FETAL CIRCULATION The fetal circulation differs from that of the post natal organism (neonates, children and adults) because of circuits which exist to circumvent the lungs, which are largely non-functional in fetal life. EMBRYOLOGY After fertilization and blastocyst formation, initial embryonic differentiation occurs to produce a bilaminar disc. Subsequent to that the amniotic cavity develops and the entire embryonic region is separated from the surrounding trophoblast by extraembryonic mesoderm. This mesoderm then degenerates, leaving only one point of connection between the embryo and the surrounding trophoblast called the connecting stalk. Gastrulation occurs at this time, resulting in a trilaminar disc, inclusive of embryonic mesoderm. The embryonic mesoderm is connected to the extraembryonic mesoderm by the connecting stalk. Embryonic mesoderm leads to formation of the fetal blood vessels and blood; the connecting stalk becomes enveloped in amnion and develops the umbilical vessels; and trophoblastic villi are ramified by extraembryonic mesoderm which also differentiate into blood vessels. Thus, there is continuity between the fetus and placental circulation via the umbilical cord and the vessels within. Fetal Circulation Oxygenated and nutrient rich blood leaves the placenta via the umbilical vein, and travels through the umbilical cord to enter the fetus. The umbilical vein divides in the liver into two trunks: the main trunk of the umbilical vein continues to the liver and then to the inferior vena cava, while the other, called the ductus venosus, carries blood directly to the inferior vena cava, and bypasses the liver altogether. Blood from the inferior and superior vena cavae both enter the right atrium of the heart. Both sides of the heart have equal pressures in fetal life, but the pulmonary circuit is collapsed, and therefore a high pressure circuit. Consequently, most blood in the right atrium bypasses the pulmonary circuit and passes directly to the left atrium through the foramen ovale, and then to the left ventricle, and out to the systemic circulation through the aorta. Some blood goes to the right ventricle and is pumped out through the pulmonary trunk; however, most of this blood is again directed away from the lungs through the ductus arteriosus which conducts it into the aorta, and out to the systemic circulation. Blood circulates through the fetus and is returned to the placenta through the umbilical arteries, which are the first branch of the internal iliac arteries in fetal life. The umbilical arteries branch throughout the placenta until capillaries enter the tertiary placental villi, which is where exchange between the maternal and fetal circulations occurs. Blood vessels from the villi then coalesce to form veins within the placenta, all of which eventually terminate in the umbilical vein to restart the cycle of circulation. Changes at Birth/Postnatal period Once the infant is born and takes the first breath, the high oxygen content of inspired air causes dilatation of the pulmonary vessels, causing the overall resistance of the pulmonary bed to fall, and becoming a low pressure circuit. As a result of this, there is increased flow through the pulmonary circuit and increased venous return from the pulmonary veins to the left atrium and ventricle. Also, at the time of birth the umbilical cord is exposed to air and cools, resulting in vasoconstriction of the umbilical vessels within. The cord is also usually clamped, and these actions disconnect the low pressure system of the placental circulation from the neonate’s systemic circulation, and leads to spontaneous constriction of the umbilical vein (and arteries) in the fetus. The reduction in flow through the umbilical vein through the ductus venosus also causes a drop in pressure in the vena cave, and in the right atrium and ventricle. The combination of increased flow to/pressure in the left atrium, and the increased systemic pressure for the left ventricle to pump against, plus the drop in right atrial pressure, results in a pressure differential between the left and right sides of the heart (left becomes more than right). This causes closure of the foramen ovale, and stops the right to left shunt at that level. Finally, the flow of blood with a high oxygen tension through the ductus arteriosus (from the aorta to the pulmonary trunk, which is now lower pressure than the systemic circuit) causes it to constrict and then to close completely, removing the shunt. This vasoconstriction is also mediated by bradykinin, which is released from the inflated lungs after birth, and which is a potent vasoconstrictor. Of note, the patency of the ductus arteriosus had been previously maintained by high levels of prostaglandin synthesis locally (prostacyclin and PGE2); these levels also fall with the onset of flow of highly oxygenated blood, further facilitating its closure. The fetal vessels and foramen ovale initially undergo a functional closure mediated by vasoconstriction, and by apposition of the septum primodium and secundum, respectively. Later anatomical closure occurs through proliferation of endothelial and fibrous tissues. The umbilical arteries become the medial umbilical ligaments The umbilical vein becomes the ligamentum teres The ductus venosus becomes the ligamentum venosum The ductus arteriosus becomes the ligamentum arteriosum The foramen ovale becomes the fossa ovalis
