FETAL CIRCULATION
Introduction : FETAL CIRCULATION
The placenta causes the circulation in fetuses to differ from that of adults. The placenta is in charge of facilitating the exchange of gasses between the mother’s and fetal blood because the lungs of the fetus are nonfunctional. So, the placenta receives blood from the right ventricle.
At the fourth week of intrauterine life, the heart completes its development and begins to beat at a pace of 65 beats per minute. The development of blood arteries occurs alongside the heart. Heart rate rises progressively until it reaches its peak just before birth, at around 140 beats per minute.
The placenta serves as a link between the mother and the fetus. Maternal blood travels via uterine vessels, whereas fetal blood travels through umbilical vessels to reach the placenta. The placenta contains these two adjacent sets of blood arteries, which allow substances from the mother’s blood to exchange with those from the fetus. But there isn’t a direct mixing of fetal and mother blood.
Blood vessels involves in Fetal Circulation :
Large blood pumps into the lungs are not necessary because the lungs of the fetus do not function.
Rather, for the purpose of substance exchange, the fetal heart pumps a significant amount of blood into the placenta. The blood, which has more oxygen and nutrients, is collected by the umbilical veins from the placenta. Hepatocellular vein traverses the liver. The umbilical vein supplies the liver with a certain volume of blood. Nonetheless, the ductus venosus allows a significant amount of blood to be transferred from the umbilical vein into the inferior vena cava. The portal vein supplies blood to the liver as well.
The inferior vena cava in the liver allows oxygenated blood to partially mix with deoxygenated blood before entering the right atrium. The foramen ovale allows the majority of blood to be transferred from the right atrium into the left atrium.
The intra-atrial septum has an aperture called the foramen ovale.
By the superior vena cava, blood from the upper body reaches the right atrium. Blood fills the right ventricle from the right atrium. Blood is circulated into the pulmonary artery until this point on. Blood reaches the systemic aorta by the ductus arteriosus from the pulmonary artery.
The amount of blood delivered to the fetal lungs is little. The left ventricle carries blood through the aorta.
Through umbilical arteries, 50% of the blood from the aorta reaches the placenta.
Blood flow chart of Fetal Circulation :
Placenta → Umbilical Vein
Umbilical Vein → Ductus Venosus → Inferior Vena Cava
Inferior Vena Cava → Right Atrium
Right Atrium → Foramen Ovale → Left Atrium
Left Atrium → Left Ventricle → Aorta → Systemic Circulation
Right Atrium(Tricuspid valve) → Right Ventricle → Pulmonary Artery
Pulmonary Artery → Ductus Arteriosus → Descending Aorta
Systemic Circulation → Umbilical Arteries → Placenta
Fetal Lungs :
Embryonic Stage (3–7 weeks): the respiratory tract and bronchial tree begin to form as the lung buds separate from the foregut endoderm.
Pseudoglandular Stage: The bronchial tree keeps branching, generating the conducting airways all the way up to the terminal bronchioles, throughout this 7–17 week period.
The growing airways are surrounded by mesenchymal tissue.
Canalicular Stage (16–26 weeks): The circulatory network starts to form around the terminal bronchioles as they split into respiratory bronchioles. Alveolar cells of Type I and Type II begin to differentiate; Type II cells produce surfactant, which is essential for lung function after birth.
Saccular Stage: (24 weeks before birth): enhanced surfactant synthesis and further terminal sac (primitive alveolar) development.
The formation of the air-blood barrier gets the lungs ready for gas exchange after birth.
The resistance to blood flow via the pulmonary vascular bed is known as pulmonary vascular resistance. The reason for the fetus’s extremely high resistance is that its lungs are still developing. The pressure in the lungs’ blood vessels is raised by the fetus’s high resistance to air. The arteriosclerosis ductus allows blood to flow into the pulmonary artery towards the aorta due to the high pressure.
Fetal Circulation - Changes In Circulation And Respiration After Birth :
1. The infant's first breathing
The lungs begin to work when the fetus emerges and the umbilical cord is severed and knotted. Acute hypoxia and hypercapnia result from the interruption of placental circulation. Now that these two stimuli have greatly activated the respiratory center, breathing begins. First, there is gasping, and then breathing returns to normal.
2. Blood flow to lungs
A baby’s first breath causes their lungs to enlarge. Lung expansion results in a rapid drop in blood vessel pressure and an instantaneous decrease in pulmonary vascular resistance. As a result, the pulmonary artery’s blood flow to the lungs rises.
3. Foramen Ovel Close :
The oxygenated blood that comes from the lungs returns to the left atrium when blood flows through the pulmonary circulation. The left-ventricular atrial pressure rises as a result. Concurrently, the inferior vena cava experiences a drop in pressure as a result of placental blood flow interruption. It causes the right atrial pressure to drop. As a result, the left atrium currently possesses a high pressure and the right atrium has a lower pressure. The foramen ovale closes as a result. The foramen ovale entirely shuts and merges with the atrial wall a few days after birth.
4. Blood flow in ductus arteriosus- Reverse :
The ductus arteriosus allows blood to flow from the pulmonary artery to the aorta in the fetus because of very high pulmonary artery pressure. In newborns, however, systemic arterial pressure is higher than pulmonary artery pressure, so blood flows in the opposite direction through the ductus arteriosus, from the systemic aorta to the pulmonary aorta. In newborns, backflow in the ductus arteriosus is heard as a continuous murmur.
5. Ductus venosus close
The ductus venosus constricts and closes as a result of smooth muscle contraction around the confluence of the umbilical vein’s lumen and ductus venosus. The ductus venosus eventually turns into a fibrous band.
6. Ductus arteriosus close
As the ductus narrows, it begins to close. After 2 days, it closes completely and the adult cycle begins. Rarely, the ductus does not close. It does not rupture and produces a constant murmur. Patent ductus arteriosus is the name given to this condition where the ductus is intact.