Fluid and Electrolyte Balance — Study Guide
Course: HyNote study pack on fluid and electrolyte balance, written for nursing students preparing for course exams and clinical check-offs. Public references: the Merck Manual professional pages on electrolyte disorders and the StatPearls acid-base and electrolyte reviews. All lab values and clinical calculations are standard teaching ranges.
Style: exam-first. The physiology is explained in plain language, then compressed into the rules, tables, and traps exams are built on. Three original cases walk the bedside logic. All scenarios are fictional and educational.
Notation: mEq/L is milliequivalents per liter (sodium, potassium, chloride, bicarbonate, magnesium). mg/dL is milligrams per deciliter (calcium). mL is milliliters. An acute weight change of 1 kg ≈ 1 L of fluid; total body water itself is about 60% of body weight, not 1 L per kg. ICF = intracellular fluid; ECF = extracellular fluid.
PART 1 — BODY FLUID COMPARTMENTS
1.1 The map
Total body water is about 60% of body weight in adult males (50% in females, 65-75% in infants). Two compartments:
- ICF (intracellular): about 40% of body weight — inside the cells. Potassium-rich, sodium-poor.
- ECF (extracellular): about 20% of body weight — outside the cells. Sodium-rich, potassium-poor.
- Interstitial fluid: about 15% of body weight (3/4 of ECF) — the fluid bathing the cells.
- Intravascular fluid: about 5% of body weight (1/4 of ECF) — plasma within the vessels.
1.2 Movement between compartments
- Osmosis: water moves across a semipermeable membrane from lower solute concentration to higher — the great regulator.
- Diffusion: particles move from higher concentration to lower until equilibrium (oxygen, CO2 across capillary walls).
- Active transport: sodium-potassium pump (Na+/K+ ATPase) — 3 Na+ out, 2 K+ in, against the gradient, using ATP. This pump is why ICF is K+-rich and ECF is Na+-rich.
- Filtration: hydrostatic pressure pushes fluid across capillary walls at the arteriolar end; oncotic pressure opposes filtration, but sustained venular reabsorption is limited in most tissues; lymphatics return most filtered fluid (the revised Starling model).
1.3 Hormonal regulation
- ADH (antidiuretic hormone / vasopressin): released by the posterior pituitary when serum osmolality rises or blood volume falls. Makes the kidneys RETAIN water → urine concentrates, blood dilutes.
- Aldosterone: released by the adrenal cortex when blood volume/Na+ falls or K+ rises. Makes the kidneys RETAIN Na+ and EXCRETE K+ → water follows Na+.
- ANP (atrial natriuretic peptide): released by the heart when atrial stretch increases (volume overload). EXCRETES Na+ and water → reduces volume.
1.4 Part 1 checklist
PART 2 — THE ELECTROLYTES AT A GLANCE
2.1 Normal serum values (memorize)
| Electrolyte | Normal range | Where most of it lives | Key job |
|---|
| Sodium (Na+) | 135-145 mEq/L | ECF | Water balance, nerve impulse |
| Potassium (K+) | 3.5-5.0 mEq/L | ICF | Cardiac rhythm, muscle contraction |
| Total calcium | 8.5-10.5 mg/dL | Bones (99% of total-body calcium) | Muscle contraction, clotting, nerve impulse |
| Magnesium (Mg2+) | 1.5-2.5 mEq/L | Bones (60%) | Enzyme cofactor, muscle and nerve function |
| Chloride (Cl-) | 98-106 mEq/L | ECF | Fluid balance, acid-base |
| Bicarbonate (HCO3-) | 22-26 mEq/L | ECF | Acid-base buffer |
2.2 The three big players in exam questions
- Sodium: the ECF cation — water follows sodium wherever it goes. Hyponatremia = water excess or Na+ deficit; hypernatremia = Na+ excess or water deficit.
- Potassium: the ICF cation — the cardiac electrolyte. Hypokalemia and hyperkalemia both threaten heart rhythm. Normal range is narrow (3.5-5.0).
