Heat is the leