Overview
This lecture introduces the bicarbonate buffering system as the primary mechanism for transporting and regulating carbon dioxide (CO₂) and blood pH in the body.
The Bicarbonate Buffering System
- The bicarbonate buffering system transports most CO₂ in the blood.
- At tissues, active cells produce CO₂ during the TCA (citric acid) cycle as a byproduct of metabolism.
- CO₂ is highly soluble, diffuses into the blood, and reacts with water to form carbonic acid.
- The enzyme carbonic anhydrase rapidly converts CO₂ and water into carbonic acid (H₂CO₃).
- Carbonic acid partially dissociates into bicarbonate ion (HCO₃⁻) and a proton (H⁺), lowering pH.
- The generated H⁺ binds to hemoglobin, while HCO₃⁻ exits the red blood cell into the plasma.
The Chloride Shift and Charge Balance
- As HCO₃⁻ leaves red blood cells, chloride ions (Cl⁻) enter to maintain electrical neutrality (the chloride shift).
- This process prevents excessive negative charge buildup inside the cell.
CO₂ Offloading at the Lungs
- In the lungs (alveoli), the reaction reverses: HCO₃⁻ re-enters red blood cells, combines with H⁺, forming CO₂ and water.
- CO₂ diffuses into the alveoli and is expelled during exhalation, keeping blood CO₂ levels low.
- The low partial pressure of CO₂ in the alveoli drives the reaction to release CO₂ from the blood.
Regulation and Effects on pH
- Continuous ventilation (breathing) is essential to maintain low CO₂, preventing acidity (low pH).
- Decreased breathing (hypoventilation) leads to CO₂ accumulation and acidosis.
- Excessive breathing (hyperventilation) reduces CO₂ too much, causing alkalosis (high pH).
- The bicarbonate buffer system links CO₂ levels to blood pH control.
Key Terms & Definitions
- Bicarbonate Buffering System — The primary mechanism for CO₂ transport and pH regulation in blood.
- Carbonic Anhydrase — Enzyme catalyzing CO₂ and water conversion to carbonic acid.
- Chloride Shift — Exchange of HCO₃⁻ out and Cl⁻ into red blood cells to balance charge.
- Alveolus (plural: Alveoli) — Lung air sac where gas exchange occurs.
- Partial Pressure — The pressure of a single gas in a mixture, important for gas exchange.
Action Items / Next Steps
- Review textbook explanations of the citric acid (TCA) cycle and its role in CO₂ production.
- Study diagrams illustrating the bicarbonate buffering system and chloride shift.
- Practice writing the chemical equations for CO₂ transport and the bicarbonate buffer reaction.