When dangerous heat waves strike, what happens inside the human body can become a life-or-death battle decided by just a few degrees. Raising a person’s ordinary core temperature by just 7 degrees Fahrenheit puts them at high risk of death. That can mean heatstroke — which the body responds to by sending blood closer to the skin to try to cool down. But that can lead to toxins escaping from internal organs and circulating where they shouldn’t. An even bigger killer in extreme heat is strain on the heart. The push of blood toward the skin requires the heart to pump harder and more often to compensation for a drop in blood pressure. That’s especially dangerous for people with heart conditions. Dehydration can also be lethal.