Oxygen Dissociation Curve of Adult Haemoglobin

This section details the oxygen dissociation curve of adult haemoglobin, a crucial concept in understanding oxygen transport in the blood. We will explore its shape, the factors influencing it, and its significance for oxygen delivery to tissues.

The Oxygen Dissociation Curve

The oxygen dissociation curve illustrates the relationship between the partial pressure of oxygen (PO₂) in the blood and the percentage of haemoglobin saturation with oxygen.

The curve is sigmoidal, meaning it has a characteristic S-shape. This shape is highly advantageous for efficient oxygen transport.

Key Features of the Curve

• Low PO₂: At low PO₂ (e.g., in tissues), haemoglobin readily binds oxygen, resulting in a steep increase in saturation.
• High PO₂: At high PO₂ (e.g., in the lungs), haemoglobin saturation levels off, indicating that further increases in PO₂ do not significantly increase oxygen binding.
• Carriers of Oxygen: The curve demonstrates that haemoglobin acts as a carrier of oxygen, facilitating its transport from the lungs to the tissues.

Factors Affecting the Oxygen Dissociation Curve

Several factors can shift the oxygen dissociation curve to the left or right, affecting the amount of oxygen delivered to tissues. A shift to the right indicates decreased oxygen delivery, while a shift to the left indicates increased oxygen delivery.

1. Bohr Effect (Effect of pH)

The Bohr effect describes the relationship between blood pH and haemoglobin's affinity for oxygen. A decrease in pH (increased acidity) shifts the curve to the right, reducing oxygen affinity. This occurs in active tissues where carbon dioxide is produced, leading to increased carbonic acid formation and lower pH.

pH	Oxygen Affinity
Decreases (more acidic)	Decreases (curve shifts right)
Increases (more alkaline)	Increases (curve shifts left)

Mechanism: Changes in pH affect the protonation state of haemoglobin's globin subunits. A lower pH promotes the release of oxygen.

2. Effect of Carbon Dioxide (CO₂)

Increased concentrations of CO₂ also shift the oxygen dissociation curve to the right. This is closely linked to the Bohr effect. CO₂ reacts with water to form carbonic acid, which further lowers the pH, leading to oxygen release.

3. Effect of Temperature

An increase in temperature shifts the oxygen dissociation curve to the right. Higher temperatures reduce haemoglobin's affinity for oxygen. This is because increased molecular motion disrupts the interactions between haemoglobin subunits and oxygen.

4. Effect of 2,3-Bisphosphoglycerate (2,3-BPG)

2,3-BPG is a molecule found in red blood cells. Increased levels of 2,3-BPG shift the oxygen dissociation curve to the right, reducing oxygen affinity. 2,3-BPG binds to haemoglobin and stabilizes the deoxy form, promoting oxygen release. 2,3-BPG levels increase in conditions of hypoxia (low oxygen levels).

Clinical Significance

Understanding the oxygen dissociation curve is vital for understanding various physiological and pathological conditions:

• Hypoxia: Conditions causing low oxygen levels (e.g., altitude sickness, lung diseases) lead to a rightward shift of the curve, reducing oxygen delivery to tissues.
• Fever: Elevated body temperature (fever) causes a rightward shift of the curve, increasing oxygen release to tissues.
• Anemia: Reduced haemoglobin concentration (anemia) can affect the overall oxygen-carrying capacity of the blood.

The oxygen dissociation curve is a fundamental concept in physiology, highlighting the intricate relationship between oxygen delivery and tissue oxygenation.