Acid Base Regulation

Hydrolysis equation, pH regulation, CO2 and HCO3- balance, protein buffer systems, and the carbonic acid-bicarbonate buffer system.

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Acid Base Regulation

Hydrolysis Equation

The hydrolysis equation is fundamental to acid-base physiology:

CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃⁻ + H⁺

Carbon dioxide combines with water to form carbonic acid, which dissociates into bicarbonate and hydrogen ion. This reversible reaction underlies all respiratory acid-base regulation.

pH Regulation

pH stands for potential of hydrogen — a measure of acidity or alkalinity. The normal blood pH range is 7.35–7.45, corresponding to approximately 0.00004 mEq/L of H⁺.

  • Low pH (< 7.35) = Acidosis — excess H⁺
  • High pH (> 7.45) = Alkalosis — deficit of H⁺

The body maintains multiple systems to regulate H⁺ concentration. The lungs and kidneys are the primary organs responsible for pH homeostasis.

CO₂ Regulation

As CO₂ increases, the hydrolysis equation shifts to the right, producing more H⁺ and causing the pH to decrease (acidosis).

As CO₂ decreases, the equation shifts to the left, consuming H⁺ and causing the pH to increase (alkalosis).

CO₂ is primarily regulated through exhalation. The pulmonary system provides rapid adjustment of CO₂ levels, making it the first line of defense against acid-base disturbances.

HCO₃⁻ (Bicarbonate) Regulation

Bicarbonate is regulated by the kidneys, which adjust more slowly than the lungs.

  • Too much H⁺ → kidneys excrete H⁺ and retain HCO₃⁻
  • Too little H⁺ → kidneys retain H⁺ and excrete HCO₃⁻

This renal compensation takes hours to days to fully adjust.

Buffer Systems

Buffer systems react with hydrogen ions to reduce their impact on pH.

Protein Buffer System: Hemoglobin and Albumin are the primary protein buffers. Both reversibly bind H⁺ — they continuously give and receive hydrogen ions as needed. Hemoglobin is particularly effective at buffering because it is present in high concentration in red blood cells and can bind excess H⁺ or release H⁺ when necessary to maintain homeostasis.

Carbonic Acid-Bicarbonate Buffer System

This system is based on the hydrolysis equation and explains how the body carries the majority of CO₂ as HCO₃⁻ (bicarbonate).

Key benefits:

  • The body can dissociate the weak acid H₂CO₃ back into CO₂ + H₂O near lung tissue, allowing CO₂ to be exhaled
  • The kidneys can excrete excess H⁺ or retain HCO₃⁻ as a base to maintain pH balance

This system is the most important buffer in the extracellular fluid and works in concert with the respiratory and renal systems to maintain acid-base homeostasis.