Overview
This lecture covers the mechanisms of oxygen transport in the body, focusing on how hemoglobin in red blood cells facilitates oxygen loading and unloading using partial pressure gradients and cooperative binding.
Oxygen Transport Process
- Oxygen diffuses from alveoli into red blood cells (RBCs) at the lungs.
- Within RBCs, oxygen binds to hemoglobin molecules for transport throughout the body.
- Hemoglobin contains four subunits, each with an iron-containing heme group that binds one oxygen molecule.
- Each hemoglobin can transport up to four oxygen molecules.
- Oxygen binding to hemoglobin is cooperative; binding of one O₂ increases affinity for subsequent O₂.
Hemoglobin Structure and Function
- Hemoglobin is a metalloprotein found in red blood cells.
- Iron (Fe) in heme groups allows oxygen molecules to bind.
- Hemoglobin also has allosteric (regulatory) sites for hydrogen ions (H⁺) and carbon dioxide (CO₂).
- Binding of H⁺ and CO₂ to hemoglobin decreases its affinity for O₂ (allosteric inhibition).
Role of Partial Pressure in Gas Exchange
- Partial pressure refers to the pressure exerted by a single gas in a mixture.
- O₂ moves from regions of high partial pressure (lungs) to low partial pressure (tissues).
- Partial pressure of O₂: highest in the alveoli, lower in RBCs, lowest in body tissues.
- Partial pressure of CO₂: highest in tissues, lowest in alveoli (opposite of O₂).
Loading and Unloading of Oxygen
- In the lungs, high O₂ partial pressure and low H⁺/CO₂ allow hemoglobin to pick up oxygen (loading).
- In tissues, low O₂ partial pressure and high H⁺/CO₂ promote oxygen release from hemoglobin (unloading).
- The presence of H⁺ and CO₂ in tissues triggers oxygen to dissociate from hemoglobin.
Oxygen Dissociation Curve
- Plots % of hemoglobin saturated with O₂ (y-axis) vs. O₂ partial pressure (x-axis).
- The curve is sigmoidal (S-shaped) due to cooperative binding.
- At high O₂ partial pressure, hemoglobin is fully saturated.
- The curve illustrates how easily hemoglobin picks up and releases oxygen under different conditions.
Key Terms & Definitions
- Hemoglobin — a protein in RBCs that carries oxygen with four heme (iron) groups.
- Heme group — iron-containing part of hemoglobin that binds oxygen.
- Cooperative binding — process where binding of one oxygen increases hemoglobin's affinity for more oxygen.
- Allosteric inhibition — regulation where molecules (e.g., H⁺, CO₂) decrease hemoglobin's oxygen affinity.
- Partial pressure — the pressure exerted by a specific gas in a mixture.
Action Items / Next Steps
- Review the oxygen dissociation curve and understand its sigmoidal shape.
- Prepare for the next lecture on carbon dioxide transport.