Sinus venosus forms the inflow tract of the primitive heart tube. On each side of the sinus venosus, the common cardinal veins open into it. Anterior and posterior cardinal veins combine to form the common cardinal vein on each side of the sinus venosus. The common cardinal vein on the right side forms part of the superior vena cava. Besides the common cardinal veins, the umbilical and the vitelline veins also drain into the sinus venosus. On the left side, these veins undergo specific remodelling. This results in formation of specific anastamosis on the left side, giving rise to left to right shunts thereby causing most of the blood to be received by the right side of the sinus venosus. Consequently, the veins of the right side start maturing and increase in size relative to their left side counterparts. As the sinus venosus also grows and matures, it becomes incorporated into the right side of the primitive atrium. Eventually the vitelline, common cardinal and the umbilical veins of the left side degenerate. The blood received by the left side of sinus venosus is greatly reduced and as a result the left side of the sinus venosus shrinks. Inferiorly the posterior cardinal veins anastamose together and form the iliac veins.
The formation of the inferior vena cava is contributed by three set of veins:
- Anterior & posterior cardinal veins
- Subcardinal veins
- Supracardinal veins (develop a little latter)
The anterior cardinal and the common cardinal veins on the left side give rise to brachicephalic and the left subclavian vein which drain into the SVC. It is important to remember that, the umbilical vein on both sides and the vitelline vein on the left side, all tend to degenerate. The vitelline vein on the right side along with the common cardinal vein of the right side together form parts of the SVC.
The subcardinal veins have an anastomosis in the middle which is referred to as subcardinal venous anastomosis. These subcardinal veins are also connected to the posterior cardinal vein and form another anastomosis which is referred to as the mesonephric shunt. Later on the inferior part of posterior cardinal vein on the left side starts degenerating, however, it’s still connected to the subcardinal vein via the mesonephric shunt. Also, there are tiny buds arising from the subcardinal veins, these form parts of the future ovarian and spermatic veins.
Subcardinal veins on the right side separate from the posterior cardinal veins and are joined in by the hepatic veins. This will later form the initial parts of the IVC. At this point it’s important to understand that an anastomosis forms between the supracardinal and the subcardinal veins, which is referred to as the supra-subcardinal anastomosis. This anastamosis becomes part of the IVC and later gives rise to the renal vein and parts of the spermatic veins. Over time the posterior cardinal vein on the right side also degenerates.
As the IVC is formed, it is composed of the following parts:
- HEPATIC PART comes from the liver sinusoids and the hepatic vein. It also has a contribution from the sinus veonsus.
- PRERENAL PART of the IVC is contributed by the subcardinal veins. Primarily the right subcardinal vein has a greater contribution in forming this part.
- RIGHT AND LEFT SUPRARENAL VEINSare also parts of the IVC. They also develop from the subcardinal veins.
- RENAL SEGMENT OF THE IVC arises from the mesonephric shunt of the right side. The mesonephric shunt itself forms as a result of the anastamosis between the subcardinal and the posterior cardinal veins. Right renal vein also arises from this renal segment of the IVC. On the left side the mesonephric shunt becomes incorporated into the IVC and gives rise to the left renal vein. The spermatic/ovarian vein on the left side drains into the left renal vein which then opens into the IVC. However, on the right side, the right spermatic/ovarian vein opens directly into the IVC.
- POSTRENAL SEGMENT OF THE IVC is the region below the renal segment of IVC. Supracardinal veins contribute to the formation of the postrenal segment of the IVC.
- ILIAC VEINS form the most inferior parts of the IVC. The inferior anastomosis between the posterior cardinal veins is responsible for the formation of iliac veins.