Heat related illness
The nervous system is what keeps the body temperature on a level which is healthy. When a person's body temperature rises, the body will transfer the heat in order to keep the temperature normal. In cases like this, in order to cool off, a person's body will sweat and there will be blood flow to the skin as well. This process is also know as thermoregulation. In a situation when a person's body cannot transfer the amount of heat which is needed to maintain the coolness, that person may experience a heat-related illness.
Hyperthermia or high body temperature can happen to a person a lot faster if he or she is in a really hot environment. For instance, a good example of such a situation is a child left in a car during a hot summer day. Attics and boiler rooms are also places where high temperature may rise up quickly because these areas are small and the ventilation is not that good. On the other hand, hyperthermia caused by fever is a different matter. Fever causes high temperature because it is a natural reaction of the body when it is infected.
Heath related illnesses
In cases when sweat ducts on the skin become blocked, a person will experience heat rash. When people sweat a lot during exercises, their muscles will lose water, salt and minerals and they will feel heat cramps in their muscles. If a person stand or sits for a long period of time in a place where the temperature is really high, he or she may experience swelling in the arms or legs. Heat tetany may happen to a person if he or she feels stressed in such a place as well. Low blood pressure may cause heat syncope or fainting because the heat makes the blood vessels expand. It is very important that a person intakes a lot of fluids if he or she is exercising in a hot environment. This is important because a lot of fluids are lost this way and they need to be replaced in order for the heat exhaustion to be avoided. The only condition where a medical help is needed is in the case of a heatstroke.
Things like age, summer heat waves and chronic diseases like cancer or diabetes can be a causing factor of a heat-related illness as well.