Formation of the inferior vena cava

The formation of the inferior vena cava is associated with the appearance of anastomoses between the posterior cardinal veins. One anastomosis, located in the iliac region, drains blood from the left lower extremity to the right posterior cardinal vein; as a result, the segment of the left posterior cardinal vein, located above the anastomosis, is reduced, and the anastomosis itself is transformed into the left common iliac vein. The right posterior cardinal vein at the site before the confluence of the anastomosis (which has become the left common iliac vein) is converted into the right common iliac vein, and from the site of the junction of both iliac veins to the confluence of the renal veins develops into the secondary inferior vena cava.

The rest of the secondary inferior vena cava is formed from the unpaired primary inferior vena cava flowing into the heart, which connects to the right inferior cardinal vein at the confluence of the renal veins (there is a 2nd anastomosis between the cardinal veins that drains blood from the left kidney). Thus, the finally formed inferior vena cava is composed of 2 parts: from the right posterior cardinal vein (before the confluence of the renal veins) and from the primary inferior vena cava (after its confluence). Since in the inferior vena cava the blood is drained from the entire caudal half of the body into the heart, the value of the posterior cardinal veins weakens, they lag behind in development and turn into v. azygos (right posterior cardinal vein) and in v. hemiazygos and v. hemiazygos accessoria (left posterior cardinal vein). V. hemiazygos falls into v. azygos through the 3rd anastomosis developing in the thoracic region between the former posterior cardinal veins. The portal vein is formed due to the transformation of the yolk veins, through which blood from the yolk sac enters the liver. Vv. omphalomesentericae in the space from the confluence of the mesenteric vein to the gate of the liver into the portal vein.