# Carbon Dioxide Transport and the Chloride Shift

> Carbon dioxide transport and chloride shift in MBBS Physiology: bicarbonate, carbamino and dissolved fractions, carbonic anhydrase, Haldane effect.

- Canonical URL: https://prepelephant.com/topics/mbbs/physiology/carbon-dioxide-transport-and-chloride-shift
- Exam / course: MBBS · Subject: Physiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Carbon Dioxide Transport and the Chloride Shift", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/carbon-dioxide-transport-and-chloride-shift

## Direct answer

Carbon dioxide returns to the lungs mainly as bicarbonate — roughly 70 per cent of the total load — with about 20-30 per cent travelling as carbamino compounds (mostly on haemoglobin) and only 5-10 per cent dissolved. Arterial blood carries 48-52 mL/dL and venous blood 52-56 mL/dL, so each decilitre releases about 4 mL in the pulmonary capillary. Inside the red cell, carbonic anhydrase hydrates CO2 several thousand times faster than the uncatalysed reaction, hydrogen ions are buffered by haemoglobin (deoxyhaemoglobin is the better buffer — the Haldane effect), and bicarbonate exits in exchange for chloride, the Hamburger phenomenon or chloride shift, which reverses direction in the lungs.

## What you must remember

- **Classic split:** bicarbonate 70 per cent, carbamino 20-30 per cent (mostly carbaminohaemoglobin on terminal amino groups), dissolved 5-10 per cent — dissolved CO2 matters more for its role in ventilation drive than for bulk transport.
- **Enzyme:** carbonic anhydrase, a zinc metalloenzyme packed in the red cell; acetazolamide inhibits it, producing bicarbonaturia, alkaline urine and a mild metabolic acidosis used therapeutically in glaucoma and high-altitude disorders.
- **Chloride shift:** bicarbonate leaves the red cell via the band 3 (AE1) exchanger for chloride; venous red cells gain chloride and water, so venous haematocrit runs about 3 per cent higher than arterial — a quotable viva fact.
- **Haldane effect:** deoxygenation of haemoglobin increases CO2 carriage (better H+ buffering and better carbamino formation); it accounts for about two-thirds of the CO2 released in the lungs, quantitatively more important for CO2 transport than the Bohr effect is for oxygen.
- **Curves differ:** the CO2 dissociation curve is far steeper and nearly linear over the physiological range, unlike the sigmoid oxygen curve.
- **Alveolar equation arithmetic:** PaCO2 = 0.863 × VCO2 / VA; with a VCO2 of 200 mL/min and alveolar ventilation of 4.3 L/min, PaCO2 lands at 40 mmHg — hypoventilation raises PaCO2 in direct proportion.
- **Clinical ceiling:** CO2 narcosis with confusion and flapping tremor typically appears beyond a PaCO2 of 70-80 mmHg.

## Walking one molecule from mitochondrion to alveolus

Follow a CO2 molecule produced in a leg muscle. It diffuses into plasma, but almost none stays there — plasma has no carbonic anhydrase — so it enters the red cell, is hydrated within milliseconds to carbonic acid, and dissociates. The hydrogen ion is mopped up by histidines on deoxyhaemoglobin, which is precisely why venous blood tolerates a PCO2 of 45-46 mmHg at a pH of only 7.37. Bicarbonate accumulates and exits for chloride; chloride enters, water follows osmotically, and the cell swells slightly. At the pulmonary capillary the sequence runs backwards: oxygenation of haemoglobin releases hydrogen ions (Haldane effect), bicarbonate re-enters in exchange for chloride, carbonic anhydrase regenerates CO2, and it diffuses out down its gradient. The system's elegance is that the oxygen and carbon dioxide transports are coupled in both directions — deoxygenation aids loading, oxygenation aids unloading.

Now apply the arithmetic clinically. Hyperventilation to an alveolar ventilation of 8 L/min drops PaCO2 toward 20 mmHg; hypoventilation at 2 L/min under opioids doubles it toward 80. Because the alveolar gas equation ties PaCO2 inversely to ventilation, PaCO2 is the single best index of alveolar ventilation — the reason an arterial blood gas reports it first.

## Where students slip

Bohr and Haldane get transposed under exam pressure. Bohr: CO2 and H+ shift the oxygen curve (effect of carbon dioxide on oxygen transport). Haldane: oxygenation displaces CO2 (effect of oxygen on carbon dioxide transport). A workable memory is that each effect is named for what it does to the other gas. The second slip is assuming the chloride shift is a one-way venous event; it reverses completely in the lungs, which is why red cell chloride, not plasma chloride, fluctuates through the circuit. Third, do not attribute the bulk of CO2 carriage to dissolved gas — dissolved CO2 is the smallest fraction, though it is the fraction the medullary chemoreceptors actually sense.

## Frequently asked questions

### In what three forms is carbon dioxide transported in blood?

As bicarbonate (about 70 per cent), carbamino compounds mainly on haemoglobin (20-30 per cent) and simple solution (5-10 per cent).

### Which enzyme accelerates CO2 transport and where does it sit?

Carbonic anhydrase, a zinc-containing enzyme concentrated in red blood cells (not plasma), catalysing the hydration of CO2 about 5000 times faster than it occurs spontaneously.

### Why does acetazolamide cause metabolic acidosis?

By inhibiting renal (and erythrocyte) carbonic anhydrase it blocks bicarbonate reabsorption, so bicarbonate is wasted in urine, which turns alkaline while plasma develops a hyperchloraemic metabolic acidosis.

### What is the difference between the Bohr and Haldane effects?

The Bohr effect is the rightward shift of the oxygen curve caused by raised CO2 and H+; the Haldane effect is the increased CO2 carriage by deoxygenated haemoglobin — each gas influencing the other's transport.

### Why is the chloride shift called the Hamburger phenomenon?

Hartog Hamburger described it: red-cell bicarbonate exchanges with plasma chloride across the band 3 anion exchanger, reversing as blood oxygenates in the lungs.