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Physiology · Respiratory

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

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

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

Oxygen–hemoglobin dissociation curve

Oxygen–hemoglobin dissociation curve: P50 26.8 mmHg 0204060801000255075100PO₂ (mmHg)Hb saturation (%)P50 2775 %97 %

P50 26.8 mmHgArterial 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

Oxygen–hemoglobin dissociation curve: P50 19.0 mmHg 0204060801000255075100PO₂ (mmHg)Hb saturation (%)P50 1988 %99 %

P50 19.0 mmHgArterial 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 contentCaO2 = 1.34 × Hb × SaO2 + 0.003 × PaO2
1.34
mL O₂ per gram of saturated Hb
Hb
hemoglobin in g/dL
SaO2
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)

ParameterValueNote
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 Bogotásomewhat higher than at sea leveladaptation to altitude

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

What shifts the curve

FactorRight shift (releases O₂)Left shift (holds O₂)
pHAcidosisAlkalosis
PCO₂HighLow
TemperatureFever, exerciseHypothermia
2,3-BPGHigh (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.

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.

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See it in 3D

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

In Biochemistry

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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 (OpenStax (Rice University), 2022); Carbon monoxide poisoning: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Methemoglobinemia: MedlinePlus Medical Encyclopedia (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.