pH Regulation in Body Fluids

pH & Buffers - Balancing Act Basics

• pH: Measure of $H^+$ concentration; $pH = -\log[H^+]$. Normal arterial blood: 7.35-7.45.
• Importance: pH homeostasis is vital for optimal enzyme activity, protein structure, and cellular processes.
• Major Buffer Systems: First-line defense, rapidly resist pH changes. 📌 Mnemonic: "Big Helpers Protect PH" (Bicarbonate, Hemoglobin, Proteins, Phosphate).
  • Bicarbonate-carbonic acid ($H_2CO_3/HCO_3^-$): Main ECF buffer. $H_2O + CO_2 \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$.
  • Hemoglobin: Significant buffer in RBCs (imidazole groups of histidine).
  • Proteins (e.g., albumin): Important ICF & plasma buffers (amino/carboxyl groups).
  • Phosphate ($HPO_4^{2-}/H_2PO_4^-$): Key in ICF & renal tubular fluid.
• Henderson-Hasselbalch Equation: $pH = pKa + \log([\text{base}]/[\text{acid}])$.
  • For bicarbonate system: $pH = 6.1 + \log([\text{HCO}_3^-] / (0.03 \times \text{PCO}_2))$.

⭐ Bicarbonate buffer: most important ECF buffer; high concentration, regulated by lungs/kidneys ($CO_2, HCO_3^-$).

בו

Respiratory Regulation - Lungs' Quick Fix

• The lungs offer a rapid mechanism for pH control by adjusting alveolar ventilation to alter systemic $PCO_2$ levels. This directly influences the carbonic acid-bicarbonate buffer system: $CO_2 + H_2O \leftrightarrow H_2CO_3 \leftrightarrow H^+ + HCO_3^-$ (enzyme: carbonic anhydrase).

• Sensing pH Changes (Chemoreceptors):

  • Central (medulla oblongata): Respond to $[H^+]$ in CSF, which is determined by arterial $PCO_2$ (as $CO_2$ diffuses easily across the blood-brain barrier).
  • Peripheral (carotid & aortic bodies): Directly sense changes in arterial $PO_2$ (especially < 60 mmHg), ↑ arterial $PCO_2$, and ↑ arterial $[H^+]$.

• Ventilatory Responses:

• Effectiveness: This system initiates a response within minutes and reaches significant effect in hours. It provides powerful but often incomplete compensation for metabolic acid-base disorders.

⭐ The respiratory system can compensate for approximately 50-75% of the pH change caused by metabolic acid-base disturbances.

Renal Regulation - Kidneys' Fine Tune

Kidneys precisely control pH by excreting $H^+$ and managing $HCO_3^-$. Slower (hours-days) but potent for full correction.

• Mechanisms:
  • $H^+$ excretion: As titratable acid ($H_2PO_4^-$) or ammonium ($NH_4^+$).
  • $HCO_3^-$ reabsorption/generation.
• Key Processes:
  • $HCO_3^-$ Reabsorption: Mainly PCT. Uses $Na^+/H^+$ exchanger (NHE3), carbonic anhydrase.
  • $H^+$ Secretion:
    • PCT: NHE3.
    • DCT & CD (intercalated cells): $H^+$-ATPase, $H^+/K^+$-ATPase. Aldosterone stimulates.
  • Fixed Acid Excretion:
    • Phosphate buffer: $HPO_4^{2-} + H^+ \rightarrow H_2PO_4^-$.
    • Ammonia buffer: $NH_3 + H^+ \rightarrow NH_4^+$. (Glutamine metabolism in PCT is $NH_3$ source).
  • New $HCO_3^-$ Generation: Linked to $H^+$ excretion with non-bicarbonate buffers.

• Renal Compensation in Acidosis:

⭐ In chronic acidosis, renal ammonia production (from glutamine in PCT) can ↑ up to 10-fold, boosting hydrogen ion excretion as ammonium.

High‑Yield Points - ⚡ Biggest Takeaways

• Henderson-Hasselbalch equation is key for pH: pH = pKa + log ([HCO₃⁻]/[0.03 x PCO₂]).
• Bicarbonate buffer (HCO₃⁻/H₂CO₃) is the main ECF buffer.
• Phosphate buffer (HPO₄²⁻/H₂PO₄⁻) is crucial in renal tubules & ICF.
• Proteins, especially hemoglobin, are important ICF buffers.
• Lungs rapidly regulate PCO₂ (volatile acid) via ventilation.
• Kidneys slowly manage balance by regulating HCO₃⁻ and H⁺ excretion.
• Normal arterial pH 7.35-7.45; PCO₂ 35-45 mmHg; HCO₃⁻ 22-26 mEq/L.