Placental Exchange via Diffusion Gradients

Steps:

• Fetal blood at X (approaching placenta) carries metabolic wastes like CO2 and urea from fetal tissues.
• In the placenta, CO2 and urea diffuse into maternal blood due to higher concentrations in fetal blood.
• This reduces CO2 and urea levels in fetal blood by Y (leaving placenta).
• Oxygen and glucose diffuse from maternal to fetal blood, increasing their concentrations from X to Y.

Why B is correct:

• CO2 and urea follow Fick's law of diffusion, moving down concentration gradients from fetal (high at X) to maternal blood, lowering levels at Y.

Why the others are wrong:

• A: Glucose diffuses into fetal blood, so higher at Y than X.
• C: Glucose is higher at Y after uptake from maternal blood.
• D: Oxygen diffuses into fetal blood, so higher at Y than X.

Final answer: B