Oxygen–Haemoglobin Dissociation Curve: Affinity, Saturation and Content

A free revision guide for Indian UG and PG medical students. Learn to read the oxygen–haemoglobin curve without confusing binding affinity with the amount of oxygen carried.

Learning objectives

  • Distinguish oxygen partial pressure, saturation and content.
  • Explain cooperative binding and the curve's changing slope.
  • Predict the effect of a shift at a fixed oxygen pressure.
  • Compare oxygen content when haemoglobin concentration changes.

1. Name the axes and quantities

The usual dissociation curve plots oxygen partial pressure (PO2, mmHg) horizontally and haemoglobin oxygen saturation (%) vertically. Saturation describes the proportion of available binding sites occupied by oxygen. Partial pressure reflects dissolved oxygen; it is not a measure of all oxygen in blood. Content is the volume of oxygen carried per volume of blood, commonly mL O2/dL.

2. Why an S-shaped curve helps

Oxygen binding changes haemoglobin's conformation and favours further binding: cooperativity produces the sigmoid relationship. On the plateau, a given pressure change produces a relatively small saturation change. On the steeper region, a comparable pressure decrease releases a larger fraction of bound oxygen. This supports loading and unloading in different environments, but does not guarantee adequate tissue oxygenation.

3. Read a shift at the same PO2

A right shift means lower affinity: at the same PO2, saturation is lower, favouring unloading. A left shift means higher affinity and greater saturation at that pressure. Increased temperature, increased CO2 and decreased pH favour a right shift; opposite changes favour a left shift. The pH/CO2 relationship to oxygen affinity is the Bohr effect. Always compare at a fixed pressure before explaining the direction.

A shift is different from moving along an unchanged curve. If PO2 falls while binding conditions stay the same, the operating point moves along the curve. If binding conditions change, the relationship itself shifts. Neither observation alone specifies tissue blood flow.

4. Worked comparison: the same saturation, different content

Use the approximate teaching equation CaO2 = 1.34 × Hb × SaO2 + 0.003 × PaO2. Hb is in g/dL; SaO2 is a fraction, not a percentage number; PaO2 is in mmHg. The constants have units mL O2/g Hb and mL O2/(dL·mmHg), respectively. The result is mL O2/dL blood.

Assume ordinary functional adult haemoglobin, no significant abnormal haemoglobin species, SaO2 = 0.98 and PaO2 = 100 mmHg in both invented samples. These are teaching inputs, not a diagnostic rule.

  • Sample A, Hb 15 g/dL: 1.34 × 15 × 0.98 + 0.003 × 100 = 19.698 + 0.300 = 20.00 mL O2/dL (rounded).
  • Sample B, Hb 8 g/dL: 1.34 × 8 × 0.98 + 0.300 = 10.5056 + 0.300 = 10.81 mL O2/dL.

The second sample carries about 54% as much oxygen per dL despite identical saturation and pressure. The dissolved component is unchanged; the bound component falls with Hb concentration. This calculation isolates concentration, rather than asserting that anaemia never alters affinity in real physiology. Whole-body oxygen delivery additionally depends on cardiac output. Constants vary slightly between teaching conventions; state the values used.

5. Common misconceptions

  • “98% saturation means oxygen content is adequate.” It does not specify haemoglobin concentration or blood flow.
  • “Right shift means more oxygen bound at the same pressure.” It means less affinity and lower saturation at that pressure.
  • “Less haemoglobin automatically shifts the saturation curve down.” Concentration reduces content; do not confuse a content graph with a saturation graph.
  • “A left shift is always beneficial.” Tighter binding can favour loading yet oppose unloading.

6. Answered revision questions

  1. What makes the curve sigmoid? Cooperative oxygen binding.
  2. PO2 decreases without a change in affinity. Shift or movement? Movement along the existing curve.
  3. At fixed PO2, lower pH has which usual effect? Lower affinity and lower saturation: a right shift.
  4. Should 98 or 0.98 enter this content equation? 0.98; using 98 exaggerates the bound term one hundredfold.
  5. Why can identical saturation coexist with different content? The number of available binding sites per dL depends on Hb concentration.
  6. Does oxygen content alone give oxygen delivery per minute? No. Delivery also requires blood flow and consistent volume units.

Continue your study

For focused reading, inspect The Oxygen-Haemoglobin Dissociation Curve chapter. Compare the broader Respiratory Physiology book if you need a system resource. Check each paid listing for scope and current price.

Explore the teaching style with the free Homeostasis sample, which covers a different foundational topic. Find more concepts in the free guide hub.

References

Original educational revision notes; use alongside your curriculum. These examples are not individual medical advice.