Acid- Base Balance

Acid-Base Balance Overview

  • Normal pH range for extracellular fluid: 7.35 to 7.45

  • Mechanisms of pH balance:

    • Generate, buffer, and eliminate acids and bases.

Definitions

Acid

  • Definition: Molecule that releases hydrogen (H+).

  • Example: Hydrochloric acid (HCl) dissociates in water to form H+ and Cl-.

Base

  • Definition: Ion or molecule that accepts or combines with hydrogen.

  • Example: Bicarbonate (HCO3-) combines with H+ to form carbonic acid (H2CO3), a weak acid.

pH Regulation Mechanisms

  1. Chemical Buffer System

    • Combines with excessive acids or bases to prevent large changes in pH.

  2. Lungs

    • Control elimination of CO2.

  3. Kidneys

    • Eliminate H+ (acidic) and reabsorb/generate new HCO3- (bicarbonate).

Mnemonics for pH Variations

  • ROAM:

    • Respiratory: Opposite

    • Metabolic: Equal

  • pH Standards:

    • Acidosis: pH < 7.35

    • Alkalosis: pH > 7.45

Details on Respiratory and Metabolic Functions

Respiratory System

  • Opposite Relation: High pH → Low CO2 (Alkalosis)

  • Indicators of Acidosis: Low pH → High CO2 (Hypoventilation)

  • Normal Respiratory Rate: 12 to 20 breaths per minute.

Metabolic System

  • Equal Relation:

    • High pH → High HCO3- (Alkalosis)

    • Low pH → Low HCO3- (Acidosis)

  • Bicarbonate Normal Range: 22 to 26 mEq/L

Arterial Blood Gases (ABG) Collection

  • Collected from radial artery using a needle at a 90-degree angle.

  • Differentiation: Arterial blood is bright red; venous blood is dark/purple.

Acid-Base Disorders

Respiratory Acidosis

  • Causes: Drug overdose, lung diseases, impaired ventilation.

  • Presentation:

    • Low pH, high CO2, normal HCO3- (uncompensated) or high HCO3- (compensated).

    • Symptoms: Headaches, confusion, drowsiness.

Respiratory Alkalosis

  • Causes: Panic attacks, hypoxia, drug reactions.

  • Presentation:

    • High pH, low CO2, normal HCO3-.

    • Symptoms: Dizziness, lightheadedness, seizures.

Metabolic Acidosis

  • Causes: Excessive production of lactic acid, renal failure, diabetic ketoacidosis.

  • Presentation:

    • Low pH, low HCO3-, normal/high CO2.

    • Symptoms: Nausea, confusion, lethargy.

Metabolic Alkalosis

  • Causes: Ingestion of antacids, GI loss of acid (vomiting), diuretics.

  • Presentation:

    • High pH, high HCO3-, normal CO2.

    • Symptoms: Confusion, hypokalemia, seizures.

Compensation Mechanisms

  • Respiratory System: Rapid response (within 30 minutes) for pH adjustment; effective but only 50-75% as a buffer.

  • Kidneys: Slower response (5-10 minutes to begin; 3-5 days to complete).

  • Compensation Levels:

    • Uncompensated: One parameter normal, one abnormal.

    • Partially Compensated: Both parameters abnormal, pH still abnormal.

    • Fully Compensated: Normal pH, both parameters abnormal.

    • Determining Compensation:

      • pH normal = fully compensated.

      • pH abnormal = partially compensated.

      • One normal parameter indicates uncompensated.

Key Values to Remember

  • Normal CO2: 35 to 45 mmHg

  • Normal HCO3-: 22 to 26 mEq/L

Summary

  • Strong emphasis on memorizing the acid-base ranges, mnemonics, and clinical presentations associated with different disorders.

  • Students encouraged to practice identifying and interpreting ABG values and understanding the primary disorder and compensation status.

  • Importance of understanding these concepts for nursing practice and exam preparation.