1. Anatomy and topography of the azygos vein
The azygos vein (v. azygos) is a large collecting vessel running along the right side of the thoracic vertebral bodies. Its location is closely related to key structures of the thoracic cavity.
Syntopy of the azygos vein:
- Posteriorly, the vessel is crossed by the right posterior intercostal arteries.
- To the left, it lies directly adjacent to the body's main lymphatic trunk — the thoracic duct (ductus thoracicus).
- Anteriorly, the vein is related to the esophagus (esophagus).
The azygos vein terminates at the level of the fourth thoracic vertebra. Here, it characteristically arches over the right main bronchus and empties into the superior vena cava (v. cava superior).
2. Hemiazygos and accessory hemiazygos veins
The left half of the thoracic cavity is drained by vessels that ultimately channel blood flow into the azygos system on the opposite side.
- Hemiazygos vein (v. hemiazygos). Runs strictly along the left side of the thoracic vertebral bodies. Topographically, this vessel is situated between the descending thoracic aorta and the left sympathetic trunk.
- Accessory hemiazygos vein (v. hemiazygos accessoria). This is an anatomically inconstant vessel localized on the left in the upper regions of the thoracic cavity. When present, it collects blood from the upper intercostal spaces and drains into the azygos vein (v. azygos). The level of confluence typically corresponds to the seventh to eighth thoracic vertebrae.
3. Vessels, nerves, and organs of the right mediastinum
Examining the topography of the mediastinal organs from the right side (viewed from the right pleural cavity) reveals an extensive picture of the relationships between the azygos vein and other anatomical structures.
Mediastinal organs: In this projection, the trachea (trachea) and esophagus (esophagus) are clearly visualized. Lower down, the right main bronchus (bronchus principalis dexter) is identified, over which the azygos vein forms its arch. The heart and pericardium (cor et pericardium) along with the diaphragmatic dome (diaphragma) also come into view.
Major vessels: In addition to the superior vena cava (v. cava superior) and the draining azygos vein (v. azygos), the following pass through here:
- Pulmonary vessels: veins (vv. pulmonales) and artery (a. pulmonalis).
- Right subclavian artery and vein (a. et v. subclavia dextra).
- Internal thoracic vessels (a. et v. thoracica interna).
- Pericardiacophrenic vessels (a. et v. pericardiacophrenica).
- Posterior intercostal arteries and veins (a., v. intercostales).
Nerve trunks and plexuses: The right mediastinum contains the path of the right vagus nerve (n. vagus dexter) and phrenic nerve (n. phrenicus). The sympathetic trunk (truncus sympathicus) extends along the spine, and the brachial plexus (plexus brachialis) is discernible in the superior aperture.
4. Clinical significance of the mediastinal veins
The azygos and hemiazygos veins are of paramount importance in providing collateral blood flow.
- Cavo-caval anastomoses: The mediastinal veins connect the basins of the superior and inferior venae cavae. This means that if outflow through one of the venae cavae is obstructed, blood can be rerouted via the vertebral and mediastinal veins.
- Porto-caval anastomoses: These veins have extensive anastomoses with the portal vein system. This plays a critical role in portal hypertension, when blood from the portal system seeks pathways to drain into the caval system.
Briefly: answers to topical questions
Where does the azygos vein originate and how does it enter the thoracic cavity?
The azygos vein originates from the right ascending lumbar vein, which ascends to the first lumbar vertebra and enters the thoracic cavity through a hiatus in the crura of the diaphragm.
- Right ascending lumbar vein (v. lumbalis ascendens dextra) — source of the azygos vein.
- Route of entry — a hiatus in the crura of the diaphragm, together with the greater splanchnic nerve (n. splanchnicus major).
- Further course — continues in the posterior mediastinum as the v. azygos, running along the right anterolateral surface of the thoracic spine.
Which visceral and parietal veins are tributaries of the azygos vein?
Tributaries of the azygos vein include:
- hemiazygos vein (v. hemiazygos);
- right superior intercostal vein (v. intercostalis superior dextra);
- right posterior intercostal veins (vv. intercostales posteriores, IV–XI), including the subcostal vein (v. subcostalis);
- superior phrenic veins (vv. phrenicae superiores);
- esophageal veins (vv. oesophageales);
- pericardial veins (vv. pericardiacae);
- bronchial veins (vv. bronchiales);
- mediastinal veins (vv. mediastinales).
At what level does the hemiazygos vein cross the spine, and where does it drain?
The provided sources do not explicitly state the level at which the hemiazygos vein crosses the spine and where it directly drains; it is only known that it runs along the left side of the vertebral bodies between the thoracic aorta and the left sympathetic trunk, and serves as a tributary of the azygos vein.
- Topography (v. hemiazygos) — left side of the vertebral bodies.
- Syntopy — between the thoracic aorta and the left sympathetic trunk.
- Connection with the azygos vein — the hemiazygos vein is indicated as its tributary.
From which abdominal vein does the hemiazygos vein originate?
The hemiazygos vein originates from the left ascending lumbar vein, which enters the thoracic cavity through a hiatus in the crura of the diaphragm.
- Left ascending lumbar vein (v. lumbalis ascendens sinistra) — source of the hemiazygos vein.
- Connection with the inferior vena cava system — the ascending lumbar veins connect the lumbar veins and continue in the thoracic cavity as the azygos (right) and hemiazygos (left) veins.
- Significance — a pathway for collateral blood flow during compression of the inferior vena cava.
The "Right-Running" rule: The Azygos vein runs on the Right side of the vertebrae, while the Hemiazygos runs on the left.
Class and exam questions
At what level does the azygos vein drain into the superior vena cava?
Confluence occurs at the level of the fourth thoracic vertebra, where the vein forms an arch over the right main bronchus.
What is located between the hemiazygos vein and the vertebral bodies on the left?
The hemiazygos vein is located on the left side of the vertebrae, running between the thoracic aorta and the left sympathetic trunk.
Where does the accessory hemiazygos vein drain?
It drains into the azygos vein (v. azygos) at the level of the seventh to eighth thoracic vertebrae.
What else to study on this topic
- Formation and Topography of the Thoracic Duct
- Mechanisms of Cava-Caval Anastomoses Functioning
- Connection of the Azygos Vein System with the Portal Vein (Portacaval Anastomoses)
- Syntopy of the Right Lung Root Elements
- Topography of the Prevertebral Fascia of the Neck and Thorax
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