Interpret a bounded placental exchange model
Invented gas-level model: oxygen maternal 80 / fetal 30; carbon dioxide maternal 20 / fetal 55. Differences guide direction, not actual transfer rates.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall cell nuclei, gametes, diffusion, respiration and the difference between a cell and an organ.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which oxygen direction does the 80-to-30 model support?
What you will learn
- Use stated differences to explain net gas transfer
- Distinguish transport evidence from rates and blood mixing
Original teaching model.
| Feature | Meaning |
|---|---|
| Oxygen difference | 80 − 30 = 50 arbitrary units |
| Oxygen direction | Toward fetal side |
| Carbon dioxide difference | 55 − 20 = 35 arbitrary units |
| Carbon dioxide direction | Toward maternal side |
Use stated differences to explain net gas transfer
Net exchange depends on differences between the supplied and receiving sides of an interface. For a simplified gas model, a higher oxygen level on the maternal side than the fetal side supports net oxygen diffusion toward fetal blood. A higher carbon dioxide level on the fetal side supports net transfer toward maternal blood. Real gas physiology is more detailed; the model supplies the conditions used here.
Dissolved nutrients also cross the placental interface by appropriate transport processes. Do not claim that every nutrient crosses only by simple diffusion or that the placenta creates glucose from nothing. The requirement is to explain supply to the fetus and waste transfer toward the mother, with the cord carrying fetal blood between the exchange interface and fetal cells.
Distinguish transport evidence from rates and blood mixing
An invented table gives maternal-side oxygen 80 and fetal-side oxygen 30 in arbitrary comparable units, and carbon dioxide 20 versus 55. The differences are 50 units toward the fetal side for oxygen and 35 units toward the maternal side for carbon dioxide in the stated simplified model. These are level differences, not flow rates, clinical measurements or proof that one transfer is 50/35 times faster.
A direction arrow alone does not give a transfer rate, and a numeric difference alone does not reveal area, thickness, blood flow or time. The two blood systems can remain separate while gas exchange occurs. State the net directions supported, preserve the invented units and limits, and do not convert a classroom table into an assessment of any pregnancy’s health.
Sources for this lesson
Pause and explain
What is missing for an actual transfer rate?
Worked example
In the invented gas model, fetal carbon dioxide is 55 and maternal is 20. Explain net direction.
Show the worked solution
- Compare the stipulated levels.
- Use the larger-to-smaller gradient in this model.
Answer Net carbon dioxide transfer is toward the maternal side; the stated difference is 35 arbitrary units, not a measured rate.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Use invented oxygen levels maternal 80 and fetal 30: find the difference and net direction.
Give me a hint
Subtract comparable units and retain their label.
Compare my reasoning
- 80 − 30 gives difference.
- Use the higher maternal level for net diffusion direction.
50 arbitrary units; net oxygen transfer toward the fetal side in the stated model.
Look for these in your work
- Calculate the difference
- Give the direction and model units
Use invented carbon dioxide levels fetal 55 and maternal 20: find the difference and net direction.
Give me a hint
This higher level is on the opposite side.
Compare my reasoning
- 55 − 20 gives difference.
- Use higher fetal level for net transfer direction.
35 arbitrary units; net carbon dioxide transfer toward the maternal side in the stated model.
Look for these in your work
- Calculate the difference
- Give the correct opposite direction
Do differences of 50 oxygen units and 35 carbon dioxide units prove oxygen transfers faster or directly measure pregnancy health? Explain.
Give me a hint
Gradient values are not amounts per time or clinical thresholds.
Compare my reasoning
- Identify missing rate measurements.
- Keep the invented-model boundary.
No. Transfer rates need amount and time plus relevant conditions; these invented gas levels neither compare actual rates nor assess pregnancy health.
Look for these in your work
- Reject a rate inference
- Reject a clinical inference
Common mistakes
- Invented levels are not clinical thresholds.
- A difference in levels is not a transfer rate.
What can you explain now?
What is missing for an actual transfer rate?
Compare with the explanation
An amount transferred and a time interval
A level difference alone is not amount per time.
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Make it stick
Explain use stated differences to explain net gas transfer and distinguish transport evidence from rates and blood mixing. Use the diagram key and check your reasoning before opening the solutions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.