Sinus venosus and atria

The development of the liver causes the omphalomesenteric veins and the umbilical veins to subdivide in the septum transversum into prehepatic and posthepatic parts. The prehepatic part of the omphalomesenteric veins first forms anastomoses around the duodenal loop before entering the liver. The portal vein develops from these anastomoses. The right umbilical vein degenerates. One branch of the left umbilical vein passes through the liver together with the omphalo-mesenteric veins; the other branch goes round the liver as the ductus venosus (by-pass of the left umbilical vein around the liver). In the posthepatic part, both the omphalomesenteric and umbilical veins are preserved on the right as the right hepato-cardiac channel. The ductus venosus also leads into the right hepato-cardiac channel, causing right-shifting of the inflow tract. This leads to regression of the left sinual horn; the remaining proximal part of the left sinual horn becomes the sinus coronarius. At S12, the opening of the sinus venosus into the right atrium (sinu-atrial orifice) is bordered by two septa, or cusps. The right cusp becomes the valve of the inferior vena cava. The left sinus valve later fuses with the septum secundum of the atrium. The two sinu-atrial valves fuse cranially to form the septum spurium. The coronary sinus also terminates in the sinu-atrial orifice.