction can severely impair pulmonary blood flow and may be life-threatening.
Pulmonary artery catheterization
- Pulmonary artery catheterization, also called right heart catheterization, involves passage of a catheter through a central vein, usually the internal jugular or subclavian vein, into the superior vena cava, right atrium, right ventricle, pulmonary trunk, and pulmonary artery.
- The procedure is used to assess right-sided cardiac function, pulmonary hemodynamics, and selected cases of severe cardiopulmonary disease.
Important Questions
- Write a short note on superior vena cava.
- Write a short note on brachiocephalic veins.
- Write short notes on ascending aorta and descending aorta.
- List the branches of arch of aorta, ascending aorta, descending thoracic aorta.
- Write a short note on aortic sinuses.
- Describe the arch of aorta under the following headings: Extent, relations, branches, applied aspects.
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