- Calcium: the muscle-and-clotting ion — About 99% of total-body calcium is stored in bone. In serum, roughly 50% is ionized (the active fraction), about 40% is protein-bound, and the remainder is complexed. Hypocalcemia causes tetany and Chvostek/Trousseau signs; hypercalcemia causes confusion and renal stones.
2.3 Part 2 checklist
PART 3 — FLUID VOLUME IMBALANCES
3.1 Three tonicity patterns
| Type | What moves | Lab signature | Classic cause |
|---|
| Isotonic fluid loss | Water and electrolytes lost proportionally | Na often normal; Hct may rise with nonblood loss, but acute hemorrhage initially leaves Hct near baseline and it falls after equilibration or fluids | Hemorrhage, burns |
| Hypotonic fluid deficit | More electrolyte than water lost | Na low; Hct rises | Excessive sweating with water replacement |
| Hypertonic fluid deficit | More water than electrolyte lost | Na high; Hct rises | Diabetes insipidus, inadequate water |
| Isotonic fluid gain | Water and electrolytes gained proportionally | Na normal; Hct falls | Normal saline overload |
| Hypotonic fluid gain | More water than electrolyte gained | Na low; Hct falls | SIADH, water intoxication |
| Hypertonic fluid gain | More electrolyte than water gained | Na high; Hct falls | Hypertonic saline infusion |
3.2 The dehydration formulas (verified)
- Acute weight change: 1 kg lost = 1 L fluid deficit. This converts a change in weight, not total body water.
- Deficit (mL) = dehydration fraction × body weight (kg) × 1,000. A 5% deficit in a 70 kg adult = 0.05 × 70 × 1,000 = 3,500 mL, the same as 50 mL/kg × 70.
- Pediatric-derived teaching bands classify about 3-5% as mild, 6-9% as moderate, and 10% or more as severe. In adults, percent dehydration is less standardized and should be estimated from a known baseline weight plus the full clinical picture rather than from these signs alone.
3.3 Edema: the excess story
Edema is excess interstitial fluid. Causes: increased hydrostatic pressure (heart failure), decreased oncotic pressure (liver failure, nephrotic syndrome), increased capillary permeability (inflammation, burns), lymphatic obstruction. Assess: pitting edema (grade 1-4), daily weights (most sensitive), I&O, lung sounds (pulmonary edema).
3.4 Part 3 checklist
PART 4 — ELECTROLYTE IMBALANCES: THE BIG THREE
4.1 Sodium (135-145 mEq/L)
Hyponatremia (< 135): headache, confusion, lethargy, seizures (cerebral edema). Causes: SIADH, water excess, diuretics, heart failure. Treatment is cause- and volume-specific: 0.9% saline for hypovolemic hyponatremia, fluid restriction often for euvolemic SIADH or hypervolemic states, and closely monitored 3% saline for acute or severely symptomatic hyponatremia.
Hypernatremia (> 145): thirst, dry mucous membranes, agitation, fever. Causes: water deficit (diabetes insipidus), excess Na+ intake, dehydration. Treatment: free water replacement with 0.45% NaCl, or with D5W (isotonic in the bag; free water once the dextrose is metabolized).
4.2 Potassium (3.5-5.0 mEq/L) — the cardiac electrolyte
Hypokalemia (< 3.5): muscle weakness, leg cramps, constipation, flat T waves, U waves, ST depression on ECG. Causes: diuretics (thiazides, loop), vomiting, diarrhea, poor intake. Treatment: oral KCl (preferred) or IV KCl (routine rate no more than 10 mEq/hr on a pump; faster central-line replacement is reserved for life-threatening cases with continuous ECG and frequent laboratory monitoring; never IV push).
