8.2.28 · Transport in Mammals

The Chloride Shift — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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8.2.2 The Chloride Shift — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how hydrogencarbonate ions are formed in red cells and how the chloride shift keeps the cell electrically balanced.Entry — carbon dioxide from respiring tissues diffuses into plasma and…

Carbonic anhydrase
Share of CO2
Hydrogen ions
Osmotic effect
Concentration gradient
Haemoglobin

Part 1 of 8Carbon dioxide in the red cell

Entry

Entry carbon dioxide from respiring tissues diffuses into plasma and then into red blood cells down a concentration gradient.

Carbonic anhydrase

Carbonic anhydrase it speeds up CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^-.
Without the enzyme the reaction is slow.
With it, CO₂ is converted efficiently, keeping its concentration inside the cell low.
This maintains the gradient for further diffusion into the cell.

Share of CO₂

Share of CO₂ about 85 per cent is carried as hydrogencarbonate.

Part 2 of 8What happens to the products

Hydrogen ions

Hydrogen ions they would make the cell acidic, so haemoglobin takes them up as a buffer, forming haemoglobinic acid (HHb).
This also promotes release of oxygen from oxyhaemoglobin (the Bohr effect).

Hydrogencarbonate ions

Hydrogencarbonate ions they diffuse out of the red cell into plasma through a transport protein in the cell surface membrane.
They move down the concentration gradient that builds up inside the cell.

Part 3 of 8The chloride shift

Problem

Problem if negative HCO₃^- ions leave alone, the inside of the cell would become positively charged from the build-up of H⁺.

Exchange

Exchange chloride ions (Cl^-) from the plasma enter the cell through the same carrier protein.

Name

Name this is the chloride shift, and it keeps the electrical balance of the cell.
- (Hamburger shift)

Osmotic effect

Osmotic effect extra ions lower the water potential inside, so water enters and venous red cells are slightly larger than arterial ones.

Part 4 of 8Sequence in tissue capillaries

Step	Where	What happens
1	Tissue capillary	CO₂ diffuses from tissue to plasma to red cell
2	Red cell	Carbonic anhydrase converts CO₂ and H₂O to H⁺ and HCO₃^-
3	Red cell	H⁺ binds to haemoglobin and O₂ is released
4	Membrane	HCO₃^- leaves and Cl^- enters (chloride shift)

Part 5 of 8Reversal in the lungs

Concentration gradient

Concentration gradient plasma CO₂ is lower than in the tissue, so HCO₃^- enters the red cell and chloride ions leave.

Haemoglobin

Haemoglobin it takes up oxygen and releases H⁺.

Release of CO₂

H⁺ combines with HCO₃^- to form carbonic acid, which carbonic anhydrase splits into CO₂ and water.

Exhalation

CO₂ diffuses from the red cell through the alveolar wall into the air to be breathed out.

Cell size

Cell size chloride leaves again and the cell returns to its previous size.

Part 6 of 8Why it matters

pH control

pH control the chloride shift and haemoglobin buffering let blood carry large amounts of CO₂ with little pH change.
Venous blood pH is only about 0.03 units lower than arterial blood.
A change of more than about 0.4 units is dangerous to enzymes.

Link to oxygen

Link to oxygen where CO₂ is high, as in active muscle, more H⁺ forms, so more oxygen is released.

Site of reaction

Site of reaction plasma has very little carbonic anhydrase, so conversion happens mainly in red cells.

Plasma buffers

Plasma buffers plasma proteins also mop up H⁺, giving blood a large buffering capacity.

Control of breathing

Control of breathing if breathing is inadequate, CO₂ accumulates and blood pH falls, and the medulla responds to the fall in pH.
- (respiratory acidosis)

Part 7 of 8More from the specification

Part 8 of 8More on this topic

Poor solubility

Poor solubility plasma alone would carry only about 3 cm³ of oxygen per litre, but haemoglobin carries about 200 cm³ per litre.

Forgetting to quote units (kPa) when reading pO_{2} from a graph.

Term	Meaning
SAN	Sinoatrial node, the heart's pacemaker.
AVN	Atrioventricular node, which delays the impulse.
Carbonic anhydrase	Enzyme converting CO_{2} and water to carbonic acid in red cells.
Chloride shift	Cl^{-} entering red cells to balance HCO_{3}^{-} leaving.
Bohr shift	Right shift of the dissociation curve when pCO_{2} is high.
Capillary	A vessel with a one-cell-thick wall where exchange occurs.

Key terms

Term	Meaning
Chloride shift	Movement of chloride ions into red cells in exchange for hydrogencarbonate ions leaving.
Carbonic anhydrase	Enzyme that catalyses the reaction of CO_{2} with water to form carbonic acid.
Hydrogencarbonate ion	HCO_{3}^{-}, the main form in which CO_{2} is carried in plasma.
Haemoglobinic acid	Haemoglobin combined with H^{+}, acting as a buffer.
Buffer	A substance that resists changes in pH.
Respiratory acidosis	A fall in blood pH caused by CO_{2} accumulating.

Avoid these mistakes

Confusing the chloride shift with the Bohr shift — the Bohr shift changes oxygen release; the chloride shift is Cl^{-} entering red cells.

Saying chloride ions leave the red cell in the tissues — they enter as HCO_{3}^{-} leaves.

Saying carbonic anhydrase works mainly in plasma — it is in red cells.

Forgetting that the carrier protein exchanges the two ions to keep the cell neutral.

Last updated: 18 October 2026