In a patient who has diarrhoea and vomiting with inadequate water, intake is suffering from –
Extracellular fluid consists of

Interstitial (intercellular fluid)
Circulating blood
Intestinal fluids and secretions
Intracellular fluid consists of fluid present within the cells.
Loss of fluid in diarrhoea and vomiting comes from the extracellular fluid. Therefore there is shrinkage or reduction in the volume of extracellular fluid.
Diarrhoea and vomiting both cause loss of Na+ rich fluid, therefore, there is a decline in the serum sodium level (hyponatremia).
These causes fall in osmolality of ECF leading to shifting of fluid from ECF to ICF causing a further decrease in ECF volume.
Intracellular dehydration with hypernatremia
Intracellular dehydration with hyponatremia
Extracellular dehydration with hyponatremia
Extracellular dehydration with hypernatremia