Water and Electrolyte Balance

Water and Electrolyte Balance of Body Fluids

I. Overview

  • Water as universal solvent:

    • Medium for biochemical reactions sustaining life.

    • Major contributor to body weight: body as organized solution of proteins, minerals, and fat in water.

    • Distribution across distinct body compartments, regulated finely.

  • Role of Electrolytes:

    • Essential cations: Na+, K+, Ca2+, Mg2+, Cl−, HCO3−.

    • Functions:

      • Maintains resting membrane potential.

      • Facilitates neurotransmission and muscle contraction.

      • Regulates myocardial electrical activity and signal transduction.

  • Importance of balance:

    • Electrolyte imbalance related to several diseases; prevalent in hospitalized patients.

    • Common electrolytes discussed: sodium and potassium.

II. Water Homeostasis

  • Body water composition:

    • 50% - 60% of body weight depending on body fat.

    • Adult males: ~60% water contributes to weight.

    • Breakdown of remaining body weight:

      • Proteins ~18%, minerals ~7%, fats ~15%.

  • A. Body Fluid Compartments:

    • Total Body Water (TBW):

      • Example: 70-kg male with 42 L TBW (60% of body weight).

      • Distribution: intracellular (ICF) and extracellular (ECF).

    • ECF components:

      • Interstitial fluid (ISF) ~10.5 L (tissue fluid), plasma ~3.5 L (from blood).

      • Other minor components: lymph, transcellular fluids (cerebrospinal, synovial, and aqueous humor).

    • Differences in electrolyte/protein compositions across compartments.

III. Water Intake and Removal

  • Daily water balance:

    • Intake: ~2300 ml (2100 ml from ingestion, 200 ml from metabolic water).

    • Removal:

      • Urine ~1400 ml/day.

      • Insensible loss (skin, lungs) ~700 ml/day.

      • Fecal loss ~200 ml/day.

    • Physiological/pathological conditions affecting balance:

      • Increased insensible loss due to exercise, fever, hyperventilation.

      • Diarrhea causing increased fecal loss.

  • Regulation mechanism:

    • Hypothalamic receptors serve as signals for fluid management.

IV. Water Balance and Osmolality

  • Osmolality:

    • Defined as the number of osmotically active particles/kg solvent (water).

    • Plasma osmolality formula:

      • Plasma osmolality = 2Na+ + Glucose + Urea.

      • Normal range: 275–295 mOsm/kg.

    • Hyperosmolality triggers osmoreceptor activation in hypothalamus leading to vasopressin (AVP) release.

      • AVP enhances water reabsorption in renal collecting ducts, increases thirst.

    • Hypovolemia and low blood pressure also stimulate osmoreceptors.

V. Vasopressin (AVP)

  • Characteristics:

    • Nonapeptide hormone, produced in hypothalamus.

    • Stored and secreted by posterior pituitary.

    • Actions mediated by V1 (vascular smooth muscle contraction) and V2 (renal water reabsorption) receptors.

  • Mechanism of action:

    • V2 receptor activation leads to aquaporin insertion into nephron cell membranes, increasing water reabsorption, thus reducing ECF osmolality and increasing ECF volume.

    • Disorders of AVP production/action lead to diabetes insipidus (increased urine volume) or SIADH (excessive water retention).

VI. Sodium and Potassium Functions

  • Sodium (Na+):

    • Principal extracellular cation; critical for water balance and physiological processes.

    • Na+-K+ ATPase functions:

      • Maintains membrane potential and drives secondary active transport.

  • Potassium (K+):

    • Major intracellular cation; vital for neuromuscular function.

    • Maintaining balance is essential; disturbances lead to dysfunctions and clinical manifestations.

VII. Disorders of Sodium and Potassium

  • Sodium disorders:

    • Hyponatremia (less than 136 mmol/L) can lead to cerebral edema, neurological symptoms.

    • Classification based on fluid status: hypovolemic, euvolemic, hypervolemic.

  • Hypernatremia (more than 145 mmol/L) causes CNS issues.

    • Classified based on fluid status too.

  • Potassium disorders:

    • Hyperkalemia (>5.0 mmol/L) vs. hypokalemia (<3.5 mmol/L) associated with cardiac risks.

VIII. Estimation of Sodium and Potassium

  • Clinical estimation uses ion-selective electrodes based on poteniometry principles.

  • Importance in monitoring and managing electrolyte imbalances.

IX. Chapter Summary

  • Water is critical to body weight and biochemical reactions.

  • Electrolyte distributions affect physiological functions and body balance mechanisms.

  • Sodium-Potassium ATPase is crucial for maintaining body homeostasis.

  • Diagnosis and management of electrolyte disorders are essential for patient care.