Why could an edematous person be considered dehydrated?

An edematous person, who appears to have swelling due to excess fluid accumulation, can still be considered dehydrated because of the relative distribution of body fluids. In the case of edema, while excess fluid is indeed retained extracellularly (outside of the cells) in the interstitial spaces, this does not necessarily mean that the overall hydration status is adequate.

When there is a significant accumulation of fluid in the extracellular spaces, particularly in conditions leading to edema, it can indicate that the fluid balance is disrupted. It's possible for the actual intravascular volume, which is the fluid within blood vessels, to be low. This means that, despite the apparent excess fluid in certain areas of the body, the fluid available within blood vessels for circulation may be insufficient, leading to symptoms of dehydration at the systemic level.

Dehydration is often defined by a lack of adequate fluid in the body's compartments that serve physiological needs, primarily within the cells and the vascular system. Therefore, a person may present with edema — suggesting fluid retention — but still have inadequate hydration that affects their health, thus illustrating the complex relationship between different fluid compartments in the body.