What is the oxygen-haemoglobin dissociation curve?
The oxygen-haemoglobin dissociation curve (ODC) plots oxygen tension (PO2, x-axis) against haemoglobin oxygen saturation (SO2, y-axis). It shows how readily haemoglobin picks up oxygen in the lungs and gives it up in the tissues.
About 98% of the oxygen in blood is bound to haemoglobin; only about 2% is dissolved in plasma (StatPearls). Each haemoglobin tetramer has four subunits, each carrying one oxygen molecule — so a fully saturated molecule carries four O2.
Why is the curve sigmoid (S-shaped)?
The sigmoid shape is due to positive cooperativity. Haemoglobin exists in two states: the T (tense, deoxygenated) state with low oxygen affinity and the R (relaxed, oxygenated) state with high affinity. Binding of the first oxygen molecule makes the remaining subunits bind oxygen more easily — this interaction between subunits is called cooperativity.
- Lower steep part — small falls in PO2 release large amounts of oxygen: the tissue (unloading) zone.
- Upper flat part (plateau) — saturation stays high even when PO2 falls a little: the lung (loading) zone, a safety margin.
- Above about 90% saturation, raising PaO2 adds little to saturation or content (Breathe 2015).
What are the key points on the curve, including P50?
P50 is the PO2 at which haemoglobin is 50% saturated. It is the standard way to describe the position of the curve. The P50 of normal adult blood is about 26–27 mmHg — Breathe gives approximately 26 mmHg and StatPearls quotes 27 mmHg. A higher P50 = right shift (lower affinity); a lower P50 = left shift (higher affinity).
| PO2 | Approximate SO2 | Meaning |
|---|---|---|
| ~100 mmHg | ~97–98% | Arterial blood — on the plateau |
| 60 mmHg | ~90% | Edge of the plateau; below this saturation falls steeply. PaO2 < 60 mmHg or SaO2 < 90% is a common definition of hypoxaemia |
| 40 mmHg | ~75% | Mixed venous blood at rest — about 75% saturated, leaving a large reserve |
| 26–27 mmHg | 50% | P50 |
What shifts the curve to the right?
A right shift means lower affinity: at any PO2, haemoglobin holds less oxygen, so unloading to the tissues is favoured and P50 rises. StatPearls lists increased PCO2, decreased pH (acidity), increased 2,3-DPG (2,3-BPG) and increased temperature as the causes — exactly the conditions in metabolically active tissue.
| Factor | Mechanism / context |
|---|---|
| ↑ CO2 | Bohr effect; CO2 also forms carbamino compounds that stabilise the T state |
| ↑ H+ (↓ pH, acidosis) | H+ binding stabilises deoxy-haemoglobin (Bohr effect) |
| ↑ 2,3-BPG | Binds the central cavity of deoxy-Hb and stabilises the T state; rises at high altitude and in chronic hypoxia |
| ↑ Temperature | Fever, exercising muscle — unloading favoured |
| Exercise | Working muscle is hot, acidic and high in CO2 — all four factors at once |
What shifts the curve to the left?
A left shift means higher affinity: haemoglobin loads oxygen easily but releases it reluctantly, so P50 falls.
| Cause | Key point |
|---|---|
| Fetal haemoglobin (HbF, α2γ2) | P50 about 19 mmHg vs 27 mmHg in adults; helps the fetus take oxygen from maternal blood |
| Carbon monoxide (COHb) | Left shift plus loss of carrying capacity |
| Methaemoglobin (Fe3+) | Allosteric change shifts the curve left; remaining Fe2+ holds oxygen more tightly |
| Alkalosis (↑ pH), ↓ PCO2 | Reverse Bohr effect |
| Hypothermia | Lower temperature → left shift |
| ↓ 2,3-BPG (e.g. stored bank blood) | 2,3-DPG depletion in stored red cells raises oxygen affinity |
What is the difference between the Bohr and Haldane effects?
| Feature | Bohr effect | Haldane effect |
|---|---|---|
| What changes | CO2 / H+ change O2 binding | O2 binding changes CO2 / H+ carriage |
| Statement | ↑ CO2 and ↓ pH lower Hb's oxygen affinity (right shift) | Deoxygenated Hb carries more CO2 (carbamino-Hb) and buffers more H+ |
| Where it matters most | Tissues — helps O2 unloading | Tissues (CO2 pick-up) and lungs (oxygenation drives CO2 off) |
| Curve affected | Oxygen dissociation curve | CO2 dissociation curve |
StatPearls defines the Bohr effect as the relationship between acidity, CO2 and haemoglobin's oxygen affinity. For the Haldane effect, it explains that deoxygenation in the periphery promotes carbaminohaemoglobin formation, binding of H+ and release of bicarbonate — the fewer oxygen molecules bound, the more H+ haemoglobin can accommodate. This buffering is why venous blood is only slightly more acidic than arterial blood.
