U01.07.004 Fetal circulation

Fetal circulation is designed to bypass non-functional fetal lungs and enable the exchange of gases and nutrients through the placenta. Below is a detailed overview of fetal circulation and its key shunts, as well as the physiological changes at birth.

Key Features of Fetal Circulation

Umbilical Vein Oxygenation:
- Blood in the umbilical vein has a Po2 of approximately 30 mm Hg and is 80% saturated with oxygen.
- This oxygen-rich blood is directed towards the fetus, providing oxygenation before birth.
Umbilical Arteries:
- Umbilical arteries carry low oxygen saturation as they return deoxygenated blood from the fetus to the placenta.

Three Important Shunts in Fetal Circulation

Ductus Venosus:
- Blood entering the fetus through the umbilical vein is shunted via the ductus venosus into the inferior vena cava (IVC), bypassing the liver.
- This allows oxygenated blood to flow directly into the right atrium, increasing the efficiency of oxygen delivery.
Foramen Ovale:
- The highly oxygenated blood reaching the heart via the IVC is directed through the foramen ovale into the left atrium.
- This allows oxygen-rich blood to enter the left side of the heart and be pumped to the brain and upper body.
Ductus Arteriosus:
- Deoxygenated blood from the superior vena cava (SVC) passes through the right atrium, right ventricle, and the main pulmonary artery, where it is shunted through the ductus arteriosus into the descending aorta.
- This bypasses the non-functional lungs due to high fetal pulmonary artery resistance (partly due to low O2 tension).

Physiological Changes at Birth

Breathing Initiation:
- At birth, the infant takes a first breath, which decreases pulmonary vascular resistance.
- Left atrial pressure increases relative to right atrial pressure, leading to the closure of the foramen ovale (it becomes the fossa ovalis).
Ductus Arteriosus Closure:
- With the increase in oxygen levels from respiration and the decrease in prostaglandins from placental separation, the ductus arteriosus closes.
- The ductus arteriosus remnant becomes the ligamentum arteriosum.

Clinical Relevance for USMLE Step 1

Patent Ductus Arteriosus (PDA):
- A patent ductus arteriosus (PDA) occurs when the ductus arteriosus fails to close after birth.
- Indomethacin can be used to help close a patent ductus arteriosus by inhibiting prostaglandin synthesis.
Prostaglandins:
- Prostaglandins E1 and E2 keep the PDA open, which is important in certain congenital conditions (e.g., congenital heart defects requiring continued shunting).

Points to Remember for USMLE Step 1

Umbilical circulation: The umbilical vein carries oxygenated blood to the fetus, while the umbilical arteries carry deoxygenated blood back to the placenta.
Three fetal shunts:
1. Ductus venosus bypasses the liver.
2. Foramen ovale allows blood to flow from the right atrium to the left atrium.
3. Ductus arteriosus bypasses the lungs by shunting blood from the pulmonary artery to the aorta.
Transition at birth:
- At birth, the foramen ovale and ductus arteriosus close due to changes in pressure and oxygen levels.
- Indomethacin helps close PDA, while prostaglandins E1 and E2 maintain it open.

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