Fetal circulation
fetal circulation diagram placenta ductus arteriosus foramen ovale
A multi-panel medical diagnostic and clinical series demonstrating congenital cardiovascular anomalies in a ctnnb1Δex3 model. Panels A and B are ultrasound images illustrating a Patent Ductus Arteriosus (PDA), with labels identifying the Left Atrium (LA), Aorta (Ao), Right Ventricle Outflow Tract (RVOT), and Pulmonary Artery (PA). An arrow points to the 'Open DA' (Ductus Arteriosus). Panel C provides an ultrasound view of the atrial septum with labels for the Right Atrium (RA), LA, and Ao, showing a patent foramen ovale. Panel D utilizes Color Doppler imaging to show a pathological right-to-left shunt of blood flow through the foramen ovale. Panel E is a clinical photograph of a dissected heart confirming the presence of an open foramen ovale, notable for the presence of dark pigmented cells (melanocytes) around the aperture. The collection demonstrates how failure of fetal cardiovascular shunts to close postnatally leads to abnormal circulation and chamber dilation.
This composite figure presents a detailed fetal echocardiographic evaluation at 35 weeks of gestation. Panels A and B (four-chamber views) demonstrate significant chamber disproportion, with a dilated right ventricle (RV) compared to the left ventricle (LV). The flap of the foramen ovale (FFo) appears relatively fixed and exhibits tense bulging throughout both systole (A) and diastole (B). Panel C (M-mode) confirms impaired RV contractility, showing a reduced shortening fraction compared to the LV. Panel D (three-vessel and trachea view) reveals a hypoplastic aortic arch (AoA) in comparison to the larger ductus arteriosus (DA). Panels E and F utilize color Doppler imaging to illustrate abnormal hemodynamics at the interatrial septum, characterized by turbulent blood flow (marked with asterisks) and restricted passage across the foramen ovale. These findings are clinically significant for identifying secondary cardiac changes due to fetal anemia and altered preload conditions, emphasizing the assessment of the foramen ovale and ventricular function in high-risk pregnancies.
This diagnostic image series utilizes 4D flow MRI-derived particle tracing to illustrate fetal circulatory dynamics in a ventral view. The visualization demonstrates the preferential streaming of blood from two distinct venous sources over one cardiac cycle (62 ms, 186 ms, 310 ms, and 434 ms). Red particles represent blood from the ductus venosus (DV), while blue particles represent blood from the distal inferior vena cava (IVCd). The series highlights the maintenance of separate flow streams within the proximal inferior vena cava. The red DV stream is directed preferentially through the foramen ovale (FO) toward the left ventricle (LV) to supply oxygenated blood to the systemic circulation. Concurrently, the blue IVCd stream is routed primarily toward the right ventricle (RV) for delivery to the main pulmonary artery. Anatomical landmarks labeled include the ductus venosus (DV), distal inferior vena cava (IVCd), foramen ovale (FO), right ventricle (RV), and left ventricle (LV). This pedagogical tool illustrates the physiological shunting essential for fetal development and the minimal mixing of nutrient-rich and nutrient-poor blood streams.
A composite medical illustration and graph depicting placental anatomy and related MRI biophysical properties. The left panel is a detailed anatomical diagram of a human placenta at approximately 32 weeks gestation. It illustrates the fetal circulation, including the umbilical cord (vein and arteries), chorionic plate, and the branching villous tree. The maternal circulation is shown via the basal plate, uterine spiral arteries, endometrial veins, and the intervillous space. The diagram highlights the proximity of maternal and fetal blood vessels at the exchange surface, separated by placental septa. The right panel is a bivariate plot used for diagnostic imaging analysis. The x-axis represents T2* relaxation time (ms) at 3T, a proxy for tissue oxygenation (SO2), ranging from deoxyhemoglobin to oxyhemoglobin. The y-axis represents the Apparent Diffusion Coefficient (ADC, mm²/s) on a logarithmic scale, indicating water mobility (restricted diffusion vs. active perfusion). Various placental tissue environments, such as septal walls, myometrium, and intervillous blood, are mapped onto the plot based on their specific oxygenation and diffusion characteristics, serving as a model for interpreting combined diffusion-relaxation MRI data.
Fetal Circulation
The Fetal Circulation Diagram
Step-by-Step Blood Flow
1. Placenta → Umbilical Vein
- The umbilical vein carries oxygenated, nutrient-rich blood (~80% O₂ saturation) from the placenta to the fetus under high pressure.
- It is the only fetal vessel that carries oxygenated blood.
2. Ductus Venosus (Shunt #1 - Liver Bypass)
- As the umbilical vein approaches the liver, approximately 50% of the blood passes directly into the ductus venosus, bypassing the liver and entering the inferior vena cava (IVC) directly.
- The remaining 50% enters the portal circulation and hepatic sinusoids, then drains into the IVC via the hepatic veins.
- A physiologic sphincter near the umbilical vein regulates this flow - it prevents cardiac overloading during high flow states (e.g., uterine contractions).
