Acid–Base Balance and Buffer Systems

Study Notes Acidosis 

1. Major Chemical Buffers in the Body

• Phosphate buffer

  • Works mainly inside cells and at the kidneys (renal tubules).

• Protein buffer

  • Found inside cells.

  • Most abundant buffer in the body.

• Bicarbonate buffer

  • Most important clinically.

  • Primary extracellular buffer system.

2. Bicarbonate Buffer System

• Buffers resist changes in pH.

• pH depends on hydrogen ion concentration:

  • ↑ H⁺ → acidic.

  • ↓ H⁺ → basic/alkaline.

Reaction:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

• H₂CO₃ = carbonic acid (weak acid).

• Splits into H⁺ (acid) and HCO₃⁻ (bicarbonate, conjugate base).

• Reversible process allows buffering in both directions.

3. Metabolism and CO₂ Production

• Food (proteins, fats, carbs) → ATP via mitochondria + electron transport chain.

• Nutrients mainly contain C, H, O (proteins also have N, but nitrogen not used for ATP).

• Hydrogens removed → carried by NADH and FADH₂ → donated to ETC → ATP formed.

• Leftover C + O → CO₂ (waste product).

• CO₂ = body’s “exhaust fumes.”

• Every tissue produces CO₂ → must be eliminated via lungs.

4. CO₂, Carbonic Acid, and Acidity

• CO₂ mixes with water in blood → forms carbonic acid.

• Carbonic acid dissociates → H⁺ ions → lowers pH.

• More CO₂ = more H⁺ = more acidic.

• Lungs excrete CO₂ → prevents buildup.

• Called a volatile acid (easily exhaled).

5. Independent H⁺ Production

• Not all H⁺ comes from CO₂.

• Example: lactic acid (exercise).

• Excess H⁺ binds to HCO₃⁻ → forms H₂CO₃ → converted to CO₂ + H₂O → exhaled.

6. Chemoreceptor Control

• Peripheral chemoreceptors:

  • Located in aortic bodies (aortic arch) + carotid bodies (carotid bifurcation).

  • Detect ↑ H⁺ and ↑ CO₂.

  • Send signals → brainstem → increase ventilation.

• Central chemoreceptors:

  • Located in brainstem.

  • Directly sense CO₂ and H⁺ in CSF.

• Response: ↑ ventilation → ↓ CO₂ → ↓ H⁺ → restores pH.

7. Respiratory Control of pH

• Holding breath → ↑ CO₂ → ↑ H⁺ → acidosis.

• Hyperventilation → ↓ CO₂ → ↓ H⁺ → alkalosis.

• Respiratory regulation = short-term pH control.

8. Respiratory Disorders

• Respiratory alkalosis

  • Cause: hyperventilation (e.g., anxiety).

  • Mechanism: ↓ CO₂ → ↓ H⁺ → blood becomes alkaline.

• Respiratory acidosis

  • Cause: inadequate ventilation (↓ CO₂ excretion).

  • Examples:

    • COPD (emphysema, chronic bronchitis).

    • Neuromuscular disease (e.g., polio).

    • Rib fractures (restrict lung expansion).

    • Pulmonary scarring or obstruction.

  • Mechanism: ↑ CO₂ → ↑ H⁺ → blood becomes acidic.

9. Renal Control of pH

• Kidneys regulate H⁺ excretion and HCO₃⁻ reabsorption/production.

• Slower than lungs → long-term control of pH.

• Compensation:

  • Acidosis → kidneys excrete more H⁺, reabsorb HCO₃⁻.

  • Alkalosis → kidneys retain H⁺, excrete HCO₃⁻.

Summary:

• Bicarbonate buffer is the most clinically important.

• Lungs = fast regulation (CO₂ control).

• Kidneys = slow regulation (H⁺ and HCO₃⁻ control).

• Disorders:

  • Respiratory alkalosis = hyperventilation.

  • Respiratory acidosis = hypoventilation/CO₂ retention.