Overview
The lecture explains fetal circulation, highlighting key shunts and the differences between fetal and postnatal blood flow, and covers changes that occur after birth.
Normal Heart Blood Flow
- Deoxygenated blood returns from the body to the right atrium via the superior and inferior vena cava.
- Blood flows from the right atrium to the right ventricle, then is pumped into the pulmonary artery toward the lungs.
- Gas exchange in the lungs removes carbon dioxide and adds oxygen to the blood.
- Oxygenated blood returns to the left atrium via the pulmonary vein, flows into the left ventricle, and is pumped out through the aorta to the body.
- There are no shunts in normal postnatal circulation; the lungs perform gas exchange.
Fetal Circulation Overview
- Fetal lungs are non-functional and filled with fluid; gas exchange occurs via the placenta.
- Most blood bypasses the lungs through specialized shunts.
- The placenta supplies oxygen and nutrients to the fetus and removes waste via the umbilical cord.
Key Fetal Shunts and Umbilical Vessels
- Three main shunts: ductus venosus (liver), foramen ovale (between atria), ductus arteriosus (pulmonary artery to aorta).
- The umbilical vein (one) carries oxygenated blood from the placenta to the fetus.
- Two umbilical arteries carry deoxygenated blood from the fetus back to the placenta.
Step-by-Step Fetal Circulation
- Oxygenated blood from the placenta enters the fetus via the umbilical vein.
- Blood mostly bypasses the liver through the ductus venosus and enters the inferior vena cava.
- In the right atrium, most oxygenated blood passes through the foramen ovale to the left atrium, then to the left ventricle and aorta.
- Deoxygenated blood returns from the body via the superior vena cava to the right atrium and is pumped to the right ventricle.
- Blood in the right ventricle is mostly shunted through the ductus arteriosus into the aorta, bypassing the lungs.
- Mixed blood flows to the body and back to the placenta via the umbilical arteries.
Changes After Birth
- Cutting the umbilical cord stops placental blood flow; the ductus venosus closes.
- Lungs inflate with the first breath, dramatically lowering resistance and right heart pressure.
- Pressure change promotes closure of the foramen ovale and ductus arteriosus.
- Loss of placental prostaglandins also helps close the ductus arteriosus.
- Shunts are no longer needed as normal postnatal circulation begins.
Key Terms & Definitions
- Shunt — a passage that allows blood to bypass certain circulatory routes.
- Ductus venosus — shunt bypassing fetal liver, directing blood from umbilical vein to inferior vena cava.
- Foramen ovale — opening between right and left atria in the fetal heart.
- Ductus arteriosus — vessel connecting the pulmonary artery and aorta in the fetus.
- Umbilical vein — vessel carrying oxygenated blood from placenta to fetus (one present).
- Umbilical arteries — vessels carrying deoxygenated blood from fetus to placenta (two present).
- Placenta — organ supplying oxygen/nutrients and removing waste for the fetus.
Action Items / Next Steps
- Take the free quiz to review and test your knowledge of fetal circulation concepts.