Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 67

The umbilical vein is a conduit vessel bringing oxygen and nutrient‐rich blood from the placenta to the fetus. The umbilical vein enters the umbilicus, courses anteriorly along the abdominal wall prior to entering the liver, and becomes the hepatic portion of the umbilical vein. The umbilical vein eventually becomes the portal vein, but first gives off the left inferior and superior portal veins, the DV, and finally the right portal vein. Approximately 50% of umbilical vein blood is directed into the DV and then to an area under the diaphragm referred to as the subdiaphragmatic vestibulum. The subdiaphragmatic vestibulum also receives blood from the inferior vena cava and blood exiting the liver via the right, middle, and left hepatic veins.

The process of preferential streaming begins in the subdiaphragmatic vestibulum with blood from the DV and the left and middle hepatic veins preferentially shunted across the foramen ovale into the left atrium and left ventricle so that the heart and head receive the most oxygenated and nutrient‐rich blood. In contrast, blood coming from the inferior vena cava and right hepatic vein are preferentially streamed into the right atrium and right ventricle. Then, after exiting through the pulmonary artery, this blood is shunted to the descending aorta via the ductus arteriosus. Blood leaves the fetus via two umbilical arteries arising from the hypogastric arteries which course around the lateral aspects of the bladder in an anterior and cephalad direction, exiting the umbilicus, returning oxygen‐reduced blood and waste products back to the placenta.

There are three primary fetal circulatory shunts that require closure after delivery for normal newborn cardiopulmonary transition to occur and for the subsequent adult circulation to be established. As mentioned above, the DV shunts blood from the umbilical vein toward the heart. The ductus arteriosus shunts approximately 90% of the blood in the main pulmonary artery to the descending aorta, leaving only 10% of pulmonary artery blood to reach the fetal lungs. The third shunt is the foramen ovale, which is maintained in a patent state in utero to allow the process of preferential streaming to occur from the right atrium to the left. Failure of any one of these shunts to close properly may result in adverse cardiopulmonary transition in the newborn.