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# The CO₂ dissociation curve and the respiratory exchange ratio

- Concept
- For completeness

Blood carries CO₂ almost in proportion to its pressure. The ratio of CO₂ given off to O₂ taken in tells you which fuel the body is burning.

Updated Nov 13, 2026

Also called: CO₂ content curve, Respiratory quotient, R value

## Key points

- The CO₂ curve has no plateau and is steep: going from PCO₂ 40 to 45 mmHg adds about 4 mL CO₂/dL.
- Deoxygenated blood holds more CO₂ at the same PCO₂ (Haldane effect): the venous curve sits above the arterial one.
- Arterial blood (PCO₂ 40) carries ≈ 48 mL CO₂/dL; venous (PCO₂ 45) ≈ 52 mL/dL, mostly as bicarbonate.
- Because of buffering and the Haldane effect, venous pH is only ≈ 0.03–0.04 lower than arterial pH.
- R ≈ 1.0 for carbohydrate, ≈ 0.7 for fat, ≈ 0.8 for protein; R above 1 occurs transiently with hyperventilation or intense exercise.

## What it is

The CO₂ dissociation curve plots total CO₂ content of blood against PCO₂. Unlike the O₂ curve, it is nearly straight over the physiological range and much steeper, so a small PCO₂ change moves a large volume of CO₂. The respiratory exchange ratio (R) is CO₂ output divided by O₂ uptake; at steady state it equals the metabolic respiratory quotient of the fuel mix, about 0.8 on a mixed diet.

## How it works, step by step

- In tissues, CO₂ enters red cells and carbonic anhydrase turns it into H⁺ and HCO₃⁻.
- HCO₃⁻ leaves the cell in exchange for Cl⁻ (chloride shift); H⁺ is buffered by deoxyhemoglobin.
- Deoxyhemoglobin binds H⁺ and CO₂ (carbamino) better than oxyhemoglobin, so more CO₂ loads as O₂ unloads.
- This buffering keeps the pH change small even though about 4 mL CO₂/dL is added.
- In the lungs the steps reverse: O₂ binding releases H⁺ and CO₂ from hemoglobin and CO₂ is exhaled.
- The amount exhaled versus O₂ taken up depends on the fuel oxidized: carbohydrate gives one CO₂ per O₂, fat less.

## Equations

**Respiratory exchange ratio**R = VCO2 / VO2

**VCO 2**
CO₂ output, mL/min

**VO 2**
O₂ uptake, mL/min

Rest: 200 / 250 = 0.8; used in the alveolar gas equation.

**Glucose oxidation**C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

**RQ**
6/6 = 1.0

Fat (e.g. palmitate) uses more O₂ per CO₂: RQ ≈ 0.7.

## Reference values (healthy adult at rest, approximate)

| Parameter | Value | Note |
| --- | --- | --- |
| Arterial CO₂ content (PCO₂ 40 mmHg) | ≈ 48 mL/dL | |
| Venous CO₂ content (PCO₂ 45 mmHg) | ≈ 52 mL/dL | |
| CO₂ unloaded in lungs | ≈ 4 mL/dL | ≈ 200 mL/min |
| Arterial vs venous pH | ≈ 7.41 vs 7.37 | |
| R on a mixed diet | ≈ 0.8 | |

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

## Respiratory quotient by fuel

| Fuel | RQ | Why |
| --- | --- | --- |
| Carbohydrate | 1.0 | Already contains enough O₂ per carbon |
| Fat | ≈ 0.7 | Little O₂ in the molecule; more O₂ needed per CO₂ |
| Protein | ≈ 0.8 | Intermediate |
| Net lipogenesis (overfeeding) | > 1 | Carbohydrate converted to fat releases CO₂ |

## Clinical notes

- Heavy carbohydrate feeding in a patient with limited ventilation raises CO₂ output and can worsen hypercapnia.
- During intense exercise, bicarbonate buffers lactic acid and releases extra CO₂, so R rises above 1.
- Ketosis and starvation, where fat is the main fuel, push R toward 0.7.

## Memory aid

Sugar is fair (1 for 1); fat is greedy for oxygen (0.7).

## Test yourself

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

## How does the shape of the CO₂ dissociation curve differ from the O₂ curve?

It is nearly linear and much steeper over the physiological range, with no plateau, so CO₂ content keeps rising with PCO₂.

## Why is venous pH only slightly lower than arterial pH although CO₂ is added?

Most added CO₂ becomes bicarbonate, and the H⁺ is buffered by deoxyhemoglobin, which is a better buffer than oxyhemoglobin.

## A person's VO₂ is 300 mL/min and VCO₂ 210 mL/min. What is R and what fuel dominates?

R = 210/300 = 0.7, suggesting fat is the main fuel (as in fasting or ketosis).

## How does the Haldane effect appear on the CO₂ dissociation curve?

The curve for deoxygenated blood lies above the curve for oxygenated blood: at the same PCO₂, deoxygenated blood carries more CO₂.

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## Connections

## Related topics 4

- [CO₂ transport and the Haldane effect](https://durgical.com/en/physiology/co-transport-and-the-haldane-effect/)
- [The alveolar gas equation and the A–a gradient](https://durgical.com/en/physiology/the-alveolar-gas-equation-and-the-a-a-gradient/)
- [Energy metabolism and basal metabolic rate](https://durgical.com/en/physiology/energy-metabolism-and-basal-metabolic-rate/)
- [Buffers and the Henderson–Hasselbalch equation](https://durgical.com/en/physiology/buffers-and-the-henderson-hasselbalch-equation/)

## Related conditions 1

- [Respiratory acidosis](https://durgical.com/en/conditions/respiratory-acidosis/)

## In Biochemistry 2

- [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/)

[← 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: [Respiratory acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000092.htm) (MedlinePlus, US National Library of Medicine, 2025); [Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test](https://medlineplus.gov/lab-tests/arterial-blood-gas-abg-test/) (MedlinePlus, US National Library of Medicine, 2025).

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Representative model, for educational purposes only.

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