Question
What is the oxygen-haemoglobin dissociation curve? Why is it sigmoidal (S-shaped) and not linear? Explain the Bohr effect. What factors shift the curve to the right, and what does a right shift mean physiologically?
(NEET pattern — conceptual reasoning + application)
Solution — Step by Step
The curve plots partial pressure of oxygen (pO₂) on the x-axis against percentage saturation of haemoglobin on the y-axis. It shows how readily haemoglobin picks up and releases oxygen at different oxygen concentrations.
Key points on the curve:
- At lungs (pO₂ ~100 mmHg): Hb is ~97-98% saturated — picks up O₂ efficiently
- At tissues (pO₂ ~40 mmHg): Hb is ~75% saturated — releases ~25% of its O₂
- At exercising muscle (pO₂ ~20 mmHg): Hb drops to ~35% saturation — releases a lot more O₂
The S-shape is due to cooperative binding. Haemoglobin has four subunits, each carrying one haem group.
- Binding of the first O₂ molecule is difficult (low affinity) — this explains the flat lower portion
- Once one O₂ binds, it changes the shape of Hb, making it easier for the second and third O₂ to bind — this creates the steep middle portion
- The fourth O₂ binding is slightly harder (fewer empty sites) — creating the plateau at top
This cooperativity ensures: Hb loads efficiently in the lungs (high pO₂) and unloads efficiently in tissues (low pO₂).
The Bohr effect: an increase in CO₂ concentration (or decrease in pH) shifts the O₂ dissociation curve to the right, meaning haemoglobin releases oxygen more readily.
Why this is brilliant: in active tissues, CO₂ production is high and pH is low. The Bohr effect ensures that Hb delivers MORE oxygen precisely where it is needed most.
| Factor | Right Shift (decreased affinity) | Left Shift (increased affinity) |
|---|---|---|
| CO₂ | Increased CO₂ | Decreased CO₂ |
| pH | Lower pH (more acidic) | Higher pH (more alkaline) |
| Temperature | Higher temperature | Lower temperature |
| 2,3-BPG | Increased | Decreased |
Right shift = Hb releases O₂ more easily (good for active tissues)
Left shift = Hb holds onto O₂ more tightly (seen in fetal Hb — helps fetus extract O₂ from maternal blood)
graph TD
A["O2 Dissociation Curve"] --> B["Sigmoidal shape"]
B --> B1["Cooperative binding of 4 O2"]
A --> C["Right Shift"]
A --> D["Left Shift"]
C --> C1["High CO2, Low pH"]
C --> C2["High temperature, High 2,3-BPG"]
C --> C3["Effect: O2 released more easily"]
D --> D1["Low CO2, High pH"]
D --> D2["Fetal Hb — grabs O2 from mother"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style C fill:#fca5a5,stroke:#000
style D fill:#93c5fd,stroke:#000Why This Works
The sigmoidal curve is a direct consequence of haemoglobin’s quaternary structure — four subunits that communicate with each other. This cooperative system creates a natural “switch” behaviour: at high pO₂ (lungs), Hb rapidly loads up; at low pO₂ (tissues), it rapidly unloads. A simple linear relationship would mean Hb releases oxygen uniformly — less efficient for targeted delivery.
The Bohr effect adds another layer of intelligence: metabolically active tissues produce more CO₂ and heat, which automatically shifts the curve right, increasing O₂ delivery exactly where needed. No nervous system control required — it is a purely chemical feedback mechanism.
Common Mistake
Students often confuse the direction of the shift. Right shift = O₂ released more easily (lower affinity, helps tissues). Left shift = O₂ held more tightly (higher affinity, helps loading). The easiest way to remember: “Right = Release.” Also, fetal haemoglobin (HbF) has a LEFT-shifted curve compared to adult Hb — this is how the fetus extracts O₂ from maternal blood across the placenta.
NEET mnemonic for right-shift factors: “CADET face Right” — CO₂ up, Acid (low pH), 2,3-DPG (BPG), Exercise, Temperature up. All these conditions are associated with active, metabolising tissue that NEEDS more oxygen — and the right shift delivers exactly that.