 You go to a run and you start overheating your body, start generating heat, and you need to expel that heat. So the body, what it does there, it starts making the heart pound harder, and it closes blood flow to many organs in a way so that all of the blood now goes to your skin, where it gets cooled down through the process of evaporation of your sweat. So the blood cools down, and then it comes back in into your body cooler. Now, the problem when it's hot outside is that the sweat doesn't evaporate. Then the blood comes back in hot.