Chapter 129. The Circulatory System Before and After Birth - Review of Medical Embryology Book

The fetal blood reaches the placenta via the umbilical arteries, which are branches of the caudal aortic system. Well-oxygenated blood from the placenta is returned by the umbilical veins, about one-half passing through the liver sinusoids, and the rest bypassing the liver via the ductus venosus into the inferior vena cava system. The blood flow is regulated by a muscular sphincter in the ductus venosus, close to the umbilical veins. The blood then passes to the right atrium. Since the inferior vena cava also receives deoxygenated blood from the lower portion of the body, the blood entering the right atrium is not as well oxygenated as that in the umbilical veins
The minor circulation (circulation through the lungs): although existing anatomically, the minor circulation is almost completely short-circuited by 2 major mechanisms

THE BLOOD FROM THE INFERIOR VENA CAVA is directed largely by the lower border of the septum secundum, through the foramen ovale, which directs blood from the right to the left side of the heart. Here it mixes with small amounts of deoxygenated blood returning from the lungs via the pulmonary veins

From the left atrium, blood goes to the left ventricle and out of the heart via the aorta to vessels of the head and neck, upper limbs, and the rest of the body. The former receive richer, well-oxygenated blood
Some oxygenated blood from the inferior vena cava stays in the right atrium, mixes with deoxygenated blood from the superior vena cava and coronary sinus, and passes to the right ventricl Blood leaves via the pulmonary trunk and passes through the ductus arteriosus, which diverts it from the pulmonary to the aortic system

Little goes to the lungs (?10%) due to pulmonary vascular resistance
The mixed blood in the descending aorta (58% saturated with oxygen) passes to the umbilical arteries and is returned to the placenta for reoxygenation

THE FETUS cannot use its pulmonary system since it lives in a liquid environment. The lungs, nevertheless, have been ready and prepared to fulfill their role from month 6 of pregnancy. It becomes effectively functional only when the fetus is born

Circulation at birth: placental circulation is interrupted. The abrupt drop in intrathoracic pressure brought about by the first respiration (aeration of the lungs) helps contribute to the initial pulmonary circulation

BLOOD PRESSURE decreases in the pulmonary artery even though its flow is increased since it supplies a capillary network considerably enlarged by expansion of the pulmonary parenchyma

As a result, blood flow decreases (even reverses momentarily) in the ductus arteriosus; its walls contract, and in a few days, it closes off completely

Closure appears to be mediated by bradykinin, released from the lungs during their initial inflation. Its action depends on the high oxygen content of the aortic blood resulting from ventilation of the lungs at birth

Similarly, influx of pulmonary blood into the left atrium causes the septum primum to be pressed against the septum secundum, and the foramen ovale is closed
The umbilical arteries also constrict at birth and prevent blood loss

THE CIRCULATORY SYSTEM at this time-resembles its adult form, with separation of the minor and major circulations. Nevertheless, occlusion of the 2 circuits is still, for some time, only physiologic (functional)
THE CIRCULATORY SYSTEM becomes fully anatomic after several weeks due to the proliferation of endothelial and fibrous tissue

Fibrous degeneration of the ductus arteriosus forms the ligamentum arteriosum, between the left pulmonary artery and the concave inferior surface of the aortic arch
Complete fusion of the septum primum to the septum secundum forms a definitive interatrial septum. Traces of the former passage, the foramen ovale, are seen as a depression, the fossa ovalis, in the right interatrial wall