Explain the chloride shift
Describe the exchange of hydrogencarbonate and chloride ions and explain how it supports carbon-dioxide transport.
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that mass transport moves substances between exchange surfaces and cells, and that diffusion occurs down a concentration or partial-pressure gradient.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
During the chloride shift in tissues, which ion enters the red blood cell as HCO₃⁻ leaves?
What you will learn
- Describe the chloride shift in tissue and lung capillaries
- Explain how the shift supports ionic balance and CO₂ transport
Build the explanation
In respiring tissues, carbon dioxide enters red blood cells and is converted to hydrogencarbonate ions. Hydrogencarbonate ions diffuse out into the plasma; chloride ions diffuse into the red blood cell to maintain electrical balance. This exchange is called the chloride shift. In the lungs the process reverses: hydrogencarbonate enters red blood cells as chloride leaves, allowing hydrogencarbonate to be converted back to carbon dioxide for exhalation.
Apply it carefully
The chloride shift is an ion exchange across the red-cell membrane, not a movement of chloride into the lungs as a gas. It helps transport carbon dioxide in the plasma mainly as hydrogencarbonate while maintaining charge balance in red cells. Track the location and direction: tissues, HCO₃⁻ out/Cl⁻ in; lungs, HCO₃⁻ in/Cl⁻ out.
Pause and explain
Which direction does the chloride shift reverse in the lungs?
Worked example
At a tissue capillary, hydrogencarbonate ions leave a red cell. Which ion moves in to balance charge?
Show the worked solution
- Hydrogencarbonate is negatively charged.
- Chloride enters as hydrogencarbonate leaves.
Answer Chloride ions enter the red blood cell.
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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.
Describe the chloride shift in a tissue capillary.
Give me a hint
Start with the red-cell product formed from CO₂.
Compare my reasoning
- CO₂ is converted to HCO₃⁻ in the red cell.
- HCO₃⁻ moves out and Cl⁻ moves in.
In tissue capillaries, carbon dioxide is converted in red cells to hydrogencarbonate. HCO₃⁻ leaves the cell and chloride enters in exchange, helping maintain ionic balance as carbon dioxide is carried in plasma.
Look for these in your work
- Tissue location specified
- HCO₃⁻ outward
- Cl⁻ inward and purpose stated
Why is the chloride shift useful for transport rather than simply storing chloride?
Give me a hint
Link the exchange to the form in which much CO₂ travels.
Compare my reasoning
- Hydrogencarbonate is the main plasma form for carrying CO₂.
- The chloride exchange allows HCO₃⁻ movement while maintaining charge balance.
The shift allows hydrogencarbonate, formed from carbon dioxide in red cells, to move into plasma for transport while chloride moves oppositely to maintain electrical balance across the red-cell membrane.
Look for these in your work
- CO₂ carriage as HCO₃⁻ included
- Charge balance purpose included
Trace the reverse chloride shift in the lungs and link it to exhalation.
Give me a hint
Reverse both ion directions before describing CO₂ formation.
Compare my reasoning
- HCO₃⁻ enters the red cell as Cl⁻ leaves.
- HCO₃⁻ is converted back through carbonic acid to CO₂ and water; CO₂ diffuses to the alveoli.
In lung capillaries, hydrogencarbonate re-enters red blood cells as chloride leaves. Hydrogencarbonate is converted through carbonic acid to carbon dioxide and water; carbon dioxide diffuses into the alveoli and is exhaled.
Look for these in your work
- Ion directions reversed
- CO₂ regeneration explained
- Diffusion to alveoli stated
Common mistakes
- Reversing tissue ion directions
- Treating chloride as the gas transported to the lungs
- Omitting ionic balance and the role of HCO₃⁻ in plasma
What can you explain now?
During the chloride shift in tissues, which ion enters the red blood cell as HCO₃⁻ leaves?
Compare with the explanation
Chloride
The exchange helps maintain electrical balance.
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Make it stick
What crosses each way during the chloride shift in tissues and lungs?
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