Vasodilation is when a dog's blood vessels expand or dilate, mainly when the dog is hot or has been exercising in warm weather. This expansion allows warm blood to flow closer to the skin's surface, where it can cool down by releasing heat into the environment. After cooling, this blood circulates back towards the heart and throughout the body, effectively reducing the dog's internal temperature. Vasodilation is a critical physiological response that regulates a dog's body temperature during hot conditions.