Purpose of Venous Anastomoses
The anatomy of the circulatory system establishes collateral pathways to ensure blood flow in case of normal vascular obstruction. In the venous system of the torso, there are three primary drainage basins: the superior vena cava (v. cava superior), the inferior vena cava (v. cava inferior), and the hepatic portal vein (v. portae hepatis).
Connections between these major vessels are called anastomoses. Normally, they are inconspicuous, but when outflow through the liver or toward the heart is impeded (e.g., in cirrhosis), blood is forced through these collaterals. Excess pressure causes their pathological dilation and leads to clinical complications.
Major Portosystemic Anastomoses
These shunts connect the portal system with tributaries of the venae cavae. According to anatomical nomenclature, there are four key zones of these connections:
- Esophageal anastomosis. Located in the cardiac part of the stomach and the lower third of the esophagus. Here, the left gastric vein and right gastric vein (v. gastrica dextra, running along the lesser curvature) meet the esophageal veins. The latter drain into the azygos (v. azygos) and hemiazygos (v. hemiazygos) veins, which empty into the superior vena cava. This connection occurs via the well-developed esophageal venous plexus (plexus venosus oesophageus). When pressure rises, it can lead to life-threatening esophageal varices.
- Rectal anastomosis. Located in the wall of the rectum, forming the rectal venous plexus (plexus venosus rectalis). The superior rectal vein (v. rectalis superior), as the origin of the inferior mesenteric vein, drains blood into the portal system. The middle rectal vein (v. rectalis media) drains into the internal iliac vein, and the inferior rectal vein drains into the internal pudendal vein, directing blood into the inferior vena cava. A clinical manifestation of this plexus dilating is hemorrhoids.
- Umbilical anastomosis. Formed on the anterior abdominal wall around the umbilicus. Paraumbilical veins (vv. paraumbilicales) travel within the round ligament of the liver directly to the portal vein. They anastomose with tributaries of the superior vena cava (superior epigastric vein, v. epigastrica superior) and tributaries of the inferior vena cava (inferior and superficial epigastric veins, v. epigastrica inferior and v. epigastrica superficialis). The dilation of subcutaneous abdominal vessels in this area is known as caput medusae.
- Retroperitoneal anastomoses (Retzius system). Located in the retroperitoneal space on the posterior abdominal wall. They provide connections between the veins of the ascending and descending colon (portal basin) and the lumbar veins (vv. lumbales), which belong to the inferior vena cava system.
Caval-Caval Anastomoses
This type of shunt ensures the redistribution of venous blood exclusively between the superior and inferior vena cava systems. Several collateral pathways exist:
- Via the azygos and hemiazygos venous systems. The ascending lumbar veins (v. lumbalis ascendens) directly connect the iliac veins to the azygos and hemiazygos veins.
- Vertebral pathway. Mediated through the extensive vertebral venous plexus (plexus venosus vertebralis).
- Deep pathway of the anterior abdominal wall. Formed by the connection between the superior epigastric vein (a tributary of the internal thoracic vein, v. thoracica interna) and the inferior epigastric vein.
- Superficial pathway of the trunk. Subcutaneous vessels interconnect via the thoracoepigastric vein (v. thoracoepigastrica) and the superficial epigastric vein (v. epigastrica superficialis).