Buffer systems (bicarbonate, phosphate, protein)

Buffer systems (bicarbonate, phosphate, protein)

Buffer systems (bicarbonate, phosphate, protein)

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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 Buffer System with Lungs and Kidneys

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.

Hemoglobin buffer system and gas exchange

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.

Practice Questions: Buffer systems (bicarbonate, phosphate, protein)

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Which factor most strongly influences protein filtration at the glomerulus?

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Flashcards: Buffer systems (bicarbonate, phosphate, protein)

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In the lumen of late distal tubule and collecting duct, secreted H+ combines with the HPO4- to form H2PO4, which is a _____ acid

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In the lumen of late distal tubule and collecting duct, secreted H+ combines with the HPO4- to form H2PO4, which is a _____ acid

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