Hyperkalemia (> 5.0): muscle weakness, paresthesias, tall peaked T waves, wide QRS, cardiac arrest risk. Causes: renal failure, ACE inhibitors, tissue damage (burns, crush injury), acidosis. Treatment: calcium gluconate for ECG toxicity (stabilizes the membrane), insulin + glucose and sometimes an inhaled beta2-agonist to shift K+ into cells, and potassium removal such as dialysis when indicated. Sodium bicarbonate has limited, delayed effect and is mainly considered when significant metabolic acidemia is present. Never give KCl IV push — lethal.
4.3 Total calcium (8.5-10.5 mg/dL)
Hypocalcemia by total calcium (< 8.5 mg/dL, assuming normal albumin): tingling fingers and lips (perioral paresthesias), Chvostek sign (facial twitch when tapping VII), Trousseau sign (carpopedal spasm with BP cuff inflated), tetany, seizures. Causes: hypoparathyroidism, vitamin D deficiency, pancreatitis, renal failure. Treatment: IV calcium gluconate for acute; oral calcium + vitamin D for chronic.
Hypercalcemia by total calcium (> 10.5 mg/dL): lethargy, confusion, constipation, renal stones, shortened QT on ECG. Causes: hyperparathyroidism, bone metastases, thiazide diuretics, prolonged immobilization. Treatment: normal saline (hydration + excretion), calcitonin, bisphosphonates.
4.4 Magnesium (1.5-2.5 mEq/L) — the quiet cofactor
Hypomagnesemia: tremor, hyperreflexia, Chvostek and Trousseau signs (like hypocalcemia), cardiac arrhythmias. Causes: alcoholism, malnutrition, diuretics. Treatment: IV magnesium sulfate.
Hypermagnesemia: hyporeflexia (lost deep tendon reflexes), respiratory depression, hypotension, cardiac arrest at severe levels. Causes: renal failure, excess Mg2+ intake (antacids). Treatment: calcium gluconate, dialysis.
4.5 Part 4 checklist
PART 5 — THE BEDSIDE: ASSESSMENT AND INTERVENTIONS
5.1 Assessment tools
- Intake and Output (I&O): every shift for at-risk patients. Output should roughly match input over 24 hours.
- Daily weights: the most sensitive indicator of fluid change. An acute change of 1 kg = 1 L. Weigh at the same time, same scale, same clothes.
- Mucous membranes: moist = adequate; dry = deficit.
- Skin turgor: tenting (slow return) suggests a deficit in children and younger adults. It is unreliable in older adults because skin elasticity is lost.
- Lung sounds: crackles can occur with pulmonary edema but are nonspecific and also occur with conditions such as atelectasis or pneumonia.
- Edema: pitting (press and release, indent remains) graded 1-4 (trace, mild, moderate, severe).
5.2 Fluid replacement by route
- Oral: first choice when the patient can swallow and is alert.
- IV: when oral is not possible or absorption is impaired. Isotonic in the bag and in the body (0.9% NaCl, lactated Ringer's) for volume. Hypotonic (0.45% NaCl) for free water. D5W is isotonic in the bag and becomes free water after the dextrose is metabolized, so it is a poor intravascular volume expander after redistribution. Hypertonic (3% NaCl) for severe symptomatic hyponatremia.
- Blood products: for hemorrhage — isotonic fluid is a bridge, not a replacement.
5.3 Safety rules for IV electrolytes
- Never give potassium IV push. Always dilute and use a pump; routine infusions should not exceed 10 mEq/hr, while faster central-line replacement is reserved for life-threatening cases with continuous ECG and frequent potassium checks.
- Calcium gluconate is given as a slow IV push, typically over 5-10 minutes, on a cardiac monitor. Do not give it as a rapid bolus; a fast injection can cause bradycardia and cardiac arrest. It is not in the same never-IV-push category as potassium.
- Assess urine output, hemodynamics, and overload risk when giving IV fluids, but do not delay indicated resuscitation solely to obtain urine output. Confirm renal function and urine output before adding potassium unless an emergency protocol directs otherwise.
