Pre-Lecture Prep

Introduction to Placental Transfer and Fetal Circulation

• Overview of the physiological processes during pregnancy and the birth of a baby.

Placental Transfer

• Definition: The process by which nutrients and gases are transferred between the maternal blood and the fetal blood across the placenta.

Factors Affecting Placental Transfer

1. Concentration of Gases:

   • Importance of the balance of oxygen (O2) and carbon dioxide (CO2) concentrations for gas diffusion across the placental barrier.

2. Area of Exchange:

   • Certain areas of the placenta allow for easier transfer of gases and nutrients compared to others. Some regions may require active transport to facilitate movement.

3. Barrier Permeability:

   • Describes how easily substances can cross the placental barrier.

   • Analogy: The permeability is compared to different types of fences (e.g., chain-link vs. concrete).

4. Size and Solubility of Molecules:

   • Smaller and more soluble molecules cross easily compared to larger ones.

   • Primary focus on O2$and$CO2 which cross freely, while other nutrients may need additional mechanisms.

Key Aspects of the Placenta

• No Mixing of Blood:

  • Maternal and fetal blood does not mix at any point during pregnancy.

• Oxygenated Blood Entry:

  • Maternal oxygenated blood enters child’s bloodstream through the placenta.

  • Key location for exchange - intervillous space (IVS):

    • Defined as the space between maternal and fetal sides of the placenta.

Chorionic Villi

• Structure:

  • Finger-like projections from the fetal side.

• Function:

  • Facilitate gas exchange where fetal blood experiences high CO2 concentration and low O2 concentration.

  • Oxygen moves from the intervillous space into the chorionic villi through diffusion (higher to lower concentration).

Fetal Circulation

Circulatory Path

• Umbilical Cord Structure:

  • Two umbilical arteries carry deoxygenated blood back to the placenta.

  • One umbilical vein carries oxygenated blood to the fetus.

Major Circulatory Shunts in Fetal Body

• Purpose: Redirect blood flow to maximize oxygen delivery where it's most needed.

1. Ductus Venosus:

   • Located in the liver; allows blood to bypass liver circulation directly to the heart.

2. Foramen Ovale:

   • A flap valve between the right and left atria of the heart; allows blood to bypass the non-functioning lungs.

3. Ductus Arteriosus:

   • Connects pulmonary artery to the descending aorta; directs blood away from the lungs to the lower body.

Blood Flow Steps

1. Oxygenated blood enters the fetus via the umbilical vein.

   • Moves toward the liver:

     • One third ($\frac{1}{3}$) directed away from liver via ductus venosus ($1^{st}$ shunt).

     • Remaining two-thirds ($\frac{2}{3}$) enter liver, then rejoin blood flow into heart via inferior vena cava (IVC).

2. In the heart:

   • Blood from IVC enters the right atrium.

   • Approximately $50-60\%$ shunted from right atrium to left atrium via foramen ovale ($2^{nd}$ shunt).

   • Left ventricle pumps oxygenated blood into the ascending aorta to upper body parts (brain, heart).

3. Remaining blood:

   • Moves from right atrium to right ventricle, then to pulmonary artery.

   • Only $10-12\%$ of blood goes to the lungs; majority diverted through ductus arteriosus ($3^{rd}$ shunt) into aorta for lower body oxygenation.

4. Return Cycle:

   • Deoxygenated blood from lower body returns via umbilical arteries back to placenta for oxygenation and $CO_{2}$ removal.

Birth Process and Transition to Postnatal Life

Changes upon Birth

• When the baby is born, several physiological changes occur to transition from placental support to independent pulmonary circulation.

1. First Breath:

   • Expansion of lungs increases blood flow to lungs, dropping pressure in the right heart.

   • Increased left heart pressure leads to closure of foramen ovale.

2. Decreased Pressure and Closure of Shunts:

   • Ductus arterioles and ductus venosus close in response to rising oxygen levels and decreasing prostaglandins.

   • The ductus venosus becomes the ligamentum venosum.

3. Timing for Closure of Shunts:

   • About $82\%$ of ductus arteriosus close within $48$ hours post-birth.

   • Most are permanently closed between $3$ weeks and $3$ months postnatal.

Conclusion

• Understanding the processes of placental transfer and fetal circulation is critical to grasp how nutrient and gas exchange happens during pregnancy and how newborns transition to breathing with their lungs.

• Additional resources for further study are recommended, including videos from Khan Academy.

NCLEX-Style Practice Questions

1. A nurse is teaching a student about the umbilical cord anatomy. Which statement by the student indicates an understanding of the structure?

   • A) "The umbilical cord contains two veins and one artery."

   • B) "The umbilical cord contains one vein and two arteries."

   • C) "The umbilical cord contains two veins and two arteries."

   • D) "The umbilical artery carries oxygenated blood to the fetus."

2. Which fetal circulatory structure is responsible for shunting oxygenated blood from the umbilical vein directly to the inferior vena cava, bypassing the liver?

   • A) Foramen ovale

   • B) Ductus arteriosus

   • C) Ductus venosus

   • D) Chorionic villi

3. A nurse explains that the foramen ovale is a flap valve located between which two chambers of the fetal heart?

   • A) Right ventricle and left ventricle

   • B) Right atrium and left atrium

   • C) Left atrium and left ventricle

   • D) Right atrium and right ventricle

4. Which physiological change occurs immediately after birth to facilitate the closure of the foramen ovale?

   • A) Decreased pressure in the left atrium

   • B) Increased prostaglandins in the blood

   • C) Increased pressure in the left atrium and decreased pressure in the right heart

   • D) Decreased oxygen concentration in the lungs

5. In fetal circulation, what percentage of blood is typically diverted to the non-functioning lungs?

   • A) $50-60\%$

   • B) 10-12\%$$

   • C) 82\%$</p></li><li><p>D)$\frac{1}{3}$</p></li></ul></li><li><p><strong>The nurse understands that the primary site of gas and nutrient exchange between the mother and fetus is the:</strong> </p><ul><li><p>A) Umbilical artery</p></li><li><p>B) Ductus venosus</p></li><li><p><strong>C) Intervillous space (IVS)</strong></p></li><li><p>D) Pulmonary artery</p></li></ul></li><li><p><strong>Which vessel is responsible for returning deoxygenated blood and$CO_{2}$from the fetus back to the placenta?</strong> </p><ul><li><p>A) Umbilical vein</p></li><li><p><strong>B) Umbilical arteries</strong></p></li><li><p>C) Inferior vena cava</p></li><li><p>D) Aorta</p></li></ul></li><li><p><strong>What does the ductus venosus become after it closes and undergoes fibrous change postnatally?</strong> </p><ul><li><p>A) Ligamentum arteriosum</p></li><li><p><strong>B) Ligamentum venosum</strong></p></li><li><p>C) Fossa ovalis</p></li><li><p>D) Coronary ligament</p></li></ul></li><li><p><strong>The ductus arteriosus connects which two major vessels to divert blood away from the fetal lungs?</strong> </p><ul><li><p>A) Umbilical vein and inferior vena cava</p></li><li><p>B) Right atrium and left atrium</p></li><li><p><strong>C) Pulmonary artery and descending aorta</strong></p></li><li><p>D) Aorta and umbilical artery</p></li></ul></li><li><p><strong>A nurse is reviewing the timing of postnatal changes. The nurse knows that approximately$82\%$of the ductus arteriosus shunts are closed within which timeframe?</strong> </p><ul><li><p>A)$24$hours</p></li><li><p><strong>B)$48$hours</strong></p></li><li><p>C)$3$weeks</p></li><li><p>D)$3$$ months