What happens when the kidneys fail?

It depends. Acute kidney failure can happen for several reasons (blood loss, obstruction, toxin damage, immune response to name a few). The kidneys lose the ability to produce urine and eliminate waste, and you can get electrolyte imbalances, uremia, and acid base imbalances. If the damage happens over a longer period of time, you can get hypertension, heart failure, bone fractures, and anemia.
Uremia develops. The body is unable to remove many of the toxins that originate from food and internal metabolism ;this leads to multiple malfunctions including hypertension, fluid retention, anemia, and eventually death from uremia.

Related Questions

What happens when your kidneys fail?

Uremia. When the kidneys fail, usually a slow process, the blood collects toxins including urea - mostly end products of metabolism and you will get extremely sick. It is commonly associated with changes in blood pressure and requires serious attention by a renal specialist. Sometimes a kidney transplantation might be needed.