Fetal Circulation

The fetal circulation system is distinctly different from adult circulation. This intricate system allows the fetus to receive oxygenated blood and nutrients from the placenta. 

Fetal circulation and placenta. Color shows relative oxygen content of blood.

It is comprised of the blood vessels in the placenta and the umbilical cord, which contains two umbilical arteries and one umbilical vein. Fetal circulation bypasses the lungs via a shunt known as the ductus arteriosus; the liver is also bypassed via the ductus venosus and blood can travel from the right atrium to the left atrium via the foramen ovale. 

📖 Beischer & MacKay’s Obstetrics, Gynaecology and the Newborn 4th Ed 

Normal fetal heart rate is between 110 and 160 peats per minute. When compared to adults, fetuses have decreased ventricular filling and reduced contractility. Fetal circulation undergoes a rapid transition after birth to accommodate extra-uterine life.  Human understanding of fetal circulation originated from fetal sheep but ultrasound and magnetic resonance imaging (MRI) during the fetal period now provide detailed information. There are distinct differences in the fetal circulation that if not appropriately formed can lead to childhood or adult diseases. 

The fetal heart initiates at 22 days; this indicates the initiation of fetal circulation. Gas exchange initially takes place in the yolk sac until the placenta entirely takes over. This transition occurs around 10 weeks gestation. Maternal oxygenated blood mixes with placental blood which is low in oxygen before heading out to the fetus. Due to this mixing, the fetus is relatively hypoxic when compared to maternal, arterial blood.

Fetal Circulation

As the baby is born, the cardiovascular system undergoes a quick, drastic change. With its first breath, the baby's pulmonary vascular resistance substantially drops, which is in response to the oxygen now present in the lungs and the physical act of breathing. With the clamping of the umbilical cord after birth, the systemic vascular resistance increases helping the blood flow towards the lungs. The ductus arteriosus has a left-to-right flow within 10 minutes. The smooth muscle in the ductus arteriosus responds to the oxygen by increasing calcium channel activity causing constriction and ultimately shunt closure. The increased systemic resistance also raises the pressure in the left atrium to be higher than the right atrium, and this causes the foramen ovale to close.

The placenta connects the fetus to the wall of the uterus. It provides oxygen and nutrients from the mother to the growing fetus and also removes metabolic wastes and carbon dioxide from the fetus via the blood vessels in the umbilical cord. The umbilical cord develops from the placenta and is attached to the fetus. Oxygenated blood from the mother in the placenta flows through the umbilical vein and into the inferior vena cava (IVC), bypassing the liver via the ductus venosus. From the IVC, oxygenated blood travels to the right atrium of the heart. There is greater pressure in the right atrium compared to the left atrium in fetal circulation; therefore most of the blood is shunted from the right atrium to the left atrium through an opening called the foramen ovale. Once in the left atrium, blood travels through the left ventricle into the aorta and the systemic circulation. The deoxygenated blood travels back to the placenta via the umbilical arteries to be oxygenated by the mother. Additionally, some oxygenated blood in the right atrium can also enter the right ventricle and then the pulmonary artery. Because there is high resistance to blood flow in the lungs, the blood is shunted from the pulmonary artery into the aorta via the ductus arteriosus, hence bypassing the lungs. Blood then enters the systemic circulation, and the deoxygenated blood is recycled back to the mother via the umbilical arteries.

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