Explain the changes that changes that occur in the cardiovascular system of the fetus upon birth.

Outline:

·        Fetal circulation

·        Changes in:

- umbilical arteries

- umbilical vein

- ductus arteriosus

- oval foramen

Essay:

            Before birth, blood from the placenta – about 80% saturated with oxygen – returns to the fetus by way of the umbilical vein which flows through the ductus venous directly into the inferior vena cava, bypassing the liver. The placental blood enters the right atrium where it is guided toward the oval foramen by the valve of the inferior vena cava, and the major portion of the bloodstream passes directly into the left atrium. From the left atrium, blood enters the left ventricle and ascending aorta. Desaturated blood from the superior vena cava flows by way of the right ventricle into the pulmonary trunk. Since resistance in the pulmonary vessels during fetal life is high, the portion of this blood passes directly through the ductus arteriosus into the descending aorta and finally to the placenta via the two umbilical arteries.

            Changes occurring in the vascular system at birth are caused by cessation of placental blood flow and the beginning of respiration. The decrease in oxygen tension causes contraction of the smooth musculature of the umbilical arteries, closing them functionally a few minutes after birth. The actual obliteration of the lumen by fibrous proliferation may take 2-3 months. Distal parts of the umbilical arteries form the median umbilical ligaments, while the proximal portions remain open as the superior vesical arteries.

            Closure of the umbilical vein and ductus venosus occurs shortly after that of the umbilical arteries. After obliteration, the umbilical vein forms the ligamentum teres in the lower margin of the falciform ligament while the ductus venosus forms the ligamentum venosum.

            Closure of the ductus arteriosus by contraction of its muscular wall occurs almost immediately after birth and is mediated by bradykinin, a substance released from the lungs during initial inflation. Complete anatomical obliteration takes 1-3 months, forming the ligamentum arteriosus.

            Closure of the oval foramen is caused by an increased pressure in the left atrium (increased flow in lungs due to closure of ductus arteriosus) and decrease in pressure on the right side (due to interruption of placental blood flow). With the first breath, the septum primum is pressed against the septum secundum. Constant apposition gradually leads to fusion of the two septa in about a year.