Fetal Circulation and Changes
at Birth
The primary vascular concept of prenatal circulation is the requirement that the intraembryonic circulation of blood bypass the nonfunctioning lungs and liver (see Plate 4-18). The placenta (villous chorion) serves the role of these organs with gas and metabolic exchange between maternal and fetal blood. The airway in the lungs is filled with amniotic fluid, and pulmonary vascular resistance is high. The lungs only receive enough blood to nourish the tissues, and pulmonary venous blood flow into the left atrium is minimal. The plan for the prenatal circulation also requires that it convert the postnatal pattern soon after the first breath of the newborn. Two lung shunts (and a liver shunt) and the design of the interatrial septum serve these needs.
The original placental circulation
consists of paired umbilical arteries that pass from the internal iliac
arteries to the placenta through the umbilical cord and paired umbilical veins
that bring highly oxygenated blood into the embryo to connect with the sinus
venosus of the developing heart tube (see Plate 4-19). The right umbilical vein disappears, and the left proximal
umbilical vein also disappears as the remaining intraembryonic segment connects
with the developing IVC under the liver. As with the lungs, blood flow in the
liver is also minimal. Most of the blood in the umbilical vein bypasses the
liver through the ductus venosus, a straight continuation of the
umbilical vein into the IVC.
After partitioning of the heart
tube, the IVC is in line with the foramen ovale in the right atrium. Its stream of blood pushes the septum primum away
from the septum secundum, and much of this oxygenated blood passes from the
right atrium to the left. This is the first lung shunt. From the left atrium,
the blood will pass into the left ventricle and out the ascending aorta.
Venous blood from the SVC is
directed at the right atrioventricular valve. Although some mixing of blood
occurs from superior and inferior venae cavae, the streams of the blood pass each other to a
degree, and the most highly oxygenated blood in the fetal heart ends up on the
left side. The blood in the right ventricle passes into the pulmonary trunk and
continues into the arch of the aorta via the ductus arteriosus (from sixth
aortic arch). This is the second lung shunt based on simple pressure
differences between the pulmonary and systemic circulations. Again, vascular
resistance in the lungs is high. Pressure is much lower in the aorta, and most
of the right ventricle blood flows to the descending aorta. The ductus
arteriosus connects with the aortic arch just past the origin of the great
arteries. The result is that the brain (and head in general) and upper
extremities receive the most highly oxygenated blood in the fetus. There is a
mixing of blood from the ductus arteriosus and ascending aorta, but its oxygen
content is still higher than typical venous blood, and it is obviously
sufficient to address the metabolic needs of the growing lower half of the
fetus.
The conversion to the
postnatal/adult configuration is triggered by the first breath. Amniotic fluid
comes out of the airway, vascular beds open in the
lungs, and blood now rushes from the ductus arteriosus into the lungs rather
than the aortic arch. Blood flow from the lungs to the left atrium increases
rapidly, and the septum primum is pressed against the septum secundum to
effectively close the foramen ovale. Over the next few weeks the ductus
arteriosus will begin to form a fibrous cord, the ligamentum arteriosum,
and the two interatrial septa will permanently fuse.
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| THREE EARLY VASCULAR SYSTEMS |
Blood in the umbilical arteries now
has a higher oxygen content, which causes the arteries to spasm. Eventually the
umbilical arteries will form fibrous cords, the medial umbilical ligaments, converging
on the umbilicus on the internal surface of the abdominal wall. The umbilical
vein collapses from lack of blood and eventually becomes the fibrous round
ligament of the liver (ligamentum teres). The ductus venosus becomes the ligamentum venosum.
