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.
Also called: CO₂ content curve, Respiratory quotient, R value
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
- VCO2
- CO₂ output, mL/min
- VO2
- O₂ uptake, mL/min
Rest: 200 / 250 = 0.8; used in the alveolar gas equation.
- 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.
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.
Connections
In Biochemistry2
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 (MedlinePlus, US National Library of Medicine, 2025); Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.