OXYGEN DISSOCIATION CURVE – NEET EXAM KEY NOTES CBSE

 

Definition:

The oxygen dissociation curve is a sigmoid (S-shaped) graph showing the relationship between

• Partial pressure of oxygen (pO₂) (x-axis) and
• Percentage saturation of haemoglobin with oxygen (y-axis).

OXYGEN DISSOCIATION CURVE

Key Points:

• At lungs (high pO₂ ~100 mm Hg):

  • Haemoglobin is almost fully saturated (~97%).

  • Favourable conditions: high pO₂, low pCO₂, lower H⁺ concentration, lower temperature.

• At tissues (low pO₂ ~40 mm Hg):

  • Haemoglobin releases O₂ → about 25% of oxygen is unloaded, 75% remains bound.

  • Conditions: low pO₂, high pCO₂, high H⁺, higher temperature → promote oxygen unloading.

Bohr Effect (Very Important for NEET):

• Definition: Decrease in oxygen affinity of haemoglobin due to increased CO₂ and H⁺ concentration.

• Effect on curve: Causes the curve to shift to the right → more oxygen released at tissues.

• Right shift = ↓ O₂ affinity (e.g., high CO₂, high H⁺, high temperature)
• Left shift = ↑ O₂ affinity (e.g., low CO₂, low H⁺, low temperature, fetal Hb)

Significance of Sigmoid Shape:

• At low pO₂, the slope is steep → small change in pO₂ = large O₂ release (useful at tissues).

• At high pO₂, the curve plateaus → ensures max loading at lungs even if pO₂ changes a little.

Typical NEET MCQs:

1. The oxygen dissociation curve is sigmoid due to…
   ✅ Cooperative binding of oxygen to haemoglobin.

2. Bohr effect refers to…
   ✅ Decreased affinity of haemoglobin for O₂ in presence of high CO₂ / H⁺.

3. Right shift of the oxygen dissociation curve indicates…
   ✅ Decreased affinity of Hb for O₂ → more O₂ delivery to tissues.

4. At a pO₂ of 100 mm Hg, Hb saturation is approximately…
   ✅ 97%

Quick Summary Table

Condition	Curve Shift	Hb Affinity	O₂ Delivery
↑ pCO₂, ↑ H⁺, ↑ Temp	Right	↓ Decreases	↑ Increases
↓ pCO₂, ↓ H⁺, ↓ Temp	Left	↑ Increases	↓ Decreases
Fetal Hb	Left	↑ Higher	—