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Fluid Homeostasis

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Pathophysiology

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Figure 18-1.
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Physiological control of ECF volume & tonicity.

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  • Maintain intravascular volume to facilitate tissue perfusion & oxygenation; maintain osmotic equilibrium → tonicity 275–290mOsmol/kg H2O; intracellular = extracellular tonicity
  • Distribution of total body water (TBW): % body weight varies with age
    • Adult ♂ = 50–60%; adult ♀ = 45–55% (↓ with age); TBW = 0.5 × weight (kg) ♀; 0.6 × weight (kg) ♂
  • Intracellular compartment fluid (ICF) = 2/3 of TBW
  • Extracellular compartment fluid (ECF) = 1/3 of TBW; interstitial fluid = ¾ ECF ∼25% of TBW; intravascular fluid = ¼ ECF ∼8% of TBW
  • Electrolyte composition of fluid compartments
    • ECF: Na+, Cl, HCO3, albumin → Na+ primary determinant of extracellular osmolality albumin provides oncotic pressure to retain intravascular volume
    • ICF: K+, Mg++, PO4 → K+ primary determinant of intracellular osmolality; physiologic control of water & sodium homeostasis

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Table Graphic Jump Location
Table 18-1 Composition & Clinical Use of Common IV Fluids
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Hyponatremia

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Pathophysiology

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  • Excess free water relative to Na+ in ECF; absolute levels of ECF water & Na+ may be ↑, ↓, or normal depending on etiology
  • Pathophysiology varies by etiology; assess measured or calculated serum osmolality → POsm = 2[Na+] + [BUN]/2.8 + [Glucose]/18; ECF volume status; measured urine [Na+] &/or measured urine osmolality
    • ↑ urine [Na+] (>20mEq/L) implicates the kidney as etiologic site (innapropriate Na+ excretion in hyponatremia); ↓ urine [Na+] indicates an appropriate renal response to extrarenal cause
  • Symptoms result from cerebral edema; severity of edema depends on rate & extent of Na+
    • Rapid ↓ (<48h) or large ↓, brain tissue remains hypertonic relative to plasma → draws water from ECF; sx of acute cerebral edema → severe HA, stupor, obtundation, coma, seizures; milder cases → confusion, dizziness, nausea, lethargy
    • Gradual Na+ ↓ allows brain tissue time to equalize ICF-ECF tonicity gradient by organic osmole production (ex. taurine); may be asymptomatic

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Diagnosis & Evaluation

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