Dehydration and volume depletion are usually considered synonymously associated with overall fluid loss. At a basic level, this correct, but as paramedics and clinicians it is important to understand the key differences in the pathophysiology of dehydration versus volume depletion, and how to best treat each condition.
Volume depletion involves a reduction of the total intravascular plasma pool; whereas dehydration is caused by loss of plasma free water (which is disproportionate to the loss of sodium). Although both volume depletion and dehydration are caused by fluid loss, it is important to identify the differences because you can have one without the other and the medical treatment should differ accordingly.
For example, a patient who has cut their leg and lost a litre of blood would be considered to have volume depletion because the same amount of plasma volume and free water have been proportionally reduced. Alternatively, the patient who has developped diabetes mellitus and subsequent osmotic diuresis, will lose large amounts of free fluid in a disproportionate amount to the loss of plasma.