Dogs pant to dissipate heat efficiently and quickly and to keep their body temperature under control. Dogs have fewer sweat glands than humans, and most of them are found on their feet.
Dogs pant with their mouth open which helps to cool the tongue, which becomes longer due to the increase in blood supply. This, in turn, cools the animal's head. Although panting keeps temperatures down, a dog that is exposed to extreme heat can suffer from heatstroke which can be catastrophic for your pet.
Dogs can soon become distressed if shut in a hot car. Temperatures can rise very quickly inside the vehicle and this could prove fatal to a dog.
Normal body temperature for a dog is between 101-102 degrees fahrenheit. Even a rise in temperature of 3 degrees can be a sign of heatstroke and panting will not be enough to stabalize the dog's body temps. Any higher temperatures need to be dealt with immediately by a vet or the dog could die.
If your dog is suffering from heatstroke, try to keep him cool with water and ice packs, but don't cover him or this will exacerbate the condition. Encourage him to drink a little water and take him to the vet as soon as possible before other complications set in.
Dogs pant naturally after exercise and on hot days, but always make sure there is a supply of fresh drinking water for your dog and a place in the shade where he can rest.