M

Water Movement Between Body Fluid Compartments

Isotonic Conditions

  • Normal ECF & ICF are isotonic ⇒ equal osmolarity; cells neither shrink nor swell.

Osmosis Basics

  • Water moves passively from higher water (lower solute) → lower water (higher solute) concentration.
  • Goal: equalize osmolarity between compartments.

Terminology

  • Hypotonic = lower solute concentration.
  • Hypertonic = higher solute concentration.
  • Water shifts from hypotonic → hypertonic compartment.

When ECF Osmolarity INCREASES (↑ [Na^+], dehydration, salty meal)

  • ECF becomes hypertonic to ICF.
  • Water exits cells → ECF; cells shrink.
  • Homeostasis: thirst + ADH retain water ⇒ ECF osmolarity falls; water re-enters cells.
  • Kidney output: small volume, concentrated urine.

When ECF Osmolarity DECREASES (excess plain water intake)

  • ECF becomes hypotonic to ICF.
  • Water enters cells; cells swell.
  • Homeostasis: ↑ blood volume triggers ANP → inhibits ADH & aldosterone ⇒ large volume, dilute urine.

Water Intoxication (Hyponatremia)

  • Cause: rapid intake of plain water > renal excretion (e.g., prolonged sweating replaced only with water, hazing).
  • Results: ↓ ECF [Na^+] ⇒ ECF hypotonic → water shifts into cells.
  • Brain cell swelling → confusion, seizures, coma, possible death.
  • Prevention/Treatment: replace water AND electrolytes (e.g., saline IV, electrolyte drinks).