Hemoglobin Oxygen Dissociation Curve

Overview of the Oxygen Dissociation Curve

The hemoglobin oxygen dissociation curve illustrates the relationship between the partial pressure of oxygen (pO2) and the percentage saturation of hemoglobin with oxygen. This curve is essential for understanding how oxygen is transported and released in the body.

• Partial Pressure of Oxygen (pO2): The pressure exerted by oxygen in a mixture of gases, measured in mmHg.

• Hemoglobin Saturation: The percentage of hemoglobin molecules bound to oxygen.

• Sigmoidal Shape: The curve is S-shaped due to cooperative binding of oxygen to hemoglobin.

Key Regions on the Curve

Different physiological sites correspond to specific regions on the curve, reflecting varying levels of hemoglobin saturation.

• Lungs (High pO2):

  • Located at the upper right portion of the curve (pO2 ≈ 100 mmHg).

  • Hemoglobin is nearly fully saturated (≈ 98-100%).

  • Example: Oxygen loading occurs in pulmonary capillaries.

• Resting Tissues (Moderate pO2):

  • Located at the middle portion of the curve (pO2 ≈ 40 mmHg).

  • Hemoglobin is partially saturated (≈ 75%).

  • Example: Oxygen is released to tissues at rest.

• Active Tissues (Low pO2):

  • Located at the lower left portion of the curve (pO2 ≈ 20 mmHg).

  • Hemoglobin saturation drops further (≈ 35%).

  • Example: Oxygen is released more readily to metabolically active tissues.

Saturation vs. Unsaturation

Saturated hemoglobin refers to hemoglobin molecules fully loaded with oxygen, while unsaturated hemoglobin has fewer oxygen molecules bound. The degree of saturation depends on the local pO2:

• High pO2 (lungs): Hemoglobin is saturated.

• Low pO2 (active tissues): Hemoglobin is unsaturated.

Changes in the Curve: Above 50 mmHg and Between 20 mmHg and 50 mmHg

The curve's slope and shape reflect hemoglobin's affinity for oxygen at different pO2 levels.

• Above 50 mmHg:

  • The curve plateaus, indicating high hemoglobin saturation.

  • Small increases in pO2 result in minimal increases in saturation.

  • Explanation: Hemoglobin's affinity for oxygen is high; most binding sites are occupied.

• Between 20 mmHg and 50 mmHg:

  • The curve is steep, showing rapid decrease in saturation with decreasing pO2.

  • Hemoglobin releases oxygen efficiently in response to tissue demand.

  • Explanation: Cooperative binding causes hemoglobin to unload oxygen quickly as pO2 drops, facilitating oxygen delivery to active tissues.

Cooperative Binding and the Sigmoidal Curve

Hemoglobin exhibits cooperative binding, meaning the binding of one oxygen molecule increases the affinity for subsequent oxygen molecules. This property is responsible for the sigmoidal (S-shaped) curve.

• Formula: The Hill equation describes cooperative binding: Where:

  • = fractional saturation

  • = partial pressure of oxygen

  • = pO2 at which hemoglobin is 50% saturated

  • = Hill coefficient (degree of cooperativity)

Summary Table: Physiological Sites and Hemoglobin Saturation

Additional info: The curve can shift right (decreased affinity) or left (increased affinity) due to factors such as pH, CO2 concentration, temperature, and 2,3-BPG levels. These shifts are known as the Bohr effect and help regulate oxygen delivery under different physiological conditions.