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Understand body fluid compartments, osmotic principles, electrolyte roles in normal physiology, fluid shifts, and acid-base foundations, all at the conceptual level without disease states.
Where the body's water lives
Approximately 60% of adult body weight is water, distributed between two main fluid compartments. Intracellular fluid (ICF) is inside cells (~40% body weight, ~2/3 of total body water). Extracellular fluid (ECF) is outside cells (~20% body weight, ~1/3 of total body water). ECF is further divided into intravascular (plasma, ~5%) and interstitial (between cells, ~15%). The distribution matters because each compartment has different electrolyte compositions that must be maintained for normal function.
Intracellular (ICF)
~2/3 of total body water. Primary cation: K⁺. Primary anion: HPO₄²⁻. Contains most of the body's potassium and phosphate. Cell function depends on this environment being tightly regulated.
Interstitial
Fluid between cells (~15% body weight). Similar electrolyte composition to plasma but with very little protein. Bathes cells and allows nutrient/waste exchange. Excess accumulation = edema.
Intravascular (Plasma)
Fluid within blood vessels (~5% body weight). Primary cation: Na⁺. Contains plasma proteins (albumin) that create oncotic pressure. This is the only compartment directly accessible for IV fluid administration.
Na⁺ is concentrated OUTSIDE cells, K⁺ is concentrated INSIDE cells. This gradient is maintained by the Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in per cycle). This concentration difference is essential for nerve impulse transmission, muscle contraction, and maintaining cell volume. Disrupting this gradient has immediate physiological consequences.
How water moves between compartments
Water moves by osmosis, the net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Water follows solute. This is a passive process requiring no energy. It always moves toward higher solute concentration. This principle governs fluid distribution between compartments and is the basis for understanding IV fluid therapy.
Osmotic pressure is created by ALL solutes (electrolytes, glucose, urea). Oncotic (colloid osmotic) pressure is the portion of osmotic pressure created specifically by plasma proteins (mainly albumin). Oncotic pressure keeps fluid inside blood vessels. When albumin is low (malnutrition, liver disease, nephrotic syndrome), oncotic pressure drops and fluid leaks into interstitial spaces → edema.
What each major electrolyte does
Each electrolyte has specific physiological roles. Understanding their normal functions helps you appreciate why imbalances cause predictable symptoms.
Sodium (Na⁺), Normal: 135–145 mEq/L
Primary ECF cation. Regulates water distribution (water follows sodium). Drives nerve impulse conduction. Major determinant of plasma osmolarity. Changes in sodium primarily affect water balance and neurological function.
Potassium (K⁺), Normal: 3.5–5.0 mEq/L
Primary ICF cation. Critical for cardiac electrical conduction (resting membrane potential), skeletal muscle contraction, and nerve transmission. Even small changes outside the narrow normal range affect cardiac rhythm. The most dangerous electrolyte to get wrong.
Calcium (Ca²⁺), Normal: 8.5–10.5 mg/dL
Muscle contraction (including cardiac), bone structure (99% stored in bone), blood clotting cascade, nerve impulse transmission, enzyme activation. Only the ionized (free) fraction is physiologically active. Albumin level affects total calcium measurement.
Magnesium (Mg²⁺), Normal: 1.5–2.5 mEq/L
Cofactor for over 300 enzyme systems. Involved in energy production (ATP requires Mg²⁺), protein synthesis, neuromuscular function, and cardiac rhythm stability. Works in tandem with calcium and potassium, deficiency of one often accompanies deficiency of others.
pH regulation at the conceptual level
The body maintains blood pH between 7.35 and 7.45. This narrow range is essential for enzyme function, protein structure, and cellular processes. pH below 7.35 = acidosis (excess H⁺). pH above 7.45 = alkalosis (deficit of H⁺). The body uses three systems to maintain this range: buffer systems (immediate), respiratory system (minutes), and renal system (hours to days). Three regulatory systems work together to maintain this balance.
Buffer Systems
Response: Immediate (seconds). Chemical buffers (bicarbonate, phosphate, protein) absorb or release H⁺ to resist pH changes. Limited capacity, buffers are consumed and must be regenerated.
Respiratory System
Response: Minutes. Controls CO₂ (which is acidic when dissolved). Hyperventilation blows off CO₂ → raises pH. Hypoventilation retains CO₂ → lowers pH. The lungs are the fast compensator.
Renal System
Response: Hours to days. Kidneys excrete H⁺ or reabsorb/generate HCO₃⁻ as needed. Most powerful but slowest compensator. Determines long-term acid-base balance.
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻. This single equation is the key to understanding acid-base balance. The left side (CO₂) is controlled by the lungs. The right side (HCO₃⁻) is controlled by the kidneys. Normal ratio of HCO₃⁻ to CO₂ is 20:1, as long as this ratio is maintained, pH stays normal.