5.4 Part 5 checklist
PART 6 — THREE ORIGINAL CASES (verified)
6.1 Case 1 — the post-operative deficit
A 60 kg patient is 3 days post-op and has not eaten or drunk. Weight has dropped 2 kg since admission. Deficit: 2 kg = 2 L (1 kg = 1 L). Signs: dry mucous membranes, poor skin turgor, heart rate 110. Nursing: establish IV access, start isotonic fluid (0.9% NaCl) at ordered rate, monitor I&O and daily weights, assess mucous membranes every shift.
6.2 Case 2 — the diuretic and the low potassium
A patient on furosemide (loop diuretic) reports muscle weakness and leg cramps. Lab: K+ 3.0 mEq/L (normal 3.5-5.0). ECG: flat T waves and U waves. Nursing: hold the diuretic per prescription, give oral KCl (preferred route), monitor cardiac telemetry, reassess K+ in 2-4 hours.
6.3 Case 3 — the renal failure and the tall T waves
A patient with chronic kidney disease has K+ 6.2 mEq/L (normal 3.5-5.0). ECG: tall peaked T waves, widening QRS. This is hyperkalemia — a cardiac emergency. Nursing: stop all K+ intake, prepare calcium gluconate (membrane stabilizer), insulin + glucose (drives K+ into cells), notify the provider, prepare for emergent dialysis.
APPENDIX A — VALUES AND RULES
| Item | Number |
|---|
| Total body water | 60% body weight (males), 50% (females), 65-75% (infants) |
| ICF / ECF split | 40% / 20% of body weight |
| Interstitial / Intravascular | 15% / 5% of body weight |
| Normal Na+ | 135-145 mEq/L |
| Normal K+ | 3.5-5.0 mEq/L |
| Normal Ca2+ | 8.5-10.5 mg/dL |
| Normal Mg2+ | 1.5-2.5 mEq/L |
| Normal Cl- | 98-106 mEq/L |
| Normal HCO3- | 22-26 mEq/L |
| Acute weight change | 1 kg = 1 L (not total body water) |
| Dehydration % to volume | 5% = 50 mL/kg = 3,500 mL (70 kg) |
| IV tonicity | Isotonic in bag and body: 0.9% NaCl, LR. D5W: isotonic in the bag, free water after metabolism. Hypotonic: 0.45% NaCl. Hypertonic: 3% NaCl |
| K+ IV infusion rate | Max 10 mEq/hr (pump only) |
| Na+/K+ pump | 3 Na+ out, 2 K+ in |
Keyword pairs: dry and tachycardic = deficit; crackles and edema = overload; peaked T waves = K+ high; U waves = K+ low; tetany = Ca2+ low.
APPENDIX B — TRAP LIST
- Acute weight change is the best fluid indicator — 1 kg = 1 L. That is not total body water (about 60% of weight). Trust the scale over lung sounds alone.
- Potassium is the cardiac electrolyte — both hypo and hyper threaten the heart.
- Never give KCl IV push. Routine infusions should not exceed 10 mEq/hr; faster central-line replacement is reserved for life-threatening cases with continuous ECG and frequent potassium checks. Calcium gluconate is a slow IV push over 5-10 minutes; a rapid bolus can cause bradycardia and arrest.
- Hypocalcemia and hypomagnesemia look identical (Chvostek, Trousseau) — check both labs.
- ADH retains water; aldosterone retains Na+ and excretes K+ — never swap them.
- Skin turgor is unreliable in elderly patients (loss of elasticity).
- Assess urine output and overload risk with IV fluids, but do not delay indicated resuscitation solely to obtain urine output; confirm output before adding potassium unless an emergency protocol directs otherwise.
- Osmosis: water moves toward the higher solute concentration — toward the salt.
These are original HyNote study notes for nursing students. They summarize public educational references and all cases are fictional. They are not official materials of any nursing organization, they are not exam questions, and they are not clinical advice.
