Acid-Base Balance

Acid-Base Balance

Introduction

  • Acid-base balance is one of the body’s most important homeostatic mechanisms.

  • It refers to the regulation of the hydrogen ion concentration of body fluids.

  • Numerous physiological processes depend on precise regulation of hydrogen ion concentrations.

  • Slight deviations from the normal pH range can lead to serious, potentially fatal health consequences.

Review of pH Concept

  • pH: A measure of the acidity or alkalinity of a solution, defined as the negative logarithm of the hydrogen ion concentration.
    ext{pH} = - ext{log}[ ext{H}^+]

  • Neutral pH: A pH of 7 indicates neutrality.

  • Acidosis: Describes an arterial blood pH less than 7.35.

  • Alkalosis: Describes an arterial blood pH greater than 7.45.

pH Range

pH Range

H⁺ Ion Concentration (moles/L)

pH Value

Acidic

[H⁺] > [OH⁻]

< 7

Neutral

[H⁺] = [OH⁻]

7

Basic (alkaline)

[H⁺] < [OH⁻]

> 7

Specific pH Values

  • Acidic Solutions:

    • pH 1: Hydrochloric acid

    • pH 2: Gastric fluid

    • pH 3: Lemon juice, vinegar, wine, orange juice, beer, tomatoes

    • pH 4: Coffee

    • pH 5: Skin film, milk, urine

  • Neutral Solutions:

    • pH 7: Distilled water, blood

  • Basic Solutions:

    • pH 8: Egg white

    • pH 9: Baking soda, pancreatic juice, borax

    • pH 10: Great Salt Lake, milk of magnesia

    • pH 11: Household ammonia

    • pH 12: Oven cleaner, lye, caustic soda

    • pH 14: Sodium hydroxide

Sources of pH-Influencing Chemicals

  • Carbonic Acid: Formed by the dissolving of carbon dioxide in water.

  • Lactic Acid: Produced during anaerobic metabolism.

  • Sulfuric Acid: Results from the oxidation of sulfur-containing amino acids.

  • Phosphoric Acid: Accumulates from the breakdown of certain phosphoproteins and ribonucleotides.

  • Acidic Ketone Bodies: Formed during fatty acid metabolism.

Types of pH Control Mechanisms

  1. Chemical Control:

    • Uses rapid-acting buffers to stabilize pH changes caused by the addition of acids or bases.

  2. Physiological Control:

    • Secondary defense mechanism involving adjustments in respiratory rate and depth to mitigate pH shifts.

  3. Homeostatic Mechanism:

    • Comprised of buffers, respiratory adjustments, and renal excretion of acids and bases to maintain equilibrium.

Buffers

  • Definition: Substances that minimize changes in pH by neutralizing excess acids or bases.

  • Composition: Typically consist of a weak acid (or its salt) combined with a basic salt of that acid.

  • Buffer Pairs Present in Body Fluids:

    • Bicarbonate pairs

    • Plasma and protein pairs

    • Hemoglobin pairs

    • Phosphate buffer pairs

Explanation of Respiratory Mechanisms

  • The amount of carbon dioxide (CO₂) in the blood directly relates to carbonic acid levels and, therefore, to the concentration of hydrogen ions (H⁺).

  • Increased respiration leads to reduced CO₂ levels in the blood, subsequently lowering carbonic acid and hydrogen ion concentrations.

  • Decreased respiration results in CO₂ retention, leading to increased carbonic acid and hydrogen ions.

Respiratory Adjustment of Blood pH

  • Changes in arterial blood CO₂ levels stimulate the respiratory center, which adjusts the rate and/or depth of respiration to maintain pH homeostasis.

Principles Relating Respiration to pH

  • Acidosis triggers hyperventilation, serving as a compensatory response to reduce CO₂ and increase pH.

    • Prolonged hyperventilation can lead to alkalosis by elevating blood pH excessively.

  • Alkalosis leads to hypoventilation, allowing CO₂ levels to rise and pH to decrease back to normal.

    • Prolonged hypoventilation may result in acidosis.

Arterial Blood Gas Analysis

  • Key Measurements:

    • Oxygen partial pressure (Po₂)

    • Oxygen saturation of hemoglobin (%So₂)

    • Blood pH

    • Concentration of bicarbonate ions (HCO₃⁻)

    • Carbon dioxide partial pressure (Pco₂)

Urinary Mechanisms That Control pH

  • The kidneys play a crucial role in acid-base balance by:

    • Eliminating excess hydrogen ions (H⁺) when pH is low (acidic).

    • Reabsorbing sodium ions when the pH tends toward the acidic side.

    • Reducing the elimination of H⁺ and sodium absorption when the pH is high (alkaline).

Regulating pH of Urine and Blood

  • The kidneys regulate pH by:

    • The distal tubules and collecting ducts secreting H⁺ ions and reabsorbing basic ions.

    • Renal tubes excreting hydrogen or potassium ions while reabsorbing sodium ions.

    • Excreting ammonia, which combines with H⁺ to form ammonium ions.

Acid-Base Imbalances

Types of Imbalances

  1. Metabolic Acidosis:

    • Characterized by large amounts of acid entering the blood, altering the bicarbonate-to-carbonic acid (BB/CA) ratio.

  2. Metabolic Alkalosis:

    • Can arise from improper use of antacids, leading to excessive base levels.

  3. Respiratory Acidosis:

    • Results from conditions such as pneumonia or emphysema, which cause carbon dioxide retention in the blood.

  4. Respiratory Alkalosis:

    • May occur due to hyperventilation triggered by fever or mental health conditions.

Conclusion

  • Maintaining acid-base balance is vital to preserve homeostasis and ensure proper physiological function. Disruptions in this balance can lead to critical health effects, necessitating immediate medical attention.