Overview
Electrolytes are charged particles (ions) dissolved in the intravascular, interstitial and intracellular fluid compartments of the body. Sodium, potassium, chloride, calcium and magnesium each perform distinct physiological roles, and even small shifts outside their normal ranges can be life-threatening.
This lesson covers the functions and clinical abnormalities of the five major electrolytes, along with the basics of arterial blood gas (ABG) testing — how samples are collected, transported and analyzed for PO2, PCO2, pH, bicarbonate and hematocrit.
Learning Objectives
After this lesson you will be able to…- List the major electrolytes and describe their physiological functions
- Differentiate hyponatremia from hypernatremia and identify their causes
- Explain the causes and dangers of hypokalemia and hyperkalemia
- Describe the functions and abnormalities of chloride, calcium and magnesium
- Outline correct arterial blood gas sample collection and common sources of error
Clinical Story
Why This MattersA 68-year-old man on long-term diuretic therapy is brought to the emergency department confused and weak. His serum potassium returns at 2.3 mEq/L — dangerously low. The medical team immediately requests cardiac monitoring, because at this level the heart's electrical conduction can fail. The laboratory's rapid, accurate electrolyte panel is the difference between a controlled correction and a cardiac arrest.
Core Concepts
Sodium is the most abundant electrolyte in the body and, in general, sodium balance defines water balance. Its functions include:
- Transmission and conduction of nerve impulses
- Maintaining the osmolality of vascular fluids
- Regulation of overall body fluid levels
- The sodium pump — Na+ shifts into cells while K+ shifts out
- Assisting acid–base balance by combining with Cl⁻ or HCO₃⁻
Hyponatremia (excessive Na loss or H₂O gain) is caused by prolonged diuretic therapy, insufficient Na intake, GI losses (suctioning, laxatives, vomiting), hypotonic fluid administration, or alcoholism. Hypernatremia (excess H₂O loss or Na retention) is caused by vomiting/diarrhea, inadequate antidiuretic hormone, or major burns, and can be fatal if untreated.
About 97% of the body's potassium is intracellular; normal extracellular K+ is 3.5–5.3 mEq/L. Functions include normal membrane excitability, nerve impulse conduction, muscle contraction, enzyme action, and acid–base maintenance. A serum K+ below 2.5 or above 7.0 mEq/L can cause cardiac arrest.
Hypokalemia results from prolonged diuretic therapy, inadequate intake, gastric suctioning/laxatives/vomiting, excess insulin, or hepatic disease — levels below 3 mEq/L produce marked neuromuscular symptoms. Hyperkalemia results from shock, severe hemolysis, tumor lysis, burns, renal failure, mineralocorticoid deficiency, potassium-sparing diuretics, acidosis, or artifactual hemolysis of the specimen — symptoms include muscle weakness, characteristic EKG changes and cardiac arrest; levels above 10 mEq/L are lethal.
Chloride (expected 118–132 mEq/L in serum) is the major extracellular anion, important for water distribution, osmotic pressure and acid–base balance. Hypochloremia occurs with fluid volume expansion, renal disease, metabolic acidosis (e.g. DKA), SIADH, or protracted vomiting. Hyperchloremia occurs with dehydration, renal tubular acidosis, bicarbonate loss (diarrhea), mineralocorticoid excess, or diabetes insipidus.
Calcium combines with phosphorus to form bone and tooth mineral salts and promotes nerve impulse transmission and muscle contraction/relaxation. Hypocalcemia (serum Ca²⁺ < 8.5 mg/dL) causes skeletal and neuromuscular abnormalities, impaired clotting and EKG changes. Hypercalcemia is often seen with malignancy, excessive vitamin D or antacid intake, prolonged immobility, or thiazide diuretics.
Magnesium plays a role in carbohydrate and protein metabolism, intracellular energy storage/use, and neural transmission, and is important for heart, nerve and muscle function.
Laboratory Principle
Arterial blood gas testing exploits the fact that dissolved gases (O₂ and CO₂), pH and bicarbonate in arterial blood accurately reflect the body's respiratory and metabolic status. Because gas concentrations change rapidly with metabolism and exposure to air, the sample must be drawn anaerobically from an artery, kept on ice, and analyzed within 10–15 minutes to give an accurate PO2, PCO2, pH, hemoglobin saturation, bicarbonate and hematocrit result.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Sodium/Potassium/Chloride Calibrators | Multi-level aqueous standard | Calibrate ion-selective electrodes | 2–8°C, protect from evaporation |
| Blood Gas Quality Control (Level 1–3) | Tonometered aqueous control at low/normal/high PCO2-PO2 | Verify blood gas analyzer accuracy | 2–8°C per manufacturer insert |
| Heparin (Lithium/Sodium) | Anticoagulant, ~50 IU per mL blood | Prevents clotting of arterial sample without diluting electrolytes significantly | Room temperature, protect from light |
Step-by-Step Procedure
Before withdrawing blood from the radial artery, check that the ulnar artery can adequately supply the hand if the radial artery is compromised.
Draw the sample into an appropriate heparinized syringe, expelling all air bubbles immediately, since air exposure alters PO2 and PCO2 readings.
