Great Vessels of the Thorax

Ascending Aorta

• Begins at the aortic valve (left ventricular outflow), within the pericardial sac, ascends to the level of the sternal angle (approximately the 2nd right costal cartilage), where it becomes the aortic arch.
• Gives rise only to the right and left coronary arteries, from the right and left aortic sinuses respectively.
• Enclosed within the fibrous pericardium along with the pulmonary trunk, both sharing a common serous pericardial sheath (relevant to the transverse pericardial sinus).

Aortic Arch

• Continues from the ascending aorta, arches posteriorly and to the left (over the left main bronchus), descending to become the descending thoracic aorta at approximately the T4 vertebral level (left side).
• Gives rise to three branches (in order, right to left): brachiocephalic trunk, left common carotid artery, left subclavian artery.
• Closely related to the left recurrent laryngeal nerve (loops beneath the arch, posterior to the ligamentum arteriosum), the ligamentum arteriosum itself (remnant of the fetal ductus arteriosus, connecting the arch to the left pulmonary artery), and the left vagus nerve (crosses the arch's lateral surface).

Descending Thoracic Aorta

• Continues from the aortic arch (T4) to the aortic hiatus of the diaphragm (T12), where it becomes the abdominal aorta.
• Lies initially to the left of the vertebral bodies, gradually shifting to a more midline/anterior position as it descends toward the hiatus.
• Gives off the posterior intercostal arteries (spaces 3–11), subcostal arteries, and small branches to the oesophagus, bronchi (bronchial arteries), pericardium, and mediastinum.

• This asymmetry (right side has a shared brachiocephalic trunk, left side has two independent branches) reflects embryological differences in aortic arch development between the two sides, and is a frequently tested anatomical point.

• Arises from the right ventricular outflow tract (pulmonary valve), ascends briefly within the pericardium (alongside the ascending aorta, sharing the transverse pericardial sinus relationship), then bifurcates into right and left pulmonary arteries at approximately the sternal angle level, beneath the aortic arch.
• Right pulmonary artery: Longer, passes posterior to the ascending aorta and SVC, anterior to the right main bronchus, to reach the right lung hilum.
• Left pulmonary artery: Shorter, passes anterior to the descending aorta and left main bronchus, connected to the aortic arch by the ligamentum arteriosum: the fibrous remnant of the fetal ductus arteriosus, which shunted blood from the pulmonary trunk directly to the aorta in utero, bypassing the non-aerated fetal lungs.

• Formation: Union of the right and left brachiocephalic veins, posterior to the right 1st costal cartilage.
• Course: Descends vertically, entirely within the superior mediastinum then continuing into the middle mediastinum, to enter the right atrium at approximately the level of the 3rd right costal cartilage.
• Tributaries: Receives the azygos vein (arching over the right lung root, joining the SVC's posterior surface at approximately the sternal angle/T4 level, another correlate of this key surface landmark) as its only major tributary below the level of the brachiocephalic vein confluence.
• Relations: Right phrenic nerve and pericardiacophrenic vessels run alongside its right lateral surface (within the pericardium, part of it); ascending aorta lies to its left; right lung/pleura lie laterally.
• The lower (intrapericardial) portion of the SVC is covered by serous pericardium anteriorly and laterally.

• Formation: Each formed by the union of the internal jugular vein and subclavian vein, on its respective side, posterior to the sternoclavicular joint.
• Course:
• Right brachiocephalic vein: Short, runs a nearly vertical course to join its left counterpart.
• Left brachiocephalic vein: Considerably longer, runs a more horizontal/oblique course across the midline, anterior to the origins of the three aortic arch branches (brachiocephalic trunk, left common carotid, left subclavian arteries) and posterior to the manubrium, a key anatomical relationship, since this vein's course places it directly behind the manubrium, at risk during central venous access or sternal procedures at this level.
• Relations: Both receive the internal thoracic veins, inferior thyroid veins, and vertebral veins as tributaries before uniting to form the SVC.

A network of longitudinally running veins in the posterior mediastinum, draining the posterior thoracic wall (and, via connections, part of the posterior abdominal wall), and providing an important collateral pathway between the SVC and IVC systems.

