Greyhounds overheat because their mechanisms for losing heat are overwhelmed by the factors tending to warm them up
A greyhound loses heat by radiating heat from the skin, by losing warm urine, faeces and saliva, but mainly by evaporating water from moisture from the lungs, airway and mouth by panting. Think how cold you can get on a hot day by standing about wet after swimming, as the water evaporates from your skin
This system works extremely efficiently, but if the external pressures are too high, such as if the outside temperature is too high or high air humidity slows t