Cap the syringe, label it with patient details, FIO2 and temperature, and transport it on ice to the laboratory within 10–15 minutes.
Run the sample as soon as it arrives; room-temperature specimens and specimens with air bubbles or improper capping give erroneous results.
Report PO2, PCO2, pH, bicarbonate, hemoglobin saturation and hematocrit together with the patient's FIO2, temperature and clinical status for correct interpretation.
Flow Diagram
Quality Control
Internal QC uses multi-level electrolyte and blood gas control materials analyzed at defined intervals; ion-selective electrodes are checked against calibrators daily and re-calibrated whenever QC drifts outside the accepted mean ±2SD range.
Laboratories participate in external proficiency testing schemes for electrolytes and blood gases, comparing results against peer laboratories using similar instrumentation to confirm long-term accuracy.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Serum K+ below 2.5 or above 7.0 mEq/L | Risk of cardiac arrest | Treat as a critical value; notify the physician immediately and repeat testing to confirm |
| Chloride outside 98–107 mEq/L | Chloride imbalance — hypochloremia or hyperchloremia | Correlate with acid–base status and GI/renal history |
| Serum Ca²⁺ below 8.5 mg/dL | Hypocalcemia — risk of tetany, prolonged PT/PTT, EKG change | Notify physician; check parathyroid function and correct urgently if symptomatic |
Common Errors & How to Avoid Them
Cause: Traumatic venipuncture, prolonged tourniquet time, or a chilled/unprocessed specimen that leaks intracellular K+ into serum
Prevention: Draw smoothly with correct needle gauge, avoid prolonged tourniquet use, and process the sample promptly without excessive chilling
Cause: Air exposure or room-temperature delay alters dissolved PO2 and PCO2 values
Prevention: Expel all air immediately after draw, cap tightly, transport on ice, and analyze within 10–15 minutes
Cause: Result reported without FIO2, temperature or clinical status, making interpretation unreliable
Prevention: Always accompany the blood gas sample with a completed request including FIO2, temperature and clinical status
Laboratory Tips from the Bench
Never draw an arterial sample from the radial artery without first confirming ulnar patency — an occluded ulnar artery risks hand ischemia if the radial artery is later compromised.
Reject any potassium sample that appears even mildly hemolyzed before running it — hemolysis is one of the most common preventable causes of a falsely critical potassium result.
Remember 'SPICM' for the five electrolytes in this lesson — Sodium, Potassium, Ionized Calcium, Chloride, Magnesium — the core panel behind fluid, nerve and muscle balance.
Important Notes
Both very low and very high potassium levels can stop the heart. Any K+ result outside roughly 2.5–7.0 mEq/L must be treated and communicated as a critical value requiring immediate physician notification.
Because most of the body's potassium sits inside red blood cells, even mild hemolysis of a blood sample releases enough intracellular K+ to produce a falsely elevated result — always assess the sample visually before reporting.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeMr. Fernandes presents to the emergency department with confusion, generalized weakness and palpitations. He has been on a thiazide diuretic for hypertension for over five years and reports poor oral intake for the past three days due to a viral illness.
Mr. Fernandes's profound hypokalemia is consistent with long-term diuretic therapy combined with several days of poor oral intake, both of which promote renal and possibly GI potassium loss. His EKG changes (flattened T waves, U waves) are classic hypokalemia findings and place him at risk of dangerous arrhythmia, requiring urgent, monitored potassium replacement.
- →Chronic diuretic therapy is a leading cause of hypokalemia and requires periodic electrolyte monitoring
- →Potassium below 2.5 mEq/L is a critical value with real risk of cardiac arrest
- →EKG changes (flattened T waves, U waves) often accompany significant hypokalemia and guide urgency of correction
Frequently Asked Questions
Potassium is concentrated inside red blood cells at roughly 20–30 times its plasma concentration, so even minor cell rupture releases enough K+ to significantly raise the measured value. Sodium is mainly extracellular, so hemolysis has little effect on its measured concentration.
Cooling slows ongoing cellular metabolism in the sample, which would otherwise continue consuming oxygen and producing carbon dioxide, distorting the PO2 and PCO2 results if the sample sits at room temperature.
Yes — 'arterialized capillary blood' from a warmed heel or earlobe may be acceptable in some cases, particularly in infants, though arterial blood remains the gold standard.
Quick Revision
10-Minute ReviewKey Takeaways
- Sodium is the major extracellular cation; potassium is the major intracellular cation — together they define the sodium pump.
- Hemolysis is a common preventable cause of falsely elevated potassium results.
- Electrolytes (Na, K, Cl, Ca, Mg) maintain fluid balance, nerve conduction, muscle contraction and acid–base status.
- Both hyponatremia/hypernatremia and hypokalemia/hyperkalemia have specific, testable causes worth memorizing.
- Potassium outside 2.5–7.0 mEq/L is a critical value requiring immediate physician notification.
- Chloride, calcium and magnesium each have their own defined normal ranges and abnormalities.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling (NIOS). Biochemistry — Module: Electrolytes and Blood Gases (Lesson 27).
- Burtis CA, Ashwood ER, Bruns DE. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics.
- CLSI. Blood Gas and pH Analysis and Related Measurements (C46).