Crystalloids, colloids, monitoring, and complications
Intravenous fluid therapy is one of the most common interventions in healthcare. Every nurse must understand why a specific fluid is chosen, how it will move through compartments, what to monitor during infusion, and how to recognize complications promptly.
Crystalloid IV Fluids — Most Common Type
0.9% Normal Saline (NS): Isotonic (~308 mOsm/L). Expands ECF/intravascular volume without causing osmotic cell shifts. Standard choice for volume resuscitation, medication administration, and blood product infusion. Risk with large volumes: hyperchloremic metabolic acidosis.
0.45% Normal Saline (1/2 NS): Hypotonic (~154 mOsm/L). Provides free water that distributes to ALL compartments, hydrating cells and ECF. Used for cellular dehydration and hypernatremia management. Risk: cerebral edema if infused too rapidly.
Lactated Ringer's (LR): Isotonic (~273 mOsm/L), contains Na⁺, K⁺, Ca²⁺, Cl⁻, and lactate (converted to bicarbonate by liver). More physiologically balanced than NS. Preferred for trauma, burns, and perioperative fluid replacement. Avoid in hyperkalemia (contains K⁺).
D5W (5% dextrose in water): Initially isotonic (~252 mOsm/L) but becomes effectively hypotonic once glucose is metabolized — dextrose provides no tonicity after metabolism. Distributes to all compartments. Used for free water replacement and as a vehicle for medications. Not used for volume resuscitation. Avoid in head injury (can worsen cerebral edema).
3% NaCl (Hypertonic Saline): Hypertonic (~1,026 mOsm/L). Draws water from ICF into intravascular space by osmosis. Reserved for symptomatic severe hyponatremia (seizures, severe confusion). Must be infused slowly on a monitored unit — rapid infusion causes osmotic demyelination syndrome.
Colloid IV Fluids — Large Molecules Stay Intravascular
Colloids contain large molecules (proteins, starches) that cannot easily cross capillary membranes, so they stay in the intravascular space and exert oncotic pressure. Albumin 5%: similar effect to isotonic saline but stays intravascular — used for hypoalbuminemia, large-volume paracentesis. Albumin 25% (hyperoncotic): strongly pulls fluid from interstitial space into intravascular — used for severe hypoalbuminemia with edema. Blood products: packed red blood cells (pRBCs) — restore oxygen-carrying capacity; fresh frozen plasma (FFP) — replaces clotting factors; platelets — treat thrombocytopenia and bleeding; cryoprecipitate — replaces fibrinogen and factor VIII. Monitor for transfusion reactions: fever, chills, back pain, hematuria during any blood product infusion — stop immediately if suspected.
IV Therapy Monitoring and Complications
Infusion site assessment at every shift and before each medication: inspect for redness (phlebitis), swelling (infiltration), pain, warmth, streak up the arm (thrombophlebitis). If infiltration: stop infusion immediately, remove IV, elevate extremity, apply warm or cool compress per facility policy, document. Fluid overload signs: new crackles in lung bases, S3 heart sound, JVD, peripheral edema, dyspnea, hypertension, bounding pulse, weight gain (>1 kg/day suggests fluid retention). Intervention: slow or stop infusion, elevate HOB, notify provider, anticipate diuretic order. Dehydration signs: poor skin turgor (tenting), dry mucous membranes, concentrated urine (dark amber, specific gravity >1.025), tachycardia, hypotension, weight loss. Track: strict I&O every shift, daily weights (same time, same scale, same clothing — 1 kg change ≈ 1 liter fluid).
ABG interpretation and clinical applications
Arterial blood gas (ABG) interpretation is a critical nursing skill. ABG results tell you how well the lungs and kidneys are maintaining pH homeostasis and whether compensation is occurring. A systematic 4-step approach eliminates guesswork.
Normal ABG Values — Memorize These
4-Step ABG Interpretation Method
Step 1: Evaluate pH. <7.35 = acidosis. >7.45 = alkalosis. Normal = compensated or no disorder.
Step 2: Evaluate PaCO₂ (respiratory component). PaCO₂ >45 with pH <7.35 = respiratory acidosis. PaCO₂ <35 with pH >7.45 = respiratory alkalosis. If CO₂ moves opposite to pH — it is the CAUSE.
Step 3: Evaluate HCO₃⁻ (metabolic component). HCO₃⁻ <22 with pH <7.35 = metabolic acidosis. HCO₃⁻ >26 with pH >7.45 = metabolic alkalosis. If HCO₃⁻ moves in the SAME direction as pH — it is the CAUSE.
