The omphalomesenteric system
Below the heart and parallel to the umbilical venous system a second venous network develops, the omphalomesenteric system. It empties into the sinus venosus. The omphalomesenteric veins are closely associated with the development of the duodenum and the liver and drain the blood of the umbilical vesicle.
With the development of the liver the omphalomesenteric veins form a thick hepatic plexus. They also anastomose with the umbilical veins. Through various transformative and degenerative processes the vena portae arises that brings the blood from the intestines into the liver. By this means the absorbed nutrients from the intestines can be taken up later through the liver and metabolized. On the other hand, a short-cut , the ductus venosus, is engendered from the thick network of anastomoses. Up to the time of birth, it takes the blood from the umbilical vein and the omphalomesenteric veins and directs it directly into the sinus venosus of the heart.
During the course of the early cardiac development the left sinus horn atrophies and with it the posthepatic part of the left omphalomesenteric vein. All of the blood from the liver is conveyed via the right omphalomesenteric vein to the sinus venosus. The prehepatic part of the omphalomesenteric veins also experiences various transformative processes in that individual parts around the duodenum atrophy and others become more pronounced.