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

# Oxygen transport and the dissociation curve

- Mechanism
- Exam priority: high

Almost all oxygen rides bound to hemoglobin. Its S-shaped curve explains why it loads in the lung and unloads in the tissues.

Updated 2026-11-01

Also called: Oxyhemoglobin dissociation curve, Bohr effect, P50

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

## Key points

- CaO₂ = 1.34 × Hb × SaO₂ + 0.003 × PaO₂; almost all of it depends on hemoglobin and saturation.
- Normal P50 ≈ 26–27 mmHg (the PO₂ at 50% saturation).
- Right shift (high P50, releases O₂): ↑H⁺, ↑PCO₂, ↑temperature, ↑2,3-BPG (Bohr effect).
- Left shift (low P50, holds O₂): HbF, CO, methemoglobin, alkalosis, hypothermia, stored blood.
- The flat part (PaO₂ > 60 mmHg) protects saturation; the steep part (20–40 mmHg) aids unloading in tissues.
- CO lowers O₂ content and also left-shifts what remains: a double hit.

## What it is

About 98% of arterial O₂ is bound to hemoglobin and only a small part is dissolved. Binding is cooperative: each O₂ that binds makes the next easier, which gives the sigmoid curve. Acid, CO₂, heat and 2,3-BPG shift the curve right and help release O₂ where the tissue needs it most.

## How it works, step by step

- In the pulmonary capillary, O₂ dissolves in plasma and enters the red cell.
- The first O₂ to bind a heme shifts hemoglobin from the T (tense) to the R (relaxed) state.
- In the R state the other hemes take up O₂ more easily: cooperative binding and an S-shaped curve.
- In active tissue, extra CO₂, H⁺ and heat stabilise the T state and hemoglobin lets O₂ go.
- Red cell 2,3-BPG binds deoxyhemoglobin and shifts the curve right; it rises at altitude and in anemia.
- HbF binds 2,3-BPG poorly, so it has higher affinity and pulls O₂ from maternal blood.

### Oxygen–hemoglobin dissociation curve

**P50** 26.8 mmHg **Arterial blood** 97 % **Mixed venous blood** 75 % **O₂ given up to the tissues** 23 %

Normal curve. Tap a point (arterial, venous or P50) or move a control.

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

Move pH, temperature or 2,3-BPG and watch the P50.

### Oxygen–hemoglobin dissociation curve

**P50** 19.0 mmHg **Arterial blood** 99 % **Mixed venous blood** 88 % **O₂ given up to the tissues** 11 %

HbF binds little 2,3-BPG: its P50 is lower, so it takes O₂ from maternal blood in the placenta.

Left shift (P50 19.0 mmHg): Hb holds O₂ more tightly and gives up less in the tissues.

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

Fetal hemoglobin: the curve sits to the left.

## Equations

**Arterial O₂ content**CaO2 = 1.34 × Hb × SaO2 + 0.003 × PaO2

**1.34**
mL O₂ per gram of saturated Hb

**Hb**
hemoglobin in g/dL

**SaO 2**
saturation as a fraction

**0.003**
mL dissolved O₂ per dL per mmHg

With Hb 15 g/dL and SaO₂ 0.98, CaO₂ ≈ 20 mL/dL.

## Reference values (sea level)

| Parameter | Value | Note |
| --- | --- | --- |
| P50 | ≈ 26–27 mmHg | |
| CaO₂ | ≈ 20 mL O₂/dL | |
| Mixed venous content | ≈ 15 mL O₂/dL | |
| Mixed venous SvO₂ | ≈ 75% | PvO₂ ≈ 40 mmHg |
| Dissolved O₂ at PaO₂ 100 mmHg | ≈ 0.3 mL/dL | |
| Hemoglobin in Bogotá | somewhat higher than at sea level | adaptation to altitude |

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

## What shifts the curve

| Factor | Right shift (releases O₂) | Left shift (holds O₂) |
| --- | --- | --- |
| pH | Acidosis | Alkalosis |
| PCO₂ | High | Low |
| Temperature | Fever, exercise | Hypothermia |
| 2,3-BPG | High (altitude, anemia) | Low (stored blood) |
| Type of hemoglobin | — | HbF, carboxyhemoglobin, methemoglobin |

## Clinical notes

- CO poisoning: carboxyhemoglobin carries no O₂ and left-shifts the curve; PaO₂ and standard pulse oximetry can read normal.
- Methemoglobinemia: ferric iron cannot bind O₂; 'chocolate-brown' cyanosis that does not improve with O₂.
- Iron deficiency anemia: low CaO₂ with normal PaO₂ and SaO₂; 2,3-BPG rises to compensate.

## Memory aid

A working muscle is 'hot, acidic and full of CO₂': those three push the curve right and hand it the O₂.

## Test yourself

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

## Why does banked blood release O₂ poorly right after transfusion?

It loses 2,3-BPG during storage; its curve shifts left and hemoglobin holds on to O₂ until 2,3-BPG is replenished.

## With Hb 10 g/dL and SaO₂ 98%, roughly what is CaO₂?

≈ 1.34 × 10 × 0.98 + 0.3 ≈ 13.4 mL/dL, against ≈ 20 mL/dL with Hb 15 g/dL.

## Why does a fall in PaO₂ from 100 to 70 mmHg barely change saturation?

That is the flat part of the curve: hemoglobin stays almost saturated (≈ 93–94%) even though PO₂ drops a lot.

## How does the Bohr effect help exercising muscle?

The CO₂ and acid made by the muscle lower local pH, shift the curve right, and hemoglobin unloads more O₂ at the same PO₂.

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

## See it in 3D

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

- [Pulmonary veins](https://durgical.com/en/anatomy/pulmonary-veins/)

## Related topics

- [CO₂ transport and the Haldane effect](https://durgical.com/en/physiology/co-transport-and-the-haldane-effect/)
- [Oxygen content, delivery and extraction](https://durgical.com/en/physiology/oxygen-content-delivery-and-extraction/)
- [Carbon monoxide, methemoglobin and cyanide](https://durgical.com/en/physiology/carbon-monoxide-methemoglobin-and-cyanide/)
- [Oxygenation: SpO₂, PaO₂, cyanosis and oxygen therapy](https://durgical.com/en/physiology/oxygenation-spo-pao-cyanosis-and-oxygen-therapy/)
- [High altitude and acclimatization](https://durgical.com/en/physiology/high-altitude-and-acclimatization/)

## Related conditions

- [Carbon monoxide poisoning](https://durgical.com/en/conditions/carbon-monoxide-poisoning/)
- [Methemoglobinemia](https://durgical.com/en/conditions/methemoglobinemia/)
- [Iron deficiency anaemia](https://durgical.com/en/conditions/iron-deficiency-anaemia/)

## In Biochemistry

- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)
- [Oxygen binding: haemoglobin versus myoglobin](https://durgical.com/en/biochemistry/topics/oxygen-binding-haemoglobin-versus-myoglobin/)
- [Glycolysis](https://durgical.com/en/biochemistry/pathways/glycolysis/)

[← 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); [Carbon monoxide poisoning: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002804.htm) (MedlinePlus, US National Library of Medicine, 2024); [Methemoglobinemia: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000562.htm) (MedlinePlus, US National Library of Medicine, 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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Representative model, for educational purposes only.

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