Highly oxygenated blood from the placenta enters the fetus via the umbilical vein. A large proportion of this blood passes into the liver to supply the hepatic sinusoids. The remainder bypasses the liver in the ductus venosus, drains into caudal vena cava and mixes with poorly oxygenated blood returning from the fetal body. The blood in the caudal vena cava, which, although mixed is still well oxygenated, drains into the right atrium of the heart.
Most of the blood entering the right atrium from the caudal vena cava is directed through the foramen ovale into the left atrium where it is mixed with a small amount of deoxygenated blood returning from the lungs. The contents of the left atrium enter the left ventricle and are expelled from the heart into the aorta.
The contents of the right atrium (which consist of some well oxygenated blood from the caudal vena cava and poorly oxygenated blood returning from the head and forelimbs via the cranial vena cava) enter the right ventricle and are expelled from the heart via the pulmonary artery. Only approximately 5 -10% of the blood in the pulmonary artery enters the lungs in the fetus due to the high resistance of their collapsed, non-aerated state. The remainder enters the ductus arteriosus which is a shunt linking the pulmonary artery and the aorta. The convergence of the poorly oxygenated pulmonary blood and the well-oxygenated aortic blood occurs after the main supply to the head and forelimbs have branched off the aortic arch. This ensures that the blood richest in oxygen reaches the developing brain.
The abdominal aorta supplies the rest of the body and gives off two umbilical arteries (branches of the internal iliac arteries) which carry poorly oxygenated blood back to the placenta.
Changes in Fetal Circulation after birth
At birth the lungs can inflate and perform their true function meaning that the fetal bypass systems are no longer required.
Umbilical vein - Constricts to form the ligamentum teres, which extends from the umbilicus to the liver. The mesentery that surrounded the umbilical vein becomes the falciform ligament.
Ductus venosus - A sphincter in the ductus venosus constricts so that all blood entering the liver passes through the hepatic sinusoids.
Foramen ovale - Due to aeration of the lungs, pulmonary resistance decreases and pulmonary blood flow increases. The increase in pulmonary blood flow causes the pressure in the left atrium to raise above that of the right which results in the valve of the foramen ovale being pushed against the septum secundum. This closes the foramen ovale and its vestge is known as the fossa ovale.
Ductus arteriosus - The change in the partial pressure of oxygen in the blood once the lungs become functional controls the constriction of the ductus arteriosus. Closure of the duct is usually complete soon after birth and its remnant is known as the ligamentum arteriosus.
Umbilical arteries- The intra-abdominal portions of the umbilical arteries constrict. Some parts remain patent supplying the urinary bladder and these are contained within the lateral vesicle ligaments which are vestiges of the mesetery surrounding the umbilical arteries.
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