The cardinal system

• The cardinal system forms the basis for the intraembryonic venous circulation
• The earliest cardinal system consists of paired anterior and posterior cardinal veins, which join near the heart to form the common cardinal vein that drains into the horns of the sinus venosus
• The left and right anterior cardinal veins drain blood into the sinus venosus via the left and right common cardinal veins. With the regression of the left horn of the sinus venosus, the left anterior cardinal vein loses its connection to the inflow end of the heart. The cranial portions of the anterior cardinal veins develop into the internal and external jugular veins that drain the head (jugulum = throat).
• An anastomosis forms between the left and right anterior cardinal veins to shunt the blood from the left side of the head and the left arm to the right once the connection of the left anterior cardinal vein to the heart is lost. This shunt becomes the left brachiocephalic vein (brachio = related to the arm, cephalic = related to the head). The small section of the right anterior cardinal vein between its junction with the left brachiocephalic anastomosis and the right atrium becomes the superior vena cava.
• The left and right posterior cardinal veins give rise to a pair of subcardinal veins an a pair of supracardinal veins, that will eventually replace the original posterior cardinal veins. The subcardinal and supracardinal veins develop in the body wall, medial to the posterior cardinal veins. These three cardinal systems form several parts of the inferior vena cava and its major branches in the thoracic and abdominal cavities.
• The subcardinal veins sprout from the base of the posterior cardinal veins and grow caudally in the dorsal body wall. They form anastomoses with the posterior cardinal veins. The left subcardinal vein regresses simultaneous to the left and right posterior cardinal veins. The right subcardinal vein develops a connection with the right vitelline vein to form the segment of the inferior vena cava between the liver and the kidneys.
• Whilst the subcardinal veins are being remodeled, yet another pair of veins sprouts from the base of the posterior cardinal veins: the supracardinal veins. These veins take over draining the body wall when the posterior cardinal veins regress. The left supracardinal vein regresses over time, whilst the right supracardinal vein develops a connection with the right subcardinal vein to form the segment of the inferior vena cava inferior to the kidneys
• The caudal most portion is the only segment of the original posterior cardinal veins to persist, forming the caudal most segment of the inferior vena cava.