A person in the desert for a long period of time without water experiences dehydration and hyperthermia. Dehydration is a shortage of water in the body and hyperthermia is an increase in the body temperature.
As the person does not consume water, the body tries to conserve water and this leads to a decrease in the production of sweat. Over time the body temperatures rises to extreme levels. If the resulting body temperatures exceed 40 degree C and this is sustained for a while the person experiences what is called a heat stroke.
A heat stroke results in the heart rate and breathing first spiking and then decreasing to a very low level accompanied by seizures, unconsciousness and finally the person going into a coma.