The majority of the body is made up of water, with up to 75% of the body’s weight due to H2O. Most of the water is found within the cells of the body (intracellular space). The rest is found in the so-called extracellular space, which consists of the blood vessels (intravascular space) and the spaces between cells (interstitial space).
Essentially, water makes the world go ’round. And by “world”, I mean, “your body”. It is in your blood, digestive system, and helps keep body parts lubricated. Water also helps to maintain the temperature of the human body, i.e. when the body starts to overheat, we sweat to cool ourselves down.
If you are not hydrated enough, and your pores can’t excrete the necessarily water/toxins in order to help you sweat (especially when you’re working out), then you risk a build up of toxins in your body, and experiencing moderate-severe edema (swelling). The same kind you experience if you take in a lot of salt, without drinking enough.