Step 4: Determine compensation. Is the non-primary component also abnormal, and in the expected direction? Full compensation = pH normal; partial compensation = pH abnormal but not as extreme. Respiratory compensation for metabolic disorders occurs in minutes to hours. Renal compensation for respiratory disorders takes 2–5 days.
ROME Mnemonic for ABG Interpretation
Respiratory Opposite: In respiratory disorders, pH and PaCO₂ move in OPPOSITE directions (↑CO₂ → ↓pH in respiratory acidosis; ↓CO₂ → ↑pH in respiratory alkalosis). Metabolic Equal: In metabolic disorders, pH and HCO₃⁻ move in the SAME direction (↑HCO₃⁻ → ↑pH in metabolic alkalosis; ↓HCO₃⁻ → ↓pH in metabolic acidosis).
Respiratory Acidosis — pH <7.35, PaCO₂ >45
Mechanism: alveolar hypoventilation → CO₂ retention → H⁺ accumulation. Causes: COPD exacerbation, opioid or sedative-induced respiratory depression, sleep apnea, neuromuscular disease (Guillain-Barré, myasthenia gravis), flail chest. Clinical presentation: bounding pulse, headache (CO₂ vasodilates cerebral vessels), confusion, somnolence, flushed warm skin, asterixis (flapping tremor). Compensation: kidneys retain HCO₃⁻ (takes 2–5 days). Treatment: improve ventilation — stimulate breathing, reduce sedation, consider non-invasive or mechanical ventilation.
Respiratory Alkalosis — pH >7.45, PaCO₂ <35
Mechanism: alveolar hyperventilation → excess CO₂ blown off → H⁺ deficit. Causes: anxiety, pain, fever, pulmonary embolism, high-altitude acclimatization, early salicylate toxicity, mechanical ventilation (over-ventilation), pregnancy. Clinical presentation: tingling fingers and perioral area, lightheadedness, carpopedal spasm (hands and feet cramp), dizziness. Compensation: kidneys excrete HCO₃⁻. Treatment: address underlying cause; for anxiety/hyperventilation — slow breathing, paper bag (controversial), anxiolytics if appropriate.
Metabolic Acidosis — pH <7.35, HCO₃⁻ <22
Mechanism: excess acid production, loss of base, or impaired acid excretion. Causes (MUDPILES mnemonic): Methanol, Uremia (renal failure), Diabetic ketoacidosis, Propylene glycol/Paraldehyde, Isoniazid/Iron, Lactic acidosis (sepsis, shock, liver failure), Ethylene glycol, Salicylates. Clinical presentation: Kussmaul respirations (deep, rapid, labored breathing — respiratory compensation attempting to blow off CO₂), fruity/acetone breath (DKA — ketones), nausea/vomiting, weakness. Compensation: lungs hyperventilate (PaCO₂ falls). Treatment: address underlying cause; bicarbonate therapy reserved for pH <7.1 or cardiac instability.
Metabolic Alkalosis — pH >7.45, HCO₃⁻ >26
Mechanism: excess base gain or loss of acid. Causes: vomiting, nasogastric suctioning (lose H⁺ and Cl⁻), diuretic use (lose H⁺ and K⁺), excessive antacid use, hypokalemia (K⁺ shifts out of cells, H⁺ shifts in). Clinical presentation: muscle cramps, weakness, hypokalemia-related dysrhythmias, tetany, mental status changes. Often coexists with hypokalemia and hypovolemia. Compensation: lungs hypoventilate — retain CO₂ (limited compensation because hypoxia also drives ventilation). Treatment: replace chloride (NS) and potassium; treat underlying cause; stop offending medications.
ABG results: pH 7.28, PaCO₂ 55, HCO₃⁻ 24. This represents:
Safe administration, monitoring, and clinical recognition of imbalances
Electrolyte replacement requires understanding not just the correct replacement agent but the correct rate, route, and monitoring parameters. Errors in electrolyte administration — particularly with potassium — are among the most dangerous medication safety events in clinical practice.
Sodium (Normal: 135–145 mEq/L) — The Slow Correction Principle
Hyponatremia (<135 mEq/L): most common electrolyte disorder. Symptoms: headache, nausea, confusion, seizures, coma (primarily neurological — brain cells swell). Severity correlates with rate of drop, not absolute value. Acute hyponatremia (developed in <48h) is more dangerous than chronic. Correction rule: no faster than 8–10 mEq/L per 24 hours total. Too-rapid correction causes osmotic demyelination syndrome (ODS) — neurons shrink faster than they can adapt, causing irreversible pontine demyelination. Treatment: mild/moderate — fluid restriction ± 0.9% NS; severe/symptomatic — 3% NaCl with close monitoring. Hypernatremia (>145 mEq/L): cells shrink (brain shrinkage can cause venous rupture). Correction rate: no faster than 10–12 mEq/L per 24 hours — too-rapid correction causes cerebral edema. Assess: LOC changes, agitation, irritability, seizures, and thirst (hypernatremia is the primary stimulus for thirst).