3. Inferior Vena Cava → Right Atrium
- Blood in the IVC is a mixture: well-oxygenated umbilical/hepatic blood mixes with poorly oxygenated blood from the lower limbs, kidneys, and pelvis.
- The IVC blood saturation at this point is approximately 67%.
4. Foramen Ovale (Shunt #2 - Lung Bypass, Right → Left Atrium)
- Blood entering the right atrium is directed by the crista dividens (inferior edge of the septum secundum).
- Most of the IVC blood passes directly through the foramen ovale into the left atrium.
- A small amount of IVC blood remains in the right atrium, where it mixes with poorly oxygenated blood from the superior vena cava (SVC) and coronary sinus.
5. Left Atrium → Left Ventricle → Ascending Aorta
- From the left atrium, blood passes to the left ventricle and out into the ascending aorta.
- This route delivers relatively well-oxygenated blood to the coronary arteries (heart) and carotid arteries (brain) - the body's most oxygen-demanding organs.
6. Right Atrium → Right Ventricle → Pulmonary Trunk
- The mixed blood that stays in the right atrium (from SVC + some IVC) passes into the right ventricle and out the pulmonary trunk.
- Pulmonary vascular resistance is high in fetal life (collapsed, fluid-filled lungs). Only ~10% of this blood reaches the lungs.
7. Ductus Arteriosus (Shunt #3 - Lung Bypass, Pulmonary Trunk → Aorta)
- ~90% of blood from the pulmonary trunk passes through the ductus arteriosus directly into the descending aorta, bypassing the lungs entirely.
- Pulmonary artery pressure in the fetus is slightly higher than aortic pressure, which drives this shunt.
- The ductus arteriosus also allows the right ventricle to strengthen and prepare for postnatal function.
8. Descending Aorta → Umbilical Arteries → Placenta
- Blood in the descending aorta is relatively deoxygenated (~60% saturation in umbilical arteries).
- 65% of descending aorta blood is returned to the placenta via the two umbilical arteries (branches of internal iliac arteries) for reoxygenation.
- The remaining 35% supplies the abdominal viscera and lower body.
Summary of the Three Fetal Shunts
| Shunt | Connection | Purpose |
|---|---|---|
| Ductus venosus | Umbilical vein → IVC | Bypasses the liver |
| Foramen ovale | Right atrium → Left atrium | Bypasses the lungs (atrial level) |
| Ductus arteriosus | Pulmonary trunk → Descending aorta | Bypasses the lungs (arterial level) |
Sites of O₂ Mixing (Blood Loses Purity)
- Liver - mixes with portal blood
- IVC - mixes with deoxygenated blood from lower limbs
- Right atrium - mixes with blood from head and arms (SVC)
- Left atrium - mixes with small amount of pulmonary venous return
- Descending aorta - where ductus arteriosus blood enters
Changes at Birth (Transitional Neonatal Circulation)
- Clamping of the umbilical cord stops placental blood flow → drops IVC and right atrial pressure.
- Lung expansion → dramatic fall in pulmonary vascular resistance → massive increase in pulmonary blood flow.
- Left atrial pressure rises (more pulmonary return) and right atrial pressure falls → the foramen ovale valve (septum primum) is pushed against septum secundum → functional closure of the foramen ovale.
- Ductus arteriosus: As pulmonary resistance drops below systemic resistance, flow reverses (aorta → pulmonary trunk). Rising arterial pO₂ stimulates muscular contraction of the ductus wall. Functional closure typically occurs within 24-48 hours (by 48 hours, ~82% are functionally closed; permanent anatomic closure by 3 weeks in most cases).
- Ductus venosus: Sphincter constricts at birth; undergoes fibrous obliteration over 2-3 months.
Adult Remnants of Fetal Structures
| Fetal Structure | Adult Remnant |
|---|---|
| Umbilical vein | Ligamentum teres (round ligament of liver) |
| Ductus venosus | Ligamentum venosum |
| Foramen ovale | Fossa ovalis |
| Ductus arteriosus | Ligamentum arteriosum |
| Umbilical arteries (proximal) | Superior vesical arteries |
| Umbilical arteries (distal) | Medial umbilical ligaments |
Key Physiological Features of Fetal Circulation
- Fetal Hb (HbF) has a higher O₂ affinity than adult HbA, with a left-shifted oxyhemoglobin dissociation curve. This allows HbF to extract more O₂ from maternal blood across the placenta even at the relatively low pO₂ of the placental sinuses.
- Right ventricular dominance: In the fetus, the right ventricle is thicker than the left (it works harder against pulmonary hypertension). By 1 month postnatal, the left ventricle becomes thicker due to the higher systemic resistance.
- Pulmonary vascular resistance is high in the fetus due to hypoxic vasoconstriction and the physical state of collapsed lungs.
- The umbilical vein is the only vessel in the fetal body carrying oxygenated blood (a notable exception to the adult rule that veins carry deoxygenated blood).