Acid-Base Balance in Physiology

Acid-Base Balance

• Acid-base balance refers to the precise regulation of free hydrogen ion (H+) concentration in body fluids.
• Maintaining acid-base balance is crucial for nerve and muscle cell excitability, enzyme activity, and proper K+ levels.
• Sources of H+ in the body include carbonic acid formation, organic acids from intermediary metabolism, and incomplete metabolism of glucose and fatty acids (lactic acid and ketoacids).
• Hydrolysis of dietary phosphate produces fixed or metabolic acid load that cannot be exhaled, requiring neutralization or excretion by the kidneys.
• Normal blood pH is maintained between 7.35 and 7.45.
• Disturbances in acid-base balance can lead to serious clinical consequences.

Types of Acid-Base Disorders

• Acidosis: A condition where the blood has too much acid (or too little base), resulting in a decrease in blood pH.
• Alkalosis: A condition where the blood has too much base (or too little acid), resulting in an increase in blood pH.
• These disorders are not diseases on their own, but rather result from various underlying conditions.

Henderson-Hasselbalch Equation

• pH = pK + log ([A-]/[HA])
• pH = -log₁₀[H⁺] (pH units)
• pK = -log₁₀K (pH units)
• [A⁻] = Concentration of base form of buffer (mEq/L)
• [HA] = Concentration of acid form of buffer (mEq/L)

Buffers

• Buffers are mixtures of a weak acid and its conjugate base or a weak base and its conjugate acid.
• They immediately combine with any acid or alkali to prevent excessive changes in [H+].

Classes of Acids

• Fixed acids: Do not leave the body in solution; remain in body fluids until eliminated by the kidneys (e.g., sulfuric acid and phosphoric acid).
• Volatile acids: Can leave the body via the lungs, e.g., carbonic acid.

Acid-Base Buffer Systems

• Chemical buffer systems (immediate): Bicarbonate buffer system, phosphate buffer system, protein buffer system.
• Physiological buffer systems (minutes-hours): Respiratory response system, renal response system.

Role of the Lungs in Acid-Base Balance

• The lungs regulate pH by excreting CO2, which affects the amount of carbonic acid in the blood.
• Changes in respiratory rate directly affect CO2 levels and thus, blood pH.
• During acidosis, the respiratory rate increases to eliminate excess CO2.
• During alkalosis, the respiratory rate decreases to retain CO2.

Role of the Kidneys in Acid-Base Balance

• The kidneys slowly adjust blood pH by excreting excess acids or bases.
• Conservation/reabsorption of HCO3⁻, generation of new HCO3⁻, and excretion of H+ as titratable acid or NH4+ are kidney functions involved in pH regulation.

Summary of Acid-Base Disorders

• Disorders are categorized as either respiratory or metabolic, and either acidosis or alkalosis.
• Compensation mechanisms involving the lungs and kidneys are indicated by arrows and descriptions of direction of change in pH.