Transition from Fetal to Neonatal Circulation: Understanding the Changes at Birth
Chapter 1: Transition from Uterine to Extrauterine Life – Placental Circulation
Placental Circulation
Survival of the fetus requires a circulatory link between the mother and the embryo. Chronic Villi: are finger-like projections that arise from the embryo’s aorta within a week of uterine implantation and invade the uterine endometrium. These projections are filled by maternal blood bathing the fetal capillaries in an oxygen and nutrient-rich environment. The maternal uterine tissues and blood vessels of the fetal chronic villi make up the placenta.
Maternal blood flows into the intervillous space through the spiral arteries. Here, the exchange of gases and metabolic products between maternal and fetal blood takes place. After the exchange, maternal blood exits through venous channels and returns to the maternal circulation.
Oxygenated fetal blood leaves the chronic villi capillaries to join placental venules, which merge to form a single umbilical vein. On the fetal side of the placenta, the chorionic and amniotic layers give rise to the umbilical cord.
The Umbilical Cord
The umbilical cord contains a single umbilical vein (returning blood to the fetus) and two umbilical arteries (carrying blood to the placenta). The umbilical vein is generally larger than the arteries but with thinner walls. For a short time after birth, the umbilical vessels remain open and can be used for blood sampling.
Abnormal implantation of the placenta, tearing of the placenta from the uterine wall, or decreased placental flow can retard intrauterine growth or cause fetal asphyxia, which places the newborn (neonate) at higher risk of respiratory distress in the immediate postnatal period. Drugs, bacteria, and viruses can cross the intervillous space and cause a variety of problems.
Oxygenation and Hemoglobin
The maternal blood in the intervillous space has a mean PaO2 of approximately 50 mmHg. Fetal blood returning to the placenta has a PaO2 of approximately 19 mmHg. Fetal hemoglobin (HbF) has increased O2 carrying capacity and is a major factor responsible for fetal survival in this relatively hypoxic environment. Increased oxygen affinity is manifested by a left shift of the fetal oxyhemoglobin dissociation curve. P50 is 6 to 8 mmHg less than adult hemoglobin (HbA). The PO2 in blood returning to the fetus through the umbilical vein is only approximately 30 mmHg.
Fetal Circulation
Oxygenated blood from the placenta is carried in the umbilical vein to the fetal circulation by the hepatic system. One-third of this blood flows to the lower trunk and extremities. The other two-thirds are shunted past the liver through the ductus venosus into the inferior vena cava. This freshly oxygenated blood mixes with the venous blood returning from the lower trunk and extremities and enters the right atrium.
- 50% of this blood is shunted from the right atrium into the left atrium through the foramen ovale. Left atrial blood goes to the left ventricle – ascending aorta – brain – brachiocephalic trunk – descending aorta.
- Venous blood from the superior vena cava (SVC) goes from the right atrium to the right ventricle, then into the main pulmonary artery. Because of the hypoxemic fetal environment, pulmonary vascular resistance is high; for this reason, the mean pulmonary artery pressure in the fetus is higher than in the aorta.
- Less than 10% of the blood entering the pulmonary artery flows into the lungs. The rest is shunted from the main pulmonary artery to the descending aorta by the ductus arteriosus. The ductus arteriosus remains open because of the volume of blood flowing through it, the reduced fetal PaO2, and circulating prostaglandins (from maternal sources).
- Shunted flow from the ductus arteriosus then mixes with the blood ejected from the left ventricle.
Cardiopulmonary Events at Birth
Before birth, the placenta provides nutrition and respiratory, digestive, and renal functions for the fetus. When the fetus and placenta separate at birth, rapid and dramatic changes must occur before breathing can begin.
The fetal lung is normally filled with a volume of fluid equal to functional residual capacity (FRC), or approximately 30 ml/kg. During normal vaginal delivery, one-third of the lung fluid is cleared by compression of the thorax in the birth canal. The pulmonary capillaries clear the remaining fluid through the lymphatics.
No air enters the newborn’s lungs until the transpulmonary pressure gradient exceeds 40 cm H2O. When the lungs expand and breathing begins, the PaO2 increases, the PaCO2 decreases, and pH rises back toward normal.
This decrease in pulmonary vascular resistance allows blood flow to increase to the lungs, and the ductus arteriosus closes due to decreased blood flow, increased PaO2, and removal of maternal prostaglandins. Constriction of the ductus arteriosus promotes increased blood flow through the lungs.
Cessation of umbilical flow causes a rapid left heart pressure rise, and now the left-side heart pressures, higher than the right-side heart pressures, promote the closing of the foramen ovale. After this, the transition from fetal to extrauterine circulation is completed.