Fetal Circulation Lecture Notes

Introduction

Placenta
- Source of nutrients and oxygen.
Umbilical Vein
- Carries oxygen-rich blood from placenta to the fetus.
Ductus Venosus
- Diverts some blood away from the liver directly into the inferior vena cava.
Foramen Ovale
- Allows blood flow from right atrium to left atrium.
Ductus Arteriosus
- Connects pulmonary artery to the aorta.
Umbilical Arteries
- Carry deoxygenated blood from the fetus back to the placenta.

Oxygen-rich blood from placenta flows through the umbilical vein into the ductus venosus.
Blood enters the right atrium, mixing with blood from the superior and inferior vena cava.
Blood flows:
- To Right Ventricle: Pumps into the pulmonary artery.
- Through Foramen Ovale: To left atrium, then left ventricle, and pumped into the aorta.
Important Note:
- This mechanism allows oxygenated blood to reach the coronary arteries and brain (most metabolically active organs).

Pulmonary Circulation: High resistance due to fluid-filled lungs.
Systemic Circulation: Low resistance.
Majority of blood from the right ventricle flows through the ductus arteriosus into the aorta.
8% of right ventricular output goes to the lungs for nutrient supply.

At Birth:
- Umbilical cord clamped and placenta removed.
- Systemic vascular resistance rises.
Breathing:
- Lungs fill with air, reducing pulmonary pressure.
- Expansion of alveoli and dilation of surrounding vessels.
- Takes 6-8 weeks for pulmonary vascular resistance to normalize.
Closure of Fetal Structures:
- Foramen Ovale: Closes functionally due to pressure changes.
- Ductus Arteriosus: Closes typically within 4 to 10 days after birth.
- Ductus Venosus: Closes within 3 to 7 days, leaving a remnant known as ligamentum venosum.
- Umbilical Vein and Arteries: Become ligaments after fibrin infiltration within a week.