Core Content: Hyponatremia

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Breaking down Physiology

  • Diffusion - the passive movement of a substance down its concentration gradient (from an area of high concentration to an area of low)

    • think: adding food coloring to glass of water

  • Osmosis - the diffusion of water from one area to another down its concentration gradient

    • With osmosis, unlike diffusion, you have to think about SOLUTES (dissolved particles)

    • Solution with a ton of solute has a lower concentration of water. So water will want to move from an area of high concentration of WATER to low concentration of WATER (high concentration of SOLUTE)

  • Osmolarity/Osmolality - “How many dissolved solute particles do I have?”

    • Osmolarity - solute particles per liter of solution

    • Osmolality - solute particles per kilogram of solution

    • We more often use osmolality

  • Tonicty - refers to how the osmotic concentration of a solution (or in the case of the human body, interstitial fluid) can affect cell volume

    • Three possibilities:

      • Hypotonic solution = LESS solute than your average cell

      • Isotonic solution = SAME amount of solute as your average cell

      • Hypertonic solution = MORE solute than your average cell

    • Example: if you place cell in a hypotonic solution, it will swell up like a balloon; if you place cell in an isotonic solution, nothing will happen; if you put cell in hypertonic solution, it will shrink like raisin

Hyponatremia

  • Normal sodium: 135-145

  • Sodium is the major extracellular cation while chloride is the major extracellular anion

    • Because it the MAJOR cation, plasma sodium and plasma osmolality usually go hand in hand (low sodium = low osmolality) which is why by calculating osmolality you can know hyponatremia is REAL

Steps to Diagnosing Hyponatremia

  1. Assess Your Patient

  2. Recheck Lab

  3. Check Plasma Osmolality

  4. Check Urine Osmolality

  5. Check Volume Status

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