Mechanisms of Fluid and Electrolyte Regulation Notes

Electrolytes

  • Molecules that dissociate into ions in H2O (e.g., NaCl → Na+ & Cl-).
  • Cations: +ve ions (e.g., Na+, K+, Ca2+, Mg2+).
  • Anions: -ve ions (e.g., Cl-, HCO3-).
  • ICF: K+; PO43−.
  • ECF: Na+; Cl-.
  • Maintained via active transport using Na+/K+ pump and ATP.

Electrolyte Composition

  • Plasma: Na+ (135-145 mmol/L), K+ (3.5-5.0 mmol/L).

Electrolyte Imbalances

  • Sodium
    • Hypernatremia (>145 mEq/L): Thirst, dry skin, reduced blood volume.
    • Hyponatremia (<135 mEq/L): Disturbed CNS function.
  • Potassium
    • Hyperkalemia (>5.0 mEq/L): Cardiac arrhythmias, muscle spasms.
    • Hypokalemia (<3.5 mEq/L): Muscular weakness and paralysis.
  • Calcium
    • Hypercalcemia (>5.3 mEq/L): Confusion, muscle pain, kidney stones.
    • Hypocalcemia (<4.3 mEq/L): Muscle spasms, convulsions.

Hormone Regulation

  • Antidiuretic Hormone (ADH)
    • Regulates fluid volume, reabsorbs H2O, vasoconstriction.
  • Aldosterone
    • Maintains ECF volume, reabsorbs Na+.
  • Natriuretic Peptides
    • Promote excretion of Na+ + H2O, decrease blood volume and BP; released in response to ↑ atrial pressure.
  • Renin-Angiotensin System
    • ↑ release of ADH + aldosterone, causes vasoconstriction; released in response to ↓ blood pressure.

Regulation of Water Balance

  • Hypothalamic-pituitary, renal, cardiac, adrenal cortical, and gastrointestinal regulation.

Regulation of Sodium

  • ADH secretion increases with increased Na+ levels in ECF and decreases with decreased Na+ levels in ECF.

Renal Control of BP

  • Decreased systemic blood pressure leads to renin release.
  • Renin leads to the formation of Angiotensin II.
  • Angiotensin II causes vasoconstriction and increased aldosterone secretion.

Disturbances in Fluid Balance

  • Volume deficit (hypovolaemia).
  • Volume excess (hypervolaemia).
  • Dehydration (loss of pure water alone).

Acid-Base Balance

  • ↑ H+ - Acidosis.
  • ↓ H+ - Alkalosis.
  • pH - 1 to 14.