Greensboro, NC -- Experts will tell you your body is basically a giant thermostat when your body feels an external temperature that's too hot or cold.

Dr. Allan Goldfarb at UNCG said when the temperatures change, "the brain starts a series of automatic reactions."

Here's how system the human body works:

• The body starts to heat.
• The heat sends a signal to circulate to warm blood up to the surface, which the blood gets closer to the skin.
• Which is being cooled because we are sweating. And the cooling process begins.

Dr. Goldfarb wants us to remember:

• When the heat index is more than 99 degrees try to stay indoors.
• If you have to be outside -- slowdown on the activities.
• And drink fluids every 15 minutes.
• And extreme heat...100 degrees or higher, this is dangerous. So listen to your body.

Dr. Goldfarb added when it's hot -- try to do activities in the morning or at night.

The elderly, infants, young children, the obese and those with chronic medical conditions are at an even greater risk for developing heat-related illnesses.