Fetal circulation refers to the unique system of blood flow in the fetus, allowing oxygen and nutrients to be supplied by the placenta. The lungs are non-functional in utero, and oxygenated blood comes from the mother.
Mechanism of Fetal Circulation
- Oxygenated
blood from the placenta enters the fetus through
the umbilical vein.
- The umbilical
vein carries blood towards the liver, where it divides into two
branches:
- A small
portion supplies the liver.
- The
majority bypasses the liver via the ductus venosus, entering the inferior
vena cava (IVC).
- Blood from
the IVC (oxygenated) mixes with deoxygenated blood from the body
and enters the right atrium.
- The
majority of this blood is directed through the foramen ovale into
the left atrium, bypassing the lungs.
- From the left
atrium, blood flows into the left ventricle and is pumped into
the ascending aorta, supplying the brain and upper body.
- Deoxygenated
blood from the upper body returns to the right atrium via the superior
vena cava (SVC).
- This blood
enters the right ventricle and is pumped into the pulmonary
artery.
- Since
fetal lungs are non-functional, most of this blood bypasses the lungs
through the ductus arteriosus, which connects the pulmonary artery
to the descending aorta.
- Blood
travels through the descending aorta, supplying the lower body.
- Deoxygenated
blood returns to the placenta through the two umbilical arteries,
where waste products are removed and blood is reoxygenated.
Summary of Fetal Circulatory
Pathway
- Placenta →
Umbilical Vein → Ductus Venosus → Inferior Vena Cava → Right Atrium
- Right
Atrium → Foramen Ovale → Left Atrium → Left Ventricle → Ascending Aorta
- Ascending
Aorta → Brain & Upper Body → Superior Vena Cava → Right Atrium
- Right
Atrium → Right Ventricle → Pulmonary Artery → Ductus Arteriosus →
Descending Aorta
- Descending
Aorta → Umbilical Arteries → Placenta
Special Features of Fetal
Circulation
- Ductus
Venosus: Bypasses the liver, directing oxygenated blood to the heart.
- Foramen
Ovale: Directs oxygenated blood from the right atrium to the left atrium,
bypassing the lungs.
- Ductus
Arteriosus: Shunts blood from the pulmonary artery to the descending aorta,
avoiding the lungs.
Changes in Circulation After
Birth
- Closure of
Umbilical Vessels:
- Umbilical
arteries become medial umbilical ligaments.
- Umbilical
vein becomes ligamentum teres (round ligament of
the liver).
- Closure of
Ductus Venosus:
- Becomes ligamentum
venosum in the liver.
- Closure of
Foramen Ovale:
- Due to
increased left atrial pressure, it closes functionally at birth and
anatomically within a few months, becoming the fossa ovalis.
- Closure of
Ductus Arteriosus:
- Due to
increased oxygen levels and decreased prostaglandins, it closes within
24-48 hours after birth and forms the ligamentum arteriosum.
Fetal vs. Postnatal Circulation
|
Feature |
Fetal Circulation |
Postnatal Circulation |
|
Gas Exchange |
Placenta |
Lungs |
|
Oxygenated Blood |
Umbilical vein |
Pulmonary veins |
|
Bypassing Liver |
Ductus Venosus |
No bypass |
|
Bypassing Lungs |
Foramen Ovale & Ductus Arteriosus |
No bypass |
|
Main Oxygen Supply |
Placenta |
Lungs |