Azygos vein:

• Formation: Variable, typically arises from the union of the right subcostal vein and the right ascending lumbar vein (an anastomotic vessel connecting to the common iliac vein/IVC system in the abdomen), though origin patterns vary.
• Course: Ascends along the right side of the vertebral bodies (right side of the descending thoracic aorta), through the aortic hiatus or occasionally penetrating the right crus of the diaphragm separately, then arches anteriorly over the root of the right lung (over the right main bronchus) at the T4/sternal angle level to join the posterior surface of the SVC.
• Drainage received: Right posterior intercostal veins (spaces 4–11, or their equivalent via the right superior intercostal vein for the upper spaces), right subcostal vein, hemiazygos and accessory hemiazygos veins (via cross-connections), oesophageal and bronchial veins.

Hemiazygos vein:

• Formation: Union of the left subcostal vein and left ascending lumbar vein (mirroring the azygos vein's formation, on the left side).
• Course: Ascends along the left side of the vertebral bodies, receiving the lower left posterior intercostal veins (typically spaces 9–11); crosses the midline (posterior to the aorta, oesophagus, and thoracic duct) at approximately the T9 level to join the azygos vein.

Accessory hemiazygos vein:

• Course: Drains the middle left posterior intercostal veins (typically spaces 4–8), descends and crosses the midline (usually at a higher level than the hemiazygos vein, around T7-T8) to join either the azygos vein directly or the hemiazygos vein.
• Together, the hemiazygos and accessory hemiazygos veins drain the left side in a pattern that funnels into the azygos vein via these midline crossing connections, since the hemiazygos system does not have its own direct connection to the SVC.

Communications with caval systems:

• Inferiorly: Via the ascending lumbar veins, connecting to the common iliac veins (part of the IVC system).
• Superiorly: The azygos vein drains directly into the SVC.
• This dual connection makes the azygos system a key collateral venous pathway if either the SVC or IVC becomes obstructed, allowing blood to reroute between the upper and lower body venous systems via the extensive azygos network.

SVC obstruction (SVC syndrome):

• Compression or invasion of the SVC (classically by a bronchogenic carcinoma, lymphoma, or mediastinal mass, given the SVC's thin walls and low intraluminal pressure, making it particularly susceptible to external compression) obstructs venous return from the head, neck, and upper limbs.
• Produces facial/neck/upper limb swelling, distended neck and chest wall veins, and (in severe cases) cerebral oedema.
• The azygos vein provides a key collateral pathway if the obstruction is above the level of the azygos vein's entry into the SVC, blood can reroute via the azygos system to reach the IVC and right atrium by an alternative route, partially compensating for the obstruction.

Aortic aneurysm relationships:

• Thoracic aortic aneurysms (ascending, arch, or descending) can compress or erode adjacent structures depending on location, an ascending aortic aneurysm may compress the SVC or distort the aortic valve (causing regurgitation); an arch aneurysm may stretch the left recurrent laryngeal nerve (causing hoarseness) or compress the trachea/oesophagus; a descending aneurysm may erode into the oesophagus or vertebral bodies.
• The aorta's close relationship to the trachea, oesophagus, and recurrent laryngeal nerve throughout its course explains this range of possible compressive symptoms.

Coarctation anatomy:

• A congenital narrowing of the aorta, most commonly located near the ligamentum arteriosum (juxtaductal), distal to the origin of the left subclavian artery.
• Produces a pressure differential between the upper body (hypertensive) and lower body (hypotensive), with collateral circulation developing via the intercostal arteries (enlarged, tortuous posterior intercostal arteries providing an alternative pathway around the coarctation, connecting to the internal thoracic artery superiorly), classically producing rib notching on chest X-ray from the pulsatile, eroding pressure of these enlarged collateral vessels against the inferior rib margins.

Azygos collateral circulation:

• Beyond SVC obstruction, the azygos system's connections to both caval systems make it a general-purpose collateral pathway in various forms of venous obstruction affecting either the upper or lower body's venous drainage, given its dual termination points.