Fig. 2

a Systemic venous system development composite schematic. The non-contemporary primitive venous network consisting of symmetrically paired venous trunks and interconnecting anastomosis, of which a dorsal systemic system, comprising of the anterior (light blue) and posterior cardinal (purple) veins, the subcardinal veins (orange), and the supracardinal veins (green), each predominating subsequently and which carries all the intraembryonic blood, and a double nutritional network, comprising the vitelline (brown) and umbilical veins (white). The precursor of the atrium, then known as the sinus venosus, is seen in (pink). b Normal systemic venous system embryology schematic. The normal end result of the complex deviations in the primitive venous network, resulting in composed single major trunks on the right side and regression of the left-sided vessels: the SVC being formed by the right anterior cardinal vein (light blue); the left brachiocephalic venous trunk by the inter-anterior cardinal anastomosis (dark blue); the suprahepatic and hepatic segments of the IVC by the right vitelline vein and right vitelline-subcardinal anastomosis (brown); the suprarenal, infrarenal, and renal segments of the IVC respectively by the right subcardinal vein (orange), the right supracardinal vein (green), and the anastomosis between them; the terminal IVC and common iliac veins by the posterior cardinal veins; and the azygos-hemiazygos venous system from the supracardinal veins, with the azygos arch resulting from an upper segment of the right posterior cardinal vein