Roles of Acids in Physiological Homeostasis: Digestion, pH Balance, and Cellular Energy

Acids are fundamental to maintaining internal stability (homeostasis) across multiple systems.

1. Digestive System

Gastric HCl (pH 1.5–3.5) activates pepsinogen → pepsin, denatures proteins, kills ingested pathogens, and stimulates pancreatic bicarbonate and bile secretion.

2. Systemic Acid-Base Balance

• Bicarbonate buffer: H₂CO₃/HCO₃⁻ instantly neutralizes H⁺/OH⁻.
• Respiratory compensation: CO₂ exhalation adjusts pH within minutes.
• Renal regulation: H⁺ excretion (titratable acid, NH₄⁺) and HCO₃⁻ reabsorption/generation over hours–days.

Arterial pH maintained at 7.35–7.45.

3. Cellular Energy Production

Mitochondrial proton gradient (H⁺) across inner membrane drives ATP synthase → ATP. Generated by electron transport chain oxidizing NADH/FADH₂ from glucose, fatty acids.

4. Kidney Function

Proximal tubule: H⁺ secretion / HCO₃⁻ reabsorption (85–90%).
Collecting duct: Intercalated cells fine-tune acid excretion; generate new HCO₃⁻ via glutamine metabolism.

5. Other Roles

• Lysosomal enzymes require low pH for degradation.
• Bone buffering: chronic acidosis releases carbonate from hydroxyapatite.
• Hemoglobin O₂ affinity modulated by H⁺ (Bohr effect).

Dysregulation (acidosis/alkalosis) disrupts enzyme function, electrolyte distribution, and cardiovascular stability.