Chapter 14

Overview of Acid-Base Balance in the Body

  • The importance of maintaining hydrogen ion (H+) concentration within a physiological range due to its critical role in metabolism and life.

Hydrogen Ion Formation

  • Sources of H+:

    • From fixed (non-volatile) and volatile acids during metabolism.

    • Significant CO2 production during tissue metabolism which is then transformed into carbonic acid (H₂CO₃) by hydration.

    • Isohydric Buffering: A process critical for maintaining pH via carbonic acid buffering.

Role of Carbonic Acid


    • The only volatile acid with physiological relevance.

    • Reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H+ + HCO₃- .

Buffering Mechanisms

Key Concepts:

  • Isohydric Buffering: Most H+ produced through metabolism is buffered immediately by hemoglobin (Hg) in red blood cells (RBC).

  • Carbonic Anhydrase: An enzyme in RBC that catalyzes the rapid conversion between CO₂, H2O, H2CO3, and subsequently H+ and HCO3-.

Buffer Solutions

  • Characteristics:

    • Composed of a weak acid and its conjugate base.

    • HCO3- buffers H+ ions effectively, minimizing pH changes during metabolic activities.

Ventilation and Acid-Base Homeostasis

  • Mechanisms of Regulation:

    • Body keeps blood pH constant through control on CO2 levels produced by metabolic activity. Increased ventilation lowers CO2, thus affecting pH positively (alkalosis) or vice versa.

Henderson-Hasselbalch Equation

  • pH = 6.1 + log([HCO₃-]/(PaCO₂ x 0.03))

Kidney Function in Acid-Base Balance

General Functions:

  • Kidneys filter and reabsorb bicarbonate (HCO3-).

  • They play a crucial role in acid excretion and regulate blood pH over a longer term.

Acidosis and Alkalosis:

  • Acidosis: Decreased bicarbonate or excess H+ results in lower pH.

  • Compensatory Mechanisms: In metabolic acidosis, respiration increases to expel CO2 (hyperventilation). In respiratory acidosis, kidneys retain HCO3- to buffer excess H+.

Disturbance Types

Primary Disturbances:

  • Can be respiratory (involving CO2) or metabolic (involving HCO3-).

  • Respiratory Acidosis: Hypoventilation leads to carbon dioxide retention.

  • Respiratory Alkalosis: Hyperventilation leads to decreased CO2 levels.

  • Metabolic Acidosis: Results from excess production of fixed acids or loss of bicarbonate.

  • Metabolic Alkalosis: Results from increased HCO3- or loss of fixed acids.

Clinical Indicators

  • Anion Gap Calculation: Important for determining the cause of metabolic acidosis and can assist in distinguishing between normal and increased anion gap acidosis.

  • Compensation Analysis: Different disease states may require different compensatory responses, often involving rapid adjustments in either the lungs or kidneys.

Treatment Considerations

  • Correcting imbalances may involve addressing underlying conditions through respiratory support (for acidosis/alkalosis) or renal support/lifestyle modifications (for metabolic disturbances).