Explain osmotic diarrhoea dehydration and ion loss
Follow chloride-driven water movement into the gut through loss from the body.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall osmosis and the preceding toxin-to-chloride sequence.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which potential is higher?
What you will learn
- Osmosis links solute and water movement
- Follow loss beyond the lumen
WHO background: cholera cause and transmission. Original classroom questions; this model does not diagnose infection.
Osmosis links solute and water movement
In the required cholera model, increased chloride secretion raises the dissolved solute concentration in the small-intestinal lumen and lowers its water potential relative to the body-fluid side. Water moves across partially permeable intestinal membranes into the gut by osmosis. The water movement follows the potential difference; the toxin does not physically push each water molecule.
For an invented potential comparison, the body-fluid side is −300 kPa and the lumen is −900 kPa. Water moves from the higher potential of −300 towards the lower potential of −900. These values are teaching assumptions rather than measurements for a patient; the negative signs must be compared correctly.
Follow loss beyond the lumen
The increased water in the gut contributes to diarrhoea. Water and ions are lost from the body in the watery output, leading to dehydration and loss of ions from the blood in the syllabus explanation. Naming only water movement into the gut leaves out the required consequences for the body.
Distinguish movement into the lumen from final loss out of the body. In an invented balance, 200 mL of body water enters the gut and 150 mL returns before output; the net retained in the gut from that exchange is 50 mL. This does not establish a complete patient fluid balance, because intake and other exchanges are not included. Real care decisions cannot be derived from these classroom figures.
Pause and explain
Which chain has the required final consequences?
Worked example
Use the teaching potentials −300 kPa body side and −900 kPa lumen to predict water direction.
Show the worked solution
- −300 is higher than −900.
- Osmosis moves water towards the lower potential.
Answer Body-fluid side → intestinal lumen.
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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.
Explain the complete cholera mechanism from bacterium to body fluid and ion loss.
Give me a hint
Include toxin, chloride, osmosis and both consequences.
Compare my reasoning
- Bacterial toxin increases chloride secretion into the small-intestinal lumen, lowering its water potential.
- Water enters by osmosis; watery diarrhoea removes water and ions.
Cholera bacterium → toxin → chloride secretion into gut → osmotic water entry → diarrhoea → dehydration and loss of ions from the blood.
Look for these in your work
- Use the correct ion and water directions
- Include diarrhoea
- Include dehydration and blood ion loss
In the invented exchange, 200 mL enters the gut from the body and 150 mL returns. Find the net transfer into the gut.
Give me a hint
Subtract the return from the entry.
Compare my reasoning
- 200 − 150 = 50.
- The result describes only this exchange.
50 mL net into the gut from the stated exchange.
Look for these in your work
- Calculate correctly
- Retain the limited model
Can that 50 mL exchange alone establish a whole-body dehydration severity or treatment plan?
Give me a hint
Other intake, losses and observations are missing.
Compare my reasoning
- It covers one invented exchange only.
- Whole-body balance and clinical evidence are not supplied.
No; one classroom exchange does not establish whole-body fluid balance, severity or a treatment plan.
Look for these in your work
- State missing scope
- Avoid clinical inference
Common mistakes
- Comparing negative potentials by magnitude only.
- Stopping the explanation before diarrhoea and blood ion loss.
What can you explain now?
Which chain has the required final consequences?
Compare with the explanation
Water enters gut → diarrhoea → dehydration and ion loss
It follows movement through loss from the body.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain the complete mechanism with arrows, including both final losses.
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.