Buffer systems (bicarbonate, phosphate, protein)

Buffer Basics - Acid-Base Guardians

• Bicarbonate Buffer System: Primary ECF buffer. Rapidly responds to metabolic acid changes. Governed by the Henderson-Hasselbalch equation:

  • $pH = pKa + log([HCO_3^-]/[0.03 * PCO_2])$
  • Maintains ECF pH near 7.4.

• Phosphate Buffer System: Key in intracellular fluid (ICF) and as a urinary buffer.

  • Uses the $H_2PO_4^-$/$HPO_4^{2-}$ pair.
  • Crucial for excreting H+ in urine.

• Protein Buffer System: Major ICF and plasma buffer.

  • Amphoteric proteins (e.g., albumin) can accept or donate H+.
  • Hemoglobin is a vital buffer in RBCs, especially for CO2 transport.

⭐ High-Yield: Hemoglobin is a particularly effective buffer due to its high concentration in blood and the numerous histidine residues, which have a pKa near physiological pH.

Bicarbonate System - The Lung-Kidney Tango

• The most critical buffer in the extracellular fluid (ECF), responsible for managing moment-to-moment pH changes.
• Equation: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$
• Lungs (Fast Control): Regulate volatile acid by adjusting $PCO_2$ via ventilation.
  • ↑Ventilation → Blow off $CO_2$ → ↓$PCO_2$ → ↑pH
• Kidneys (Slow Control): Regulate fixed acids by adjusting $[HCO_3^-]$ reabsorption and generation.
  • Slower, more powerful response (hours to days).

⭐ The system's pKa is 6.1, which is not ideal for buffering at physiological pH (7.4). Its effectiveness stems from the independent and powerful regulation of $CO_2$ by the lungs and $HCO_3^-$ by the kidneys.

Phosphate & Proteins - The Supporting Cast

• Phosphate Buffer System: A key player in intracellular fluid (ICF) and renal tubules.

  • Components: Dihydrogen phosphate ($H_2PO_4^−$, the weak acid) and monohydrogen phosphate ($HPO_4^{2−}$, the conjugate base).
  • Reaction: $H_2PO_4^− \leftrightarrow H^+ + HPO_4^{2−}$
  • Has an effective pKa of 6.8, making it an efficient buffer in urine and ICF.

• Protein Buffer System: The most abundant buffer system, particularly intracellularly.

  • Hemoglobin (Hb) in RBCs is the most significant protein buffer.
  • Proteins are amphoteric: carboxyl groups (-COOH) donate $H^+$, while amino groups (-NH₂) accept $H^+$.
  • The amino acid Histidine is the most effective amino acid buffer.

⭐ High-Yield: The Haldane Effect is critical for $CO_2$ transport. Deoxygenated hemoglobin in the tissues is a better proton acceptor (stronger base) than oxygenated Hb. It readily binds to $H^+$ ions formed when $CO_2$ is converted to bicarbonate, thus facilitating the loading and transport of $CO_2$ from tissues to the lungs.

High‑Yield Points - ⚡ Biggest Takeaways

• The bicarbonate buffer system is the primary ECF buffer, dynamically regulated by the lungs (CO2) and kidneys (HCO3⁻).
• Proteins are the most plentiful buffers, with hemoglobin being crucial in RBCs and albumin in plasma.
• The phosphate buffer system is most effective in the ICF and renal tubules.
• The Henderson-Hasselbalch equation is fundamental for understanding the bicarbonate system's function.
• All buffers are linked by the isohydric principle, maintaining a common pH.