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unit 2 chemistry part -2 Electrolytes – Testing & Levels : Comprehensive Study Notes

Core Concepts: Electrolytes

  • Electrically-charged minerals dissolved in body fluids
    • Exist as ions: cations (+) and anions (–)
    • Control fluid distribution, acid–base balance, membrane potentials, muscle & nerve excitability, enzyme activity
  • Major laboratory focus: Na^+,\;K^+,\;Cl^-,\;HCO_3^- because alterations in one usually affect the others
  • Body must maintain overall electrical neutrality ⇒ sum of all cations = sum of all anions
  • Principal regulating organs
    • Kidneys – filtration, selective re-absorption & secretion
    • Lungs – CO$_2$ exchange (acid–base impact)

Sodium (Na$^+$)

  • Major extracellular cation (highest [ ] outside cells)
  • Key roles
    • Maintains osmotic pressure & blood volume
    • Acid–base balance via relationship with Cl^- & HCO_3^-
  • Normal serum reference 135\text{–}145\;mmol/L
    Critical values:
  • Dysnatremias
    • Hyponatremia
    • Causes: severe polyuria, metabolic acidosis, Addison disease, diarrhea, some renal tubular d/o
    • Hypernatremia
    • Causes: dehydration (primary H$_2$O loss), Cushing syndrome, brain injury, excess Na$^+$ therapy, insulin-treated diabetic coma
  • Regulation mechanisms
    • Water intake driven by thirst (plasma osmolality sensor in hypothalamus)
    • Water excretion governed by ADH (vasopressin)
    • Na$^+$ excretion influenced by aldosterone, angiotensin II, atrial natriuretic peptide (volume sensors)
  • Body-fluid content examples
    • CSF 138\text{–}150\;mmol/L
    • Urine 24 h: 40\text{–}220\;mmol/L (diet dependent; diurnal ↓ at night)

Potassium (K$^+$)

  • Major intracellular cation (≈100\;mmol/L inside vs 4\text{–}5\;mmol/L outside)
  • Essential for cardiac & skeletal muscle contraction, nerve impulse transmission
  • Normal serum reference 3.5\text{–}5.0\;mmol/L
    Critical values:
  • Dyskalemias
    • Hypokalemia – prolonged vomiting/diarrhea, inadequate intake; kidneys still excrete K$^+$ despite deficiency
    • Hyperkalemia – renal failure/obstruction, adrenal insufficiency, acidosis; removed via dialysis if severe
  • Renal handling
    • Filtered K$^+$ almost completely re-absorbed in proximal tubule
    • Secreted in distal tubule in exchange for Na$^+$ under aldosterone control
    • Factors influencing distal secretion: Na$^+$ & K$^+$ intake, tubular flow rate, mineralocorticoids, acid–base status
  • Body-fluid content examples
    • Urine 24 h: 25\text{–}125\;mmol/L (diet dependent)
    • CSF ≈70\% of plasma value

Sodium–Potassium Cellular Relationship

  • Na$^+$–K$^+$ ATPase pump maintains gradients
    • Intracellular: K^+ \approx 100\;mmol/L,\;Na^+ \approx 10\;mmol/L
    • Extracellular: K^+ \approx 5\;mmol/L,\;Na^+ \approx 150\;mmol/L
    • Active transport essential for excitability & volume control

Chloride (Cl$^-$)

  • Major extracellular anion; counter-balances Na$^+$
  • Normal reference 98\text{–}108\;mmol/L
  • Physiology
    • Participates in osmotic pressure & electrical neutrality
    • Reciprocal relationship with HCO_3^- (↓ one ⇒ ↑ other)
    • Involved in CO$_2$ transport – “chloride shift” between plasma & RBCs during oxygenation/de-oxygenation
  • Distribution: ⅔ plasma, ⅓ RBC → assay plasma/serum only
  • Abnormal values
    • Hyperchloremia – dehydration, ↓ renal blood flow (CHF), excessive Cl$^-$ intake/therapy
    • Hypochloremia – salt loss (chronic pyelonephritis)
  • Pre-analytical caveat – separate cells promptly; standing whole blood allows intracellular Cl$^-$ ↔ plasma exchange

Bicarbonate (HCO$_3^-$)

  • Second most abundant extracellular anion
  • Formed via dissociation of carbonic acid; reversible conversion with CO$2$ + H$2$O
  • Normal reference 22\text{–}30\;mmol/L
  • Central to buffering system; evaluated with pH & blood gases (arterial preferred)

Calcium (Ca$^{2+}$)

  • Functions – myocardial & skeletal muscle contraction, coagulation, bone mineralization
  • Three regulators: PTH (↑), vitamin D (↑), calcitonin (↓)
  • Specimen notes
    • Total Ca$^{2+}$ – serum preferred
    • Ionized Ca$^{2+}$ – keep tube uncapped until analysis; CO$_2$ loss ↑ pH → alters protein binding
  • Reference (total) 2.10\text{–}2.60\;mmol/L
    Critical
  • Low ionized Ca$^{2+}$ ⇒ tetany, impaired cardiac function

Magnesium (Mg$^{2+}$)

  • 4th most abundant cation; 2nd intracellular
  • Roles – cofactor for ATP reactions, bone structure, neuromuscular stability, glycemic control
  • Distribution – high in bone & muscle; <1 % in serum/RBCs
  • Dietary deficiency common (esp. females) → supplements may be required
  • Reference 0.65\text{–}1.05\;mmol/L
    Critical
  • Measurement (serum)
    • Colorimetric methods: calmagite, formazan dye, methylthymol blue