Potassium (Normal: 3.5–5.0 mEq/L) — The High-Alert Electrolyte
Potassium is a HIGH ALERT medication in all healthcare settings. Hypokalemia (<3.5 mEq/L): causes — diuretics (loop and thiazide), vomiting, diarrhea, NG suction, alkalosis (K⁺ shifts into cells as H⁺ moves out). Symptoms: weakness, fatigue, constipation, ECG changes (flattened T waves, U waves), dysrhythmias. Replacement: oral route preferred for mild deficits (K-Dur, K-Lor tabs or liquid) — better tolerated and less dangerous. IV potassium: ALWAYS dilute. Maximum concentration via peripheral IV: 40 mEq/L. Maximum concentration via central line: 80–100 mEq/L. Maximum infusion rate: 10–20 mEq/hour (NEVER exceed without continuous cardiac monitoring). NEVER give IV potassium as an IV push or bolus — cardiac arrest. IV K⁺ burns and stings the peripheral vein — warn the patient this is expected. Hyperkalemia (>5.0 mEq/L): causes — AKI/CKD, ACE inhibitors, potassium-sparing diuretics, metabolic acidosis, cell lysis (rhabdomyolysis, hemolysis, massive transfusion). Symptoms: peaked T waves (earliest ECG change), widened QRS, then sine wave pattern, then asystole or VF. Treatment cascade: calcium gluconate (cardiac membrane stabilization — does not lower K⁺), insulin + dextrose (shifts K⁺ into cells), sodium bicarbonate (shifts K⁺ into cells in acidosis), kayexalate/patiromer (gut removal), dialysis (definitive).
Calcium (Total: 8.5–10.5 mg/dL; Ionized: 1.1–1.35 mmol/L)
Total calcium is affected by albumin (40% of total Ca²⁺ is albumin-bound and inactive). Corrected calcium = measured Ca²⁺ + 0.8 × (4.0 − albumin). Always check ionized calcium for true physiological assessment. Hypocalcemia (<8.5 mg/dL total or <1.1 mmol/L ionized): causes — hypoparathyroidism, post-thyroidectomy/parathyroidectomy, vitamin D deficiency, pancreatitis, hypomagnesemia, massive blood transfusion (citrate in blood products chelates Ca²⁺). Clinical signs: Chvostek sign (tap facial nerve just anterior to ear — positive if facial twitch occurs); Trousseau sign (inflate BP cuff above systolic for 3 minutes — positive if carpal spasm); perioral and fingertip paresthesias; tetany; laryngospasm; seizures; QT prolongation (dysrhythmia risk). Treatment: IV calcium gluconate preferred (safer peripheral administration) vs. calcium chloride (3× more elemental calcium, caustic — administer centrally or in emergencies only). Hypercalcemia (>10.5 mg/dL): "Bones, Stones, Groans, Psychic Moans" — bone pain, kidney stones, abdominal pain/nausea, and confusion/depression. Treatment: IV NS hydration, furosemide, bisphosphonates, treat underlying cause.
Magnesium (Normal: 1.7–2.2 mg/dL) — The Forgotten Electrolyte
Hypomagnesemia (<1.7 mg/dL): often unrecognized. Causes: malnutrition, alcoholism, prolonged diarrhea, diuretic use, proton pump inhibitor (PPI) use, DKA. Critical clinical point: hypomagnesemia causes refractory hypokalemia — the kidney cannot retain potassium without adequate magnesium. If a patient's potassium does not respond to replacement, check and correct magnesium first. Symptoms: irritability, tremors, seizures, prolonged QT interval, torsades de pointes (polymorphic VT). Treatment: oral supplementation for mild; IV magnesium sulfate for severe — administer slowly (rapid IV MgSO₄ causes hypotension, flushing, bradycardia). Hypermagnesemia (>2.2 mg/dL): most common in patients with renal failure receiving Mg-containing antacids or laxatives (Milk of Magnesia, Maalox). Toxicity sequence: deep tendon reflex (DTR) loss is the FIRST sign (occurs at 4–7 mEq/L — check DTRs before each dose during IV magnesium infusion); respiratory depression (>7 mEq/L — the respiratory rate must be ≥12/min before administering IV Mg²⁺); cardiac arrest. Antidote: calcium gluconate — antagonizes magnesium at the cellular level.
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Which fluid compartment contains the most body water?
A nurse prepares to administer IV potassium at 40 mEq in 250 mL NS via peripheral IV. The maximum safe infusion rate is:
