Foetal (Fetal) Circulation | Before and At Birth | Cardiac Physiology | Embryology

Summary

This video from Bite Size Med explores fetal circulation, highlighting the unique differences from adult circulation due to the non-functional lungs and reliance on the placenta for gas exchange. It details the three key shunts in the fetal circulatory system: the ductus venosus, foramen ovale, and ductus arteriosus, which facilitate blood flow bypassing the lungs. The video also explains the physiological changes at birth that lead to the closure of these shunts, transitioning the baby's circulation to an adult pattern.

Takeaways

• 🌟 Fetal circulation is distinct from adult circulation due to the non-functional lungs and reliance on the placenta for gas exchange.
• 🔄 The umbilical cord has two arteries and one vein, with the arteries carrying deoxygenated blood and the vein carrying oxygenated blood.
• 🚦 Three shunts are crucial for fetal circulation: the ductus venosus, foramen ovale, and ductus arteriosus, allowing blood to bypass the liver, lungs, and directing it towards vital organs.
• 📍 The ductus venosus shunts blood from the umbilical vein to the inferior vena cava, bypassing the liver.
• 🫀 The foramen ovale allows oxygenated blood to pass from the right atrium to the left atrium, bypassing the lungs.
• 🩸 The ductus arteriosus channels blood from the pulmonary artery to the aorta, ensuring only a small portion goes to the lungs.
• 🌡️ High pulmonary vascular resistance in the fetus due to collapsed lungs and high blood flow to the placenta due to low systemic vascular resistance.
• 👶 At birth, the first breath decreases pulmonary vascular resistance, leading to closure of the shunts as the circulatory system adapts to lung breathing.
• ⏳ After birth, the functional closure of the ductus arteriosus occurs within hours to days, followed by anatomical closure over months.
• 🔗 The remnants of fetal circulatory structures persist into adulthood as ligaments and other anatomical markers.

Q & A

What is the primary difference between fetal and adult circulation?

The primary difference is that in fetal circulation, the lungs are non-functional, and the placenta performs the gas exchange, whereas in adults, the lungs are fully functional.

Why does the umbilical vein carry oxygenated blood and the umbilical arteries carry deoxygenated blood?

The umbilical vein carries oxygenated blood because it transports blood from the oxygenating placenta to the fetus. The umbilical arteries carry deoxygenated blood because they transport blood from the fetus to the placenta for oxygenation.

What are the three shunts in fetal circulation and their functions?

The three shunts are the ductus venosus, the foramen ovale, and the ductus arteriosus. The ductus venosus allows blood from the placenta to bypass the liver. The foramen ovale allows blood to pass from the right atrium to the left atrium, bypassing the lungs. The ductus arteriosus allows blood to bypass the lungs by shunting from the pulmonary artery to the aorta.

How does the blood from the umbilical vein enter the fetal heart?

The blood from the umbilical vein enters the inferior vena cava (IVC) and then bypasses the liver through the ductus venosus, entering the IVC and right atrium.

What directs the oxygenated blood towards the foramen ovale in the right atrium?

The Eustachian valve at the junction of the IVC and the right atrium directs the more oxygenated blood towards the foramen ovale.

Why does the majority of oxygenated blood from the left ventricle go to the brain and coronary circulation?

The majority of oxygenated blood from the left ventricle goes to the brain and coronary circulation because these are vital structures that require a good supply of oxygen.

What causes the right-to-left shunt in the fetal circulation?

The right-to-left shunt in fetal circulation is caused by the high pulmonary vascular resistance due to the collapsed lungs and the lower systemic vascular resistance in the placenta, leading to higher pressures on the right side of the heart compared to the left.

What changes occur in the fetal circulation at birth?

At birth, the baby takes a breath, the lungs expand, and the pulmonary vascular resistance falls. This causes the pressures in the left side of the heart to become higher than the right, reversing the shunts and closing them.

How do the shunts close after birth?

The foramen ovale closes due to the left atrial pressure becoming higher than the right, causing a flap to close. The ductus arteriosus closes due to high oxygen tension and loss of prostaglandin E2, leading to constriction and eventual fibrous tissue growth. The ductus venosus closes as the umbilical vein flow stops and portal venous pressure rises.

What are the remnants of the fetal circulation structures in adults?

In adults, the remnants are the ligamentum teres (from the umbilical veins), the ligamentum venosum (from the ductus venosus), the ligamentum arteriosum (from the ductus arteriosus), and the fossa ovalis (from the foramen ovale).