Phosphate (PO$_4^{3-}$)

  • Mentioned as part of unmeasured anions influencing anion gap; essential for bone, cell growth/repair
  • Reference 0.80\text{–}1.50\;mmol/L
    Critical

Osmolality

  • Definition – number of dissolved particles per kg of water
  • Normal serum 280\text{–}300\;mOsm/kg; urine varies widely & is more accurate than specific gravity for concentration status
  • Significance
    • ↓ serum osmolality ⇒ excess water (over-hydration, edema)
    • ↑ serum osmolality ⇒ water deficit (dehydration); hypothalamus triggers thirst & ADH release
  • Water moves freely across membranes to equalize osmotic gradients → reflects total body hydration (homeostasis)
  • Testing
    • Measured by freezing-point depression osmometer (e.g., OsmoTECH)
    • Calculated osmolality (common formula)
      Calc\;Osm = 2(Na^+) + \frac{Glucose}{18} + \frac{BUN}{2.8} (units in mg/dL)
    • Osmolal gap = measured – calculated; normal 0\text{–}10\;mOsm/kg
    • ↑ gap ⇒ presence of unmeasured solutes (ketones, ethanol/methanol, ethylene glycol, mannitol)

Anion Gap (AG)

  • Purpose – checks analytical accuracy, screens acid–base disorders, infers unmeasured ions
  • Equations
    AG = (Na^+ + K^+) - (Cl^- + HCO3^-) Reference 10\text{–}20\;mmol/L AG = Na^+ - (Cl^- + HCO3^-) Reference 5\text{–}15\;mmol/L (without K$^+$)
  • Interpretation
    • ↑ AG – accumulation of unmeasured anions: ketoacidosis, lactic acidosis, renal failure (sulfates/phosphates), salicylate or methanol poisoning, high plasma proteins
    • ↓ AG – rare; may indicate lab error, hypoalbuminemia, or paraproteinemia
    • Always correlate with clinical picture & repeat if unexpected
  • Healthy blood pH 7.35\text{–}7.45

Reference Ranges & Critical Values Summary

  • Sodium: 135\text{–}145 (Crit
  • Potassium: 3.5\text{–}5.0 (Crit
  • Chloride: 98\text{–}108
  • Bicarbonate: 22\text{–}30
  • Magnesium: 0.65\text{–}1.05 (Crit
  • Calcium (total): 2.10\text{–}2.60 (Crit
  • Phosphate: 0.80\text{–}1.50 (Crit

Specimen Collection & Handling

  • General
    • Centrifuge within 2 h; separate serum/plasma from cells promptly (unless gel barrier)
  • Sodium
    • Specimens: lithium-heparin plasma, serum, urine (24 h), CSF
    • Avoid gross hemolysis/lipemia; store/transport 2–8 °C
    • DO NOT use sodium-heparin tubes (false ↑ Na$^+$)
  • Potassium
    • Heparinized plasma or serum (SST); anticoagulant must be K$^+$-free
    • Pre-analytical errors causing false ↑ K$^+$:
    • Hemolysis, delayed or repeat centrifugation, removing stoppers pre-spin, tube → tube transfer, refrigeration pre-analysis, IV contamination
  • Chloride
    • Lithium or sodium heparin plasma; SST acceptable
  • Bicarbonate
    • Arterial or venous lithium/sodium-heparin plasma or serum
    • Analyze immediately after uncapping; pH & HCO$3^-$ change rapidly with CO$2$ loss

Quantitative Measurement Methods

  • Core electrolyte profile (Na$^+$, K$^+$, Cl$^-$, HCO$_3^-$) – Ion-Selective Electrode (ISE) potentiometry
    • Na$^+$: glass ion-exchange membrane
    • K$^+$: valinomycin carrier membrane
    • Cl$^-$: Ag/AgCl or Ag$_2$S ISE
    • HCO$3^-$: measured as total CO$2$; sample acidified → releases gaseous CO$_2$
  • Osmolality – freezing point depression osmometer
  • Magnesium – colorimetric dyes (calmagite, formazan, methylthymol blue)

Clinical & Practical Connections

  • Heart failure, liver ascites, nephrotic syndrome → Na$^+$ retention
  • Renal tubular acidosis & metabolic/respiratory acid-base disorders affect K$^+$ secretion/retention
  • Dialysis therapy partly aimed at K$^+$ removal
  • Diuretics, IV fluids, endocrine disorders (Addison, Cushing) profoundly influence electrolyte status
  • Timely & accurate electrolyte testing guides emergent management (e.g., hyperkalemic arrhythmia, hyponatremic seizures)

Ethical / Institutional Note

  • Content ©2022 Anderson College MLA/T Program; sharing beyond Anderson College educational use is prohibited.

Quick Study Checklist

  • Memorize reference & critical values (see table above)
  • Know hormone regulators: ADH, aldosterone, angiotensin II, ANP, PTH, vitamin D, calcitonin
  • Practice calculations: anion gap, calculated osmolality, osmolal gap
  • Recall specimen pitfalls (e.g., sodium-heparin tube, hemolysis → ↑K$^+$)
  • Understand pathophysiologic patterns (dehydration ↑Na$^+$ & osmolality; renal failure ↑K$^+$; ketoacidosis ↑AG)