Physiology · Renal, fluids and acid–base
Reasoning through fluid and electrolyte imbalances !
- Table to know
- Exam priority: high
Each electrolyte problem makes sense with three questions: did it move into or out of cells, what did the kidney do, and which tissue suffers? Here is the map.
Updated 2026-11-01
Also called: Electrolyte disorders, Sodium, potassium, calcium
What it is
Sodium, potassium and calcium disorders look like lists of causes, but they come from a few mechanisms: a water problem (sodium), a shift between cells and plasma or a renal failure (potassium), and a PTH–vitamin D problem (calcium). This page sorts those mechanisms and the classic signs so you can reason instead of memorise.
Key facts
- Sodium step 1: measure osmolality (rules out hyponatremia from hyperglycemia or pseudohyponatremia).
- Step 2: assess extracellular volume (hypovolemia, euvolemia, edema) to learn what total sodium is doing.
- Step 3: urine (osmolality and urine sodium) to see whether ADH and the kidney respond as they should.
- Potassium: rule out hemolysis, then look for a cell shift and finally for failed renal excretion.
- Calcium: correct for albumin or measure ionized calcium, then look at PTH (high, low or inappropriately normal).
Adult reference ranges (vary by laboratory)
| Parameter | Value | Note |
|---|---|---|
| Sodium | 135–145 mEq/L | |
| Potassium | 3.5–5.0 mEq/L | |
| Total calcium | 8.5–10.5 mg/dL | |
| Ionized calcium | ≈ 4.5–5.3 mg/dL | ≈ 1.1–1.3 mmol/L |
| Magnesium | ≈ 1.7–2.2 mg/dL | |
| Plasma osmolality | ≈ 285–295 mOsm/kg |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Map of the main imbalances
| Disorder | Core mechanism | Typical causes | Classic sign |
|---|---|---|---|
| Hyponatremia (< 135 mEq/L) | Excess water relative to sodium, almost always with active ADH | SIADH, heart failure, cirrhosis, thiazides, losses replaced with plain water | Headache, confusion, seizures (brain edema) |
| Hypernatremia (> 145 mEq/L) | Lack of water or of access to it | Older adults or infants without access to water, diabetes insipidus, fever, diarrhea | Thirst, irritability, lethargy |
| Hypokalemia (< 3.5 mEq/L) | Renal or GI loss, or shift into cells | Diuretics, vomiting, diarrhea, insulin, β2 agonists | Weakness, ileus, U waves |
| Hyperkalemia (> 5.0 mEq/L) | Shift out of cells or low renal excretion | Kidney disease, ACE inhibitors/ARBs, acidosis, insulin lack, lysis | Peaked T, wide QRS, arrhythmias |
| Hypocalcemia | Lack of PTH or vitamin D, or calcium binding | Hypoparathyroidism (after thyroidectomy), vitamin D deficiency, pancreatitis, alkalosis | Tetany, Chvostek, Trousseau, long QT |
| Hypercalcemia | Excess PTH or bone resorption | Primary hyperparathyroidism, cancer | Stones, bone pain, polyuria, short QT |
Clinical notes
- In acute childhood diarrhea (common in the tropics), replacing losses with plain water can cause hyponatremia; oral rehydration solutions balance sodium and glucose.
- On the NCLEX, Chvostek and Trousseau signs, peaked T waves and U waves are classic nursing questions.
- Long-standing very high or very low sodium is corrected slowly: the brain has adapted and a rapid change injures it.
Test yourself
Think of the answer, then tap the question to see it.
Why can severe hyperglycemia lower measured sodium without excess body water?
Glucose is osmotically active and draws water from cells into plasma, diluting sodium: a hypertonic hyponatremia.
Why can hyperventilation cause tingling and tetany?
Respiratory alkalosis increases calcium binding to albumin; ionized calcium falls and nerves become hyperexcitable.
Why does hypocalcemia make nerves more excitable?
Extracellular Ca²⁺ stabilises voltage-gated Na⁺ channels; with less Ca²⁺ they open with a smaller stimulus and neurons fire on their own.
Why won't hypokalemia correct if hypomagnesemia is also present?
Magnesium restrains K⁺ exit through ROMK in the distal nephron; without it, the kidney keeps losing K⁺ despite replacement.
Related topics
- ADH, thirst and water balance
- Sodium and extracellular volume regulation (with natriuretic peptides)
- Potassium balance
- Renal handling of calcium, phosphate and magnesium
- Fluid shifts between compartments
- IV fluids: tonicity and where they go
- The ECG in potassium and calcium disorders
- Calcium regulation: PTH, vitamin D and calcitonin
Related conditions
- Hyponatraemia
- Hypernatraemia
- Hypokalaemia
- Hyperkalaemia
- Hypocalcaemia
- Hypercalcaemia
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 26.3 Electrolyte Balance (OpenStax (Rice University), 2022); Low blood sodium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); High potassium level: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Dehydration: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 25). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.