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[Physiology](https://durgical.com/en/physiology/) · [Respiratory](https://durgical.com/en/physiology/#s-resp)

# CO₂ transport and the Haldane effect

- Mechanism
- Exam priority: high

CO₂ travels in blood mostly as bicarbonate. When hemoglobin gives up O₂, it picks up more CO₂ and acid.

Updated 2026-11-01

Also called: Chloride shift, Haldane effect, Carbonic anhydrase

**See it in 3D:** [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/)

## Key points

- Forms in arterial blood: bicarbonate ≈ 70%, carbamino ≈ 20–23%, dissolved ≈ 7–10%.
- Carbonic anhydrase (red cell): CO₂ + H₂O ⇄ H₂CO₃ ⇄ H⁺ + HCO₃⁻.
- Chloride shift: HCO₃⁻ leaves the red cell in exchange for Cl⁻ (band 3 exchanger).
- Haldane effect: oxygenating hemoglobin in the lung releases CO₂; deoxygenating it in tissues takes CO₂ up.
- Bohr = CO₂ changes O₂ binding; Haldane = O₂ changes CO₂ binding.

## What it is

CO₂ made by the tissues travels in three forms: as bicarbonate (most), bound to hemoglobin as carbamino compounds, and dissolved. Red cell carbonic anhydrase speeds the conversion of CO₂ to bicarbonate and hydrogen ions. The Haldane effect says deoxygenated hemoglobin carries more CO₂ and buffers more H⁺; in the lung the reverse happens.

## How it works, step by step

- In the tissue, CO₂ diffuses into plasma and into the red cell.
- Carbonic anhydrase rapidly turns it into carbonic acid, which splits into H⁺ and HCO₃⁻.
- Deoxyhemoglobin buffers the H⁺ and also forms carbamino more readily (Haldane effect).
- HCO₃⁻ moves out to plasma in exchange for Cl⁻ moving in (chloride shift).
- In the lung, hemoglobin is oxygenated, releases H⁺ and loses affinity for CO₂.
- The freed H⁺ combines with HCO₃⁻, which re-enters the red cell, and CO₂ is re-formed.
- CO₂ diffuses into the alveolus and is removed by ventilation.

### Oxygen–hemoglobin dissociation curve

**P50** 38.5 mmHg **Arterial blood** 93 % **Mixed venous blood** 53 % **O₂ given up to the tissues** 40 %

Right shift (P50 38.5 mmHg): Hb has less affinity and gives up more O₂ in the tissues. Acid, CO₂, heat and 2,3-BPG cause it.

- Current curve
- Normal curve (pH 7.40 · 37 °C)
- Arterial blood (PO₂ 100)
- Mixed venous blood (PO₂ 40)
- O₂ given up to the tissues

More CO₂ and lower pH in active tissue: the curve shifts right.

## Equations

**Henderson–Hasselbalch (bicarbonate)**pH = 6.1 + log (HCO3- / (0.03 × PCO2))

**6.1**
pKa of the carbonic acid/bicarbonate system

**0.03**
CO₂ solubility, mmol/L per mmHg

If PCO₂ rises with no change in bicarbonate, pH falls.

## Reference values

| Parameter | Value | Note |
| --- | --- | --- |
| PaCO₂ (sea level) | 35–45 mmHg | Bogotá ≈ 30–35 mmHg |
| Mixed venous PvCO₂ | ≈ 45–46 mmHg | |
| Arterial HCO₃⁻ | 22–26 mEq/L | somewhat lower at altitude |
| CO₂ solubility | ≈ 20 times that of O₂ | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Forms of CO₂ transport

| Form | Share (arterial) | Detail |
| --- | --- | --- |
| Bicarbonate | ≈ 70% | Made in the red cell, carried in plasma |
| Carbamino (mostly Hb) | ≈ 20–23% | Bound to globin amino groups; more on deoxygenated Hb |
| Dissolved | ≈ 7–10% | The fraction that sets the measured PCO₂ |

## Clinical notes

- COPD on high-flow O₂: PaCO₂ can rise, partly through the Haldane effect and partly through worsening V/Q.
- Acute respiratory acidosis: PaCO₂ rises and pH falls before the kidney can retain bicarbonate.
- Carbonic anhydrase inhibitors lower plasma bicarbonate and cause a mild metabolic acidosis.

## Memory aid

Haldane is about the 'Hold': when hemoglobin fills with O₂ it loses its hold on CO₂.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does venous blood carry more CO₂ than arterial blood at the same PCO₂?

The Haldane effect: deoxyhemoglobin buffers more H⁺ and forms more carbamino, so it accepts more CO₂.

## Which ion enters the red cell in tissue capillaries, and why?

Chloride: it comes in as bicarbonate goes out, keeping the cell electrically neutral (band 3 exchanger).

## Why is venous hematocrit slightly higher than arterial?

As Cl⁻ enters and more solute forms inside the red cell, water follows by osmosis and the cell swells slightly.

## Which enzyme limits bicarbonate formation in plasma, and why is it slow there?

Carbonic anhydrase: plasma has almost none, so the conversion happens inside the red cell.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/co-transport-and-the-haldane-effect/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Pulmonary arteries](https://durgical.com/en/anatomy/right-pulmonary-artery/)

## Related topics

- [Oxygen transport and the dissociation curve](https://durgical.com/en/physiology/oxygen-transport-and-the-dissociation-curve/)
- [Dead space and alveolar ventilation](https://durgical.com/en/physiology/dead-space-and-alveolar-ventilation/)
- [Respiratory acidosis and alkalosis](https://durgical.com/en/physiology/respiratory-acidosis-and-alkalosis/)
- [Control of breathing, chemoreceptors and lung reflexes](https://durgical.com/en/physiology/control-of-breathing-chemoreceptors-and-lung-reflexes/)
- [Reading an arterial blood gas step by step](https://durgical.com/en/physiology/reading-an-arterial-blood-gas-step-by-step/)

## Related conditions

- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)
- [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/)
- [Respiratory alkalosis](https://durgical.com/en/conditions/respiratory-alkalosis/)

## In Biochemistry

- [Bicarbonate–CO₂ buffer system](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Acid–base compensation rules](https://durgical.com/en/biochemistry/topics/acid-base-compensation-rules/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [22.5 Transport of Gases](https://openstax.org/books/anatomy-and-physiology-2e/pages/22-5-transport-of-gases) (OpenStax (Rice University), 2022); [Respiratory acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000092.htm) (MedlinePlus, US National Library of Medicine, 2024); [Chronic Obstructive Pulmonary Disease (COPD)](https://www.ncbi.nlm.nih.gov/books/NBK559281/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 41). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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