An older dog’s circulatory system is not as efficient at maintaining body temperature as it once was. When he is too warm, he’ll pant to expel heat from his body via evaporation of moisture from his tongue, thereby lowering his body temperature. If your dog is suffering from a heart or lung problem, panting is a result of him trying to draw enough oxygen into his body to survive. If he’s stressed, he’ll use up more oxygen, so will pant to provided himself with more. If your pet appears distressed when panting, seek veterinary advice.