Na⁺, K⁺, Ca²⁺, PO₄³⁻, Mg²⁺, Acid–Base + Endocrine links (PTH, Vit D, Aldosterone, Cortisol, ADH).

Sodium- Water + Osmolality

Primary controller: ADH & Kidney

Function: Osmolality + Brain perfusion

Danger: Neuro symptoms first

• Chronic hyponatremia → slow correction (risk ODS)

• Chronic hypernatremia → slow correction (risk cerebral oedema)

Potassium - Cell excitability + Acid-base

Primary Controller: Aldosterone

Function: Cardiac rhythm + Membrane potential

Danger: Arrhythmia first

• Acidosis: H⁺ enters cells → K⁺ exits → hyperkalemia

• Alkalosis: H⁺ exits cells → K⁺ enters → hypokalemia

• Insulin & β-agonists: ↓ K (into cells)

Calcium - Neuromuscular stability

Primary Controller: PTH + Calcitriol

Function: Muscle contraction, neuronal firing, coagulation

Danger: Seizure, Arrhythmia

• ↓ Ca: Tetany, QT prolongation

• ↑ Ca: Stones, Psychiatry