If a patient is in need of hydration then it implies that initially the patient's cells were in a hypertonic solution and their internal cell's environment was hypotonic. A hypotonic solution is a solution which has low solute concentration which results in lower osmotic pressure. In such condition, solvent moves from high concentration towards lower concentration through a semi permeable membrane by osmosis so as to make osmotic pressure equal on both the sides. So, when the patient's cells were kept in a hypertonic solution their cell's water moved outside and they became dehydrated. This is the reason why they need to be rehydrated to be normal again. Cells which become dehydrated are also known as flaccid cells whereas cells which have higher solvent concentration than the environment are also known as turgid cells.