Brain responses to dehydration and rehydration
An elderly person confined to an unairconditioned room in the summer may suffer from dehydration that develops over hours or day : ECF volume decreases and its osmolarity increases.
Brain shrinkage will occur and may be so severe that there is intracerebral hemorrhage from torn blood vessels.
Regulatory volume increase of brain celle involves synthesis and accumulation of additional intracellular organic osmolyte.
If the patient is brought into the ER, the decreased volume and increased osmotic concentration of his ECF may be rapidly corrected.
Rapid water entry into brain cells will then cause drastic swelling and possible death.
Moral : osmotic of extracellular fluid must be corrected slowly