How does dehydration kill?

Dehydration occurs when the body excretes fluids (via sweat and urine) that is not sufficiently replaced. Illness can hasten dehydration just as can hot/dry conditions and intense exertion, as diarrhoea and vomiting accelerate the loss of fluids and hinder re-absorption.

As electrolyte levels change, cells toxify or become turgid and fail. Blood becomes less efficient; the heart struggles to compensate as your muscles and nervous system begin to falter. You eventually stop sweating as you lose more fluid; your organs begin to overheat and tissues start to die.

Eventually, organ failure resulting from heat stroke and massive cell death leads to the loss of life functions.