127. Development of The Venous System: The Portal System and Pulmonary Veins

  1. The portal system: the 3-week-old embryo is characterized by the existence of 3 pairs of veins: the vitelline or omphalomesenteric veins carrying blood from the yolk sac to the heart, the umbilical veins originating in the chorionic villi of the placenta and carrying oxygenated blood to the embryo, and the cardinal veins draining the body of the embryo. The portal system relates to the first 2
    1. THE VITELLINE OR OMPHALOMESENTERIC VEINS enter the body of the embryo via the yolk sac stalk, form an anastomotic network around the duodenum of the digestive tract, and then enter the septum transversum, which they cross on their way to the heart
      1. Proliferation of the entodermal liver cords, which form the liver primordium, fragments the vitelline veins to form a vascular labyrinth, the so-called hepatic sinusoids
        1. After the yolk sac disappears, the vitelline veins regress almost completely and persist only in their mesenteric branches
      2. Cranial to the liver, the vitelline veins open into the right and left horns of the sinus venosus
        1. When the left horn of the sinus venosus disappears, the right vitelline trunk receives the anastomosis of the inferior vena cava and becomes its terminal portion or suprahepatic portion of the inferior vena cava
      3. Caudal to the liver, the vitelline vein anastomotic network around the duodenum fuses to form a single trunk, the portal vein, partly by obliteration and partly by growth of different portions
        1. The superior mesenteric vein which drains the primitive intestinal loop is considered to be the successor of the right vitelline vein
        2. The distal portion of the left vitelline vein disappears completely
    2. THE UMBILICOALLANTOIC (UMBILICAL) VEINS enter the embryo by way of the connecting stalk and then course through the mesoderm of the septum transversum toward the heart. They lie more lateral than the vitelline veins but are also fragmented by the development of the liver and are connected to the hepatic sinusoids
      1. The proximal portions of both umbilical veins as well as the remainder of the right umbilical vein ultimately disappear, so that only the left umbilical vein continues to drain the blood coming from the placenta to the liver
      2. As a result of the marked increase of the placental circulation during further development, a short circuit (direct communication) is temporarily established between the left umbilical vein and the inferior vena cava, namely, the ductus venosus, which bypasses the sinusoidal plexus of the liver
        1. After birth, the left umbilical vein and the ductus venosus are obliterated and form the ligamentum teres hepatis and the ligamentum venosum, respectively
  2. The pulmonary veins: at about the 4 mm stage, the common pulmonary vein is visible as an evagination of the dorsal wall of the atrium
    1. THE BUD subsequently grows out into the dorsal mesocardium in the direction of the primitive foregut, which gives rise to the lung buds
      1. As the atrial cavity continues to develop, the stem of the pulmonary vein is progressively incorporated into the left atrial wall
      2. The incorporation of the pulmonary veins continues until 2 right and 2 left branches of the pulmonary stem enter the atrial cavity

development of the venous system:  the portal system and pulmonary veins: image #1