Changes in the vascular system at birth are caused by cessation of placental blood flow and the beginning of respiration. Since the ductus arteriosus closes by muscular contraction of its wall, the amount of blood flowing through the lung vessels increases rapidly. This, in turn, raises pressure in the left atrium. Simultaneously, pressure in the right atrium decreases as a result of interruption of placental blood flow. The septum primum is then apposed to the septum secundum, and functionally the oval foramen closes.
To summarize, the following changes occur in the vascular system after birth (Fig. 11.48):
Closure of the umbilical arteries, accomplished by contraction of the smooth musculature in their walls, is probably caused by thermal and mechanical stimuli and a change in oxygen tension. Functionally the arteries close a few minutes after birth, although the actual obliteration of the lumen by fibrous proliferation may take 2 to 3 months. Distal parts of the umbilical arteries form the medial umbilical ligaments, and the proximal portions remain open as the superior vesical arteries (Fig. 11.48).
Closure of the umbilical vein and ductus venosus occurs shortly after that of the umbilical arteries. Hence blood from the placenta may enter the newborn for some time after birth. After obliteration, the umbilical vein forms the ligamentum teres hepatis in the lower margin of the falciform ligament. The ductus venosus, which courses from the ligamentum teres to the inferior vena cava, is also obliterated and forms the ligamentum venosum.
Closure of the ductus arteriosus by contraction of its muscular wall occurs almost immediately after birth; it is mediated by bradykinin, a substance released from the lungs during initial inflation. Complete anatomical obliteration by proliferation of the intima is thought to take 1 to 3 months. In the adult the obliterated ductus arteriosus forms the ligamentum arteriosum.
Closure of the oval foramen is caused by an increased pressure in the left atrium, combined with a decrease in pressure on the right side. The first breath presses the septum primum against the septum secundum. During the first days of life, however, this closure is reversible. Crying by the baby creates a shunt from right to left, which accounts for cyanotic periods in the newborn. Constant apposition gradually leads to fusion of the two septa in about 1 year. In 20% of individuals, however, perfect anatomical closure may never be obtained (probe patent foramen ovale).
Ligamentum arteriosum Superior vena cava
Closed oval foramen
Inferior vena cava
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.