How does the myoglobin curve differ from haemoglobin?
Myoglobin is a single-subunit oxygen-binding protein in muscle, so it does not show cooperativity. Non-cooperative binding gives a hyperbolic curve rather than a sigmoid one. Myoglobin has a very high oxygen affinity, so its curve lies far to the left of haemoglobin's — the affinity difference is why myoglobin in cardiac and skeletal muscle takes up oxygen handed over by arterial haemoglobin.
| Feature | Haemoglobin (HbA) | Myoglobin |
|---|---|---|
| Structure | Tetramer (α2β2), 4 haem groups | Monomer, 1 haem group |
| O2 molecules per molecule | 4 | 1 |
| Cooperativity | Yes | No |
| Curve shape | Sigmoid | Hyperbolic |
| Affinity | Moderate (P50 ~26–27 mmHg) | Very high — curve far to the left |
What do carbon monoxide, methaemoglobin and anaemia do to the curve?
Carbon monoxide binds haemoglobin with an affinity about 200–250 times that of oxygen, forming carboxyhaemoglobin (COHb). This reduces oxygen-carrying capacity and causes a leftward shift, so the oxygen that is carried is released poorly — a double hit to tissue oxygen delivery.
- PaO2 is normal in CO poisoning (dissolved oxygen is unaffected).
- Standard pulse oximetry cannot tell oxyhaemoglobin from COHb and gives falsely reassuring readings — CO-oximetry is needed.
- COHb half-life: 4–6 hours on room air, about 60–90 minutes on 100% oxygen, and about 20–30 minutes with hyperbaric oxygen.
Methaemoglobin contains ferric (Fe3+) iron that cannot carry oxygen; the allosteric change also shifts the curve left, increasing the affinity of the remaining ferrous haem for oxygen. Typical triggers include nitrites, dapsone and topical benzocaine; methylene blue is the antidote.
| Condition | PaO2 | SaO2 (true) | O2 content | Curve |
|---|---|---|---|---|
| Anaemia | Normal | Normal | ↓ (proportional to Hb) | Saturation curve unchanged; content curve scaled down |
| CO poisoning | Normal | ↓ (COHb) | ↓ | Left shift |
| Methaemoglobinaemia | Normal | ↓ | ↓ | Left shift |
How is the curve used at the bedside?
Most patients are monitored with pulse oximetry (SpO2) rather than arterial PO2, so the curve is how clinicians translate one into the other. Breathe 2015 notes that pulse oximeters are generally reliable when saturation is above about 88% and less so below that; skin pigmentation, nail varnish, methaemoglobin and carboxyhaemoglobin also affect readings.
- On the plateau (PaO2 well above 60 mmHg), a large fall in PaO2 barely moves SpO2 — a normal oximeter reading can hide a falling PaO2 in a patient on supplemental oxygen.
- On the steep part (below about 60 mmHg), a small further fall in PaO2 produces a large drop in saturation and oxygen delivery.
- Very high PaO2 adds almost nothing to content once saturation approaches 100%; only dissolved oxygen keeps rising.
- Oxygen targets are therefore usually set around SpO2 of 90% or more, or 88–92% in patients at risk of hypercapnia (StatPearls — Home Oxygen Therapy).
How is the dissociation curve asked in NEET PG and INI-CET?
- Shape and basis — sigmoid due to cooperative binding; myoglobin hyperbolic.
- P50 value — about 26–27 mmHg in adults; about 19 mmHg for fetal Hb.
- Which of the following shifts the curve to the left? — HbF, CO, methaemoglobin, alkalosis, hypothermia, stored blood.
- Right shift — ↑ CO2, acidosis, ↑ 2,3-BPG, fever, exercise, high altitude.
- Bohr vs Haldane — which gas affects which: CO2/H+ on O2 binding (Bohr) vs O2 on CO2 carriage (Haldane).
- Graph reading — 60 mmHg ≈ 90%, 40 mmHg ≈ 75% (mixed venous).