Ductus Venosus

Definition & Anatomy

• Creasy & Resnik's Maternal-Fetal Medicine, p. 4025
• The Developing Human: Clinically Oriented Embryology, p. 4800

Role in Fetal Circulation

1. Oxygenated blood (~O₂ saturation 80–85%) returns from the placenta via the umbilical vein.
2. ~50% of umbilical blood flow passes through the ductus venosus directly into the IVC, bypassing the largely non-functional liver.
3. The remaining ~50% enters the hepatic-portal system and perfuses the liver.
4. Blood from the ductus venosus merges with IVC blood (~19% of combined cardiac output, CCO) draining the lower limbs, kidneys, and splanchnic organs — so ~69% of CCO enters the right atrium.
5. The well-oxygenated ductus venosus stream occupies the dorsal and leftward portion of the IVC and preferentially streams through the foramen ovale into the left atrium → left ventricle → ascending aorta, ensuring the brain and myocardium receive the most oxygenated blood.

• Medical Physiology (Boron & Boulpaep), p. 182
• Creasy & Resnik's Maternal-Fetal Medicine, p. 4031

Physiologic Significance

• The ductus venosus is the only fetal vessel that can actively dilate in response to hypoxemia, providing a regulatory mechanism for oxygen-sensitive flow centralization.
• In healthy fetuses, it regulates distribution of oxygen and placental nutrients by restricting centralization of flow.
• High-velocity flow through the ductus venosus makes it easily identifiable on Doppler ultrasound.

Doppler Assessment (Clinical Use)

• Normal: triphasic, pulsatile high-velocity waveform with forward flow throughout.
• Absent or reversed flow during atrial contraction (the "a-wave") signals increased cardiac afterload and fetal compromise.
• Abnormal ductus venosus pulsation is also a marker in first-trimester Down syndrome screening, with detection rates of 65–75%.

Closure After Birth

• Guyton & Hall Textbook of Medical Physiology, p. 1725
• Medical Physiology (Boron & Boulpaep), p. 301
• The Developing Human, p. 4800

Summary Table

The Core Answer: The Right Atrium Receives TWO Different Streams

What Enters the Right Atrium?

So Does ANY Blood Go to the Lungs?

• The ductus arteriosus is so large and low-resistance that it accepts ~90% of right ventricular output and dumps it into the descending aorta.
• Only ~10% trickles into the pulmonary vessels — just enough to nourish the developing lung tissue, not for gas exchange (because the lungs aren't doing gas exchange yet).
• The lungs are fluid-filled and have high vascular resistance in utero, which is why blood preferentially avoids them.

So Why Do You Need BOTH Shunts? What's the Difference?

Foramen Ovale — Shortcut BEFORE the right ventricle

• The well-oxygenated IVC stream enters the RA and immediately crosses to the left atrium, skipping the right ventricle and pulmonary artery entirely.
• This blood → Left ventricle → Ascending aorta → Brain and heart (coronaries)
• Purpose: Deliver the best, most oxygenated blood directly to the most oxygen-sensitive organs (brain and heart)

Ductus Arteriosus — Shortcut AFTER the right ventricle

• The deoxygenated SVC stream goes: RA → Right ventricle → Pulmonary artery → Ductus arteriosus → Descending aorta → Lower body and back to placenta
• This blood supplies the lower body and then returns to the placenta via umbilical arteries to get re-oxygenated
• Purpose: Bypass the non-functional, high-resistance lungs

The Complete Fetal Flow Map

Placenta (oxygenated)
        ↓
   Umbilical vein
        ↓
   Ductus venosus → IVC
        ↓
   RIGHT ATRIUM  ─── splits here ───────────────────────┐
        │                                               │
   (Well-oxygenated IVC stream)              (Deoxygenated SVC stream)
        ↓                                               ↓
  FORAMEN OVALE                              Right Ventricle
        ↓                                               ↓
   Left Atrium                             Pulmonary Artery
        ↓                                    ↙         ↘
   Left Ventricle                    10% to lungs    90% via DUCTUS ARTERIOSUS
        ↓                            (nourishment)          ↓
  Ascending Aorta                                   Descending Aorta
        ↓                                                   ↓
BRAIN + HEART ✓                                    Lower body → Umbilical arteries
(best blood goes here)                                        → Placenta (re-oxygenate)

The Developing Human: Clinically Oriented Embryology — Fetal Circulation. Red = high O₂, purple = medium O₂, blue = low O₂. Note how the foramen ovale and ductus arteriosus handle completely separate blood streams.

Summary in One Line

The foramen ovale diverts the best blood to the brain and heart before it reaches the lungs. The ductus arteriosus diverts the remaining blood away from the lungs after it's already been through the right ventricle. They are complementary, not redundant.

• Medical Physiology (Boron & Boulpaep), pp. 182–192
• The Developing Human: Clinically Oriented Embryology, pp. 4691–4716