Electrolytes, Fluids, and Acid–Base – Study Notes

Body Fluid Compartments

• Total Body Water (TBW) = ~60% body weight.

  • Intracellular Fluid (ICF) = 40%.

  • Extracellular Fluid (ECF) = 20% (Interstitial 15% + Plasma 5%).

• Example: 70 kg adult → TBW ≈ 42 L (ICF 28 L, ECF 14 L).

Plasma Expansion with IV Fluids

• Normal saline distributes in ECF only.

• Plasma:Interstitial ratio = 1:3.

• To increase plasma volume by 1 L → need 4 L saline infusion.

Electrolyte Distribution

Intracellular (ICF) – “P rule”

• Potassium (K⁺)

• Phosphate (PO₄³⁻)

• Proteins

• (+ Magnesium also inside cells).

Extracellular (ECF)

• Na⁺ – major cation.

• Cl⁻ & HCO₃⁻ – major anions.

Key Transport Systems

• Na⁺/K⁺ ATPase: pumps 3 Na⁺ out, 2 K⁺ in → net +1 out.

  • Maintains: Na⁺ high outside, K⁺ high inside.

• Cl⁻ follows Na⁺ (electrochemical balance).

• Phosphate stays inside for ATP production, glycolysis, DNA/RNA.

Clinical Correlations

Refeeding Syndrome

• Malnourished patient → refeeding → insulin surge.

• Rapid shift of phosphate into cells → hypophosphatemia.

• Consequences: ↓ ATP → organ failure, fatal if unrecognized.

Electrolyte Functions & Disorders

• Na⁺ (sodium)

  • Function: fluid balance, CNS effects.

  • Disturbance → brain cell swelling/shrinking → mental status changes.

• K⁺ (potassium / kalium)

  • Function: resting membrane potential.

  • Disturbance → cardiac arrhythmias.

  • Mnemonic: Kalium → Cardiac.

• Ca²⁺ (calcium)

  • Function: excitability regulator.

  • Hypocalcemia → ↑ Na⁺ permeability → hyperexcitability (tetany, seizures).

  • Hypercalcemia → ↓ excitability (fatigue, constipation).

  • Mnemonic: Calcium = Contra-excitability.

• Mg²⁺ (magnesium)

  • Stored in bone; regulates Na⁺/K⁺ transport.

  • Deficiency → tremor, delirium, arrhythmias, prolonged QT.

• Cl⁻ (chloride)

  • Major ECF anion; follows sodium.

• Phosphate (PO₄³⁻)

  • Intracellular anion → ATP, DNA, RNA, bone.

  • Hypophosphatemia → ↓ ATP, infection, bleeding.

  • Hyperphosphatemia → binds Ca²⁺ → tissue deposits.

Acid–Base Basics

• Metabolism → acids (lactic acid, ketoacids, CO₂).

• CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

  • CO₂ = volatile acid → lungs excrete.

  • HCO₃⁻ = alkaline reserve → kidneys regulate.

• ICF = more acidic (site of metabolism).

• ECF = more alkaline (buffered by bicarbonate).

• Normal pH = 7.4 (slightly alkaline).

Lab Measurement Units

• Concentration = Amount ÷ Volume.

• Electrolytes measured in:

  • mEq/L = milliequivalents per liter.

  • mmol/L (for monovalent ions, values match).

  • For divalent ions (e.g., Mg²⁺, Ca²⁺) → mEq/L = 2 × mmol/L.

Normal Serum Values – “Rule of 4”

• Na⁺ = 140 mEq/L

• Cl⁻ = 104 mEq/L

• K⁺ = 4 mEq/L

• HCO₃⁻ = 24 mEq/L

• pH = 7.4

Law of Electro-Neutrality

• In any solution, total cations = total anions.

• Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).

  • Represents “unmeasured anions” (lactate, ketones, proteins, etc.).

Osmolality

• ECF osmolality = ICF osmolality (~290 mOsm/kg).

• Maintains fluid balance; prevents constant water shifts.

• Osmosis = water moves across semipermeable membrane to equalize solute concentration.