Traces the route of blood through the fetal circulation, from the umbilical vein and ductus venosus through the foramen ovale and ductus arteriosus, then describes how the circulation changes after the first breath. It also explains the two shunts, the mixing of oxygenated and CO₂-rich blood in the fetal heart, and the reasons for the closures at birth.
Topic 58 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 4 sections, about 498 words (4 min of reading), with 12 flashcards.
In this note
Topic 58- Embryonal blood circulation
Before Birth
After Birth
Fetal Circulation System
Before Birth
Blood from the placenta is carried to the fetus by the umbilical vein.
Less than one-third of this blood enters the fetal ductus venosus and is carried to the inferior vena cava, while the rest enters the liver proper from the inferior border of the liver.
The branch of the umbilical vein that supplies the right lobe of the liver first joins with the portal vein.
Blood then moves to the right atrium of the heart.
In the fetus, the foramen ovale connects the right and left atria, allowing most blood to flow directly from the right atrium into the left atrium, bypassing pulmonary circulation.
Continuation of flow: left atrium → left ventricle → aorta → body.
Some blood moves from the aorta through the internal iliac arteries to the umbilical arteries, returning to the placenta for CO₂ and waste exchange.
Some blood entering the right atrium flows into the right ventricle, then pulmonary artery.
In the fetus, the ductus arteriosus connects the pulmonary artery to the aorta, diverting most blood away from the lungs, which are non functional and filled with amniotic fluid.
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