Fetal Physiology, Fetal Circulation, and the Changes of the Fetal Circulation at Birth

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1. Fetal Physiology

Fetal physiology revolves around how the fetus grows and develops within the uterus. Key aspects include:

1. Oxygen and Nutrient Exchange

   • The fetus relies on the placenta for oxygen and nutrients.
   • The mother’s blood supplies nutrients and removes fetal waste products via the placental barrier.

2. Fetal Hemoglobin (HbF)

   • Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin (HbA).
   • This helps the fetus extract oxygen from maternal blood efficiently.

3. Fetal Growth Factors

   • Hormones such as human placental lactogen (hPL), estrogen, progesterone, and cortisol are crucial for fetal growth and metabolic regulation.
   • The fetus also produces its own hormones like fetal thyroid hormones (for brain development) and fetal insulin (for growth).

4. Amniotic Fluid

   • Cushions the fetus and allows for symmetrical growth.
   • Fetal kidneys contribute to amniotic fluid production by excreting urine into the amniotic sac.

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2. Fetal Circulation

Fetal circulation is uniquely adapted to maximize oxygen delivery and bypass non-functional lungs in utero. There are three main shunts that help redirect blood flow:

1. Ductus Venosus

   • Shunts blood from the umbilical vein directly to the inferior vena cava (IVC), bypassing most of the liver.
   • Ensures well-oxygenated blood from the placenta quickly enters systemic circulation.

2. Foramen Ovale

   • An opening between the right atrium (RA) and the left atrium (LA).
   • Allows oxygen-rich blood entering the RA to flow directly into the LA, bypassing the right ventricle and lungs.

3. Ductus Arteriosus

   • Connects the pulmonary artery to the descending aorta.
   • Diverts blood away from the lungs (which are not yet in use) into systemic circulation.

Mnemonic for Fetal Shunts

A quick way to remember the three fetal shunts is:

“D.O.V.” or “DOV”

• D = Ductus Venosus
• O = (Foramen) Ovale
• V = Ductus Arteriosus (think “vessel”)

(Another memory trick is to note that “Foramen Ovale” starts with an O, so you keep the three shunts in mind as DV, FO, DA. You can adapt whichever you find simpler!)

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3. Flow Chart of Fetal Circulation

Below is a simplified flow chart showing how blood travels through the fetal body.

flowchart LR
    A((Placenta)) --> B[Umbilical Vein]
    B --> C[Ductus Venosus] --> D[IVC]
    D --> E[Right Atrium]
    E --(Most blood)--> F[Foramen Ovale] --> G[Left Atrium] --> H[Left Ventricle] --> I[Aorta --> Body]
    E --(Remaining blood)--> J[Right Ventricle] --> K[Pulmonary Artery]
    K --(Bypass Lungs)--> L[Ductus Arteriosus] --> M[Descending Aorta] --> N[Umbilical Arteries] --> A

1. Placenta → Umbilical Vein
2. Umbilical Vein → Ductus Venosus → IVC
3. IVC → Right Atrium → (Most) Foramen Ovale → Left Atrium → Left Ventricle → Aorta
4. Right Atrium → (Remaining) Right Ventricle → Pulmonary Artery → Ductus Arteriosus → Descending Aorta
5. Descending Aorta → Umbilical Arteries → Placenta

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4. Changes at Birth

When a baby takes its first breath, the lungs expand and the circulatory system undergoes rapid changes:

1. Umbilical Cord Clamping

   • Stops blood flow from the placenta.
   • Increases systemic vascular resistance in the neonate.

2. Lung Expansion

   • Decreases pulmonary vascular resistance.
   • More blood flows to the lungs for oxygenation.

3. Closure of Fetal Shunts

   • Foramen Ovale → Functionally closes as left atrial pressure exceeds right atrial pressure.
   • Ductus Arteriosus → Begins to constrict due to increased oxygen tension (and decreased prostaglandins).
   • Ductus Venosus → Closes as blood no longer flows from the umbilical vein.

Mnemonic for Changes at Birth

“CRISP Lungs”

• C = Closure of shunts (DA, FO, DV)
• R = Resistance in lungs decreases
• I = Increase in systemic vascular resistance
• S = Stop placental circulation (cord clamped)
• P = Prostaglandins drop (helps close ductus arteriosus)
• Lungs = They expand and take over gas exchange

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5. Making It Stick: Key Points

• Fetal Hemoglobin has higher O2 affinity → ensures oxygen uptake from maternal blood.
• Three Shunts (DV, FO, DA) → help bypass liver and lungs.
• At Birth → Shunts close, lungs inflate, and the neonate’s circulation shifts to an adult-type pattern.

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Putting It All Together (Attractive Summary)

Fetal Circulation in a Nutshell

1. Placenta acts as the lung, kidney, and digestive organ for the fetus.
2. Blood bypasses the liver via the Ductus Venosus.
3. Blood bypasses the lungs via the Foramen Ovale and Ductus Arteriosus.

At Birth

• The baby’s lungs take over oxygenation.
• The shunts close, redirecting blood to flow through the lungs and liver.
• The baby is now independent of the placenta.

Visual Tip:

• Picture a road map: certain “exits” (shunts) are open in the fetus; at birth, those “exits” get blocked off, and traffic (blood) follows the main roads (through the lungs and liver).

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Final Eye-Catching Takeaway

“Before birth, the fetus doesn’t breathe air—so nature designed shortcuts (shunts) to bypass the lungs. Once the baby cries, these shortcuts close, and the lungs finally take center stage!”

Use the mnemonics, flow chart, and colorful diagrams in your notes to remember these vital steps in fetal physiology and circulation changes at birth. Good luck with your studies!