Physiological Adaptation

135–145 mEq/L. Sodium is the major extracellular cation and the main determinant of serum osmolality.

Low serum sodium pulls water into brain cells by osmosis, causing cerebral edema — leading to headache, confusion, lethargy, and seizures when severe.

Excess water relative to sodium: SIADH, excessive plain water intake, diuretics, vomiting/diarrhea with hypotonic fluid replacement, and heart failure dilution.

Seizure precautions — pad rails, suction and oxygen at the bedside — because cerebral edema lowers the seizure threshold. Restrict free water as ordered.

Thirst, dry sticky mucous membranes, restlessness, agitation, and twitching — think 'big and bloated or dry and agitated.' Cells shrink as water shifts out of them.

Water loss or sodium gain: dehydration, fever, diabetes insipidus, inadequate water intake (e.g., impaired thirst in elderly), and excess sodium intake.