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[Physiology](https://durgical.com/en/physiology/) · [Renal, fluids and acid–base](https://durgical.com/en/physiology/#s-renal)

# 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

## Key points

- Sodium = water: hyponatremia is almost always excess water with active ADH; hypernatremia is lack of water or of access to it.
- Sodium symptoms are neurological (neuronal swelling or shrinking); potassium symptoms are cardiac and muscular.
- Ionized calcium stabilises Na⁺ channels: low calcium = hyperexcitability (tetany, Chvostek, Trousseau); high = lethargy, polyuria and constipation.
- Alkalosis lowers ionized calcium (more albumin binding) and moves K⁺ into cells; acidosis does the opposite.
- Correct calcium for albumin: each 1 g/dL fall in albumin lowers total calcium by ≈ 0.8 mg/dL without changing the ionized fraction.
- Hypomagnesemia prevents correction of hypokalemia and hypocalcemia.

## 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 (145 mEq/L)](https://durgical.com/en/conditions/hypernatraemia/) | Lack of water or of access to it | Older adults or infants without access to water, diabetes insipidus, fever, diarrhea | Thirst, irritability, lethargy |
| [Hypokalemia (5.0 mEq/L)](https://durgical.com/en/conditions/hyperkalaemia/) | Shift out of cells or low renal excretion | Kidney disease, ACE inhibitors/ARBs, acidosis, insulin lack, lysis | Peaked T, wide QRS, arrhythmias |
| [Hypocalcemia](https://durgical.com/en/conditions/hypocalcaemia/) | Lack of PTH or vitamin D, or calcium binding | Hypoparathyroidism (after thyroidectomy), vitamin D deficiency, pancreatitis, alkalosis | Tetany, Chvostek, Trousseau, long QT |
| [Hypercalcemia](https://durgical.com/en/conditions/hypercalcaemia/) | 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.

## Memory aid

'Sodium head, potassium heart, calcium nerve': sodium upsets the brain, potassium the rhythm and calcium excitability.

## 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.

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## Related topics

- [ADH, thirst and water balance](https://durgical.com/en/physiology/adh-thirst-and-water-balance/)
- [Sodium and extracellular volume regulation (with natriuretic peptides)](https://durgical.com/en/physiology/sodium-and-extracellular-volume-regulation/)
- [Potassium balance](https://durgical.com/en/physiology/potassium-balance/)
- [Renal handling of calcium, phosphate and magnesium](https://durgical.com/en/physiology/renal-handling-of-calcium-phosphate-and-magnesium/)
- [Fluid shifts between compartments](https://durgical.com/en/physiology/fluid-shifts-between-compartments/)
- [IV fluids: tonicity and where they go](https://durgical.com/en/physiology/iv-fluids-tonicity-and-where-they-go/)
- [The ECG in potassium and calcium disorders](https://durgical.com/en/physiology/the-ecg-in-potassium-and-calcium-disorders/)
- [Calcium regulation: PTH, vitamin D and calcitonin](https://durgical.com/en/physiology/calcium-regulation-pth-vitamin-d-and-calcitonin/)

## Related conditions

- [Hyponatraemia](https://durgical.com/en/conditions/hyponatraemia/)
- [Hypernatraemia](https://durgical.com/en/conditions/hypernatraemia/)
- [Hypokalaemia](https://durgical.com/en/conditions/hypokalaemia/)
- [Hyperkalaemia](https://durgical.com/en/conditions/hyperkalaemia/)
- [Hypocalcaemia](https://durgical.com/en/conditions/hypocalcaemia/)
- [Hypercalcaemia](https://durgical.com/en/conditions/hypercalcaemia/)
- [Syndrome of inappropriate antidiuretic hormone secretion (SIADH)](https://durgical.com/en/conditions/syndrome-of-inappropriate-antidiuretic-hormone-secretion/)

## In Biochemistry

- [Serum osmolality and osmolar gap](https://durgical.com/en/biochemistry/topics/serum-osmolality-and-osmolar-gap/)
- [Magnesium](https://durgical.com/en/biochemistry/topics/magnesium/)
- [Calcium and phosphate](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/26-3-electrolyte-balance) (OpenStax (Rice University), 2022); [Low blood sodium: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000394.htm) (MedlinePlus, US National Library of Medicine, 2026); [High potassium level: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001179.htm) (MedlinePlus, US National Library of Medicine, 2026); [Dehydration: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000982.htm) (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.

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