Patients who lose enough extracellular fluid (ECF) volume develop skin tenting (loss of skin elasticity), flat neck veins, and orthostatic or frank tachycardia and dizziness or fainting when standing up due to orthostatic hypotension, are often said to be dehydrated or dry.
The hallmarks of dehydration include thirst and neurological changes such as headaches, general discomfort, loss of appetite, decreased urine volume (unless polyuria is the cause of dehydration), confusion, unexplained tiredness, purple fingernails and even seizures.