Fluid & Electrolytes (Clinical Points)
1. Body Fluids
•Total body water ≈ 50–60% of body weight
•Intracellular fluid (ICF): ~2/3 of body fluid (inside cells)
•Extracellular fluid (ECF): ~1/3 (plasma, interstitial, transcellular)
2. Fluid Balance
•Maintained by intake vs output
•Regulated by:
Kidneys
Hormones: ADH, Aldosterone, ANP
Thirst mechanism
3. Movement of Fluids
•Diffusion: movement from high → low concentration
•Osmosis: water moves through semipermeable membrane
•Filtration: movement due to pressure differences
•Active transport: movement against gradient (needs energy)
4. Osmolality & Tonicity
•Isotonic: no net fluid shift (0.9% NS)
•Hypotonic: water moves into cells → swelling
•Hypertonic: water moves out of cells → shrinkage
5. Electrolytes
Electrolytes carry electrical charge and regulate body functions.
Major Electrolytes & Functions
Sodium (Na⁺)
•Normal: 135–145 mEq/L
•Main ECF cation
•Regulates fluid balance, nerve impulses
•↓ Hyponatremia: confusion, seizures
•↑ Hypernatremia: thirst, dry mucosa
Potassium (K⁺)
•Normal: 3.5–5.0 mEq/L
•Main ICF cation
•Important for heart & muscle function
•↓ Hypokalemia: weakness, arrhythmia
•↑ Hyperkalemia: cardiac arrest
Calcium (Ca²⁺)
•Normal: 8.5–10.5 mg/dL
•Bone formation, muscle contraction
•↓ Hypocalcemia: tetany, spasms
•↑ Hypercalcemia: kidney stones, lethargy
Magnesium (Mg²⁺)
•Normal: 1.5–2.5 mEq/L
•Enzyme activity, neuromuscular function
•↓ causes tremors
•↑ causes respiratory depression
Chloride (Cl⁻)
•Normal: 96–106 mEq/L
•Works with sodium for fluid balance
Phosphate (PO₄³⁻)
•Energy production (ATP), bones
•Inverse relationship with calcium