Overview
This lecture covers physiological buffer systems in the body that maintain pH balance, focusing on their mechanisms, main buffer types, and related clinical conditions.
Buffer System Basics
- Buffers maintain equilibrium by resisting changes in pH in body fluids.
- Intracellular buffer systems, like phosphate, operate inside cells; extracellular systems, like the carbonic acid-bicarbonate, operate outside cells.
- The protein buffer system interacts with both intracellular (ICF) and extracellular (ECF) fluid.
Major Physiological Buffers
- The carbonic acid-bicarbonate buffer system is the primary ECF buffer, regulating blood pH.
- The phosphate buffer system regulates ICF and urine pH.
- Hemoglobin acts as a buffer for carbon dioxide and hydrogen ions in red blood cells.
Carbonic Acid-Bicarbonate Buffer System
- CO₂ reacts with water to form carbonic acid, which dissociates into bicarbonate and protons (acidic).
- The reaction is catalyzed by the enzyme carbonic anhydrase.
- Net reaction: CO₂ (gas) + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺.
- Buffer capacity is optimal near blood pH of 7.4.
Buffer System Regulation
- Kidneys regulate bicarbonate (long-term pH control) by reabsorbing or excreting it in urine.
- Lungs regulate CO₂ levels (short-term pH control) through breathing.
Acidosis and Alkalosis
- Alkalosis (high blood pH) can result from hyperventilation, excessive vomiting, or diuretic use.
- Combating alkalosis: breathe into a CO₂-rich bag or infuse NH₄Cl.
- Metabolic acidosis (low blood pH) occurs with diabetes (ketoacidosis), starvation, high protein diets, or lactic acid buildup.
- Respiratory acidosis occurs when CO₂ is retained due to poor lung function.
- Bicarbonate infusion can treat acidosis; ethanol can reduce toxic glycolic acid after antifreeze ingestion.
Hemoglobin and Phosphate Buffer Systems
- Hemoglobin transports O₂ and CO₂, buffering carbon dioxide and hydrogen ions.
- 20% of CO₂ binds to hemoglobin, 70% forms bicarbonate, 10% dissolves in plasma.
- The phosphate buffer system uses dihydrogen and monohydrogen phosphate; pKa ≈ 7.2, close to blood pH.
Buffering in Urine
- Urine buffering involves carbonic acid-bicarbonate, phosphate, and ammonia buffer systems.
- Ammonia (NH₃) generated from glutamine binds H⁺ to form ammonium (NH₄⁺) for excretion.
Key Terms & Definitions
- Buffer — a substance that minimizes pH changes in a solution.
- ICF (Intracellular Fluid) — fluid inside cells.
- ECF (Extracellular Fluid) — fluid outside cells.
- Carbonic Anhydrase — enzyme catalyzing CO₂ + H₂O ⇌ H₂CO₃.
- pKa — the pH at which a buffer is most effective.
- Acidosis — blood pH below 7.35.
- Alkalosis — blood pH above 7.45.
- Hemoglobin Buffer System — buffering of CO₂ and H⁺ in red blood cells.
- Phosphate Buffer System — intracellular and urinary buffer with pKa ~7.2.
- Ammonia Buffer System — urinary buffer using NH₃/NH₄⁺ pair.
Action Items / Next Steps
- Review buffer system equations and mechanisms.
- Know normal blood pH values and the consequences of deviations.
- Understand how kidneys and lungs regulate acid-base balance.