Oxygen-Hemoglobin Dissociation Curve

Overview

This lecture explains the oxygen-hemoglobin dissociation curve, detailing how oxygen binds to hemoglobin, the shape of the curve, and its role in oxygen transport from lungs to tissues.

Hemoglobin and Oxygen Binding

• Each hemoglobin molecule contains four heme groups, each binding one oxygen molecule.
• At low partial pressure of oxygen (pO₂), hemoglobin’s affinity for oxygen is low.
• As pO₂ increases, more oxygen binds to hemoglobin, especially in the middle pressure range.
• Binding of oxygen increases the affinity of hemoglobin for more oxygen, a phenomenon called positive cooperativity.
• Hemoglobin becomes saturated at high pO₂, so additional oxygen binding slows down.

Oxygen-Hemoglobin Dissociation Curve

• The curve plots pO₂ (x-axis) against percentage hemoglobin saturation (y-axis).
• At low pO₂, the curve is flat due to minimal oxygen binding.
• The curve steepens as positive cooperativity increases oxygen binding in the middle range.
• The curve flattens again at high pO₂ as hemoglobin nears full saturation.
• The curve has an S-shape (sigmoidal) because of cooperative binding.

Oxygen Transport: Lungs to Tissues

• In lungs (pO₂ ≈ 100 mmHg), hemoglobin is about 97% saturated with oxygen.
• In peripheral tissues (pO₂ ≈ 40 mmHg), hemoglobin saturation drops to about 75% as oxygen is released.
• Only about 22% of hemoglobin’s oxygen is unloaded at tissues under normal conditions.
• The cycle repeats as hemoglobin travels between lungs and tissues.

Oxygen Content in Blood

• At 97% saturation, arterial blood contains about 20 ml oxygen per 100 ml blood.
• At 75% saturation, venous blood contains about 15 ml oxygen per 100 ml blood.
• Approximately 5 ml oxygen is delivered to tissues per 100 ml blood in each cycle.

Key Terms & Definitions

• Partial Pressure of Oxygen (pO₂) — the pressure exerted by oxygen in a mixture of gases or in blood.
• Hemoglobin (Hb) — a protein in red blood cells responsible for carrying oxygen.
• Affinity — the strength of binding between hemoglobin and oxygen.
• Positive Cooperativity — increased binding affinity of hemoglobin for oxygen as more oxygen molecules are bound.
• Oxygen-Hemoglobin Dissociation Curve — a graph showing the relationship between pO₂ and hemoglobin saturation.
• Saturation — the percentage of hemoglobin binding sites occupied by oxygen.

Action Items / Next Steps

• Review factors that affect hemoglobin’s affinity for oxygen (to be covered in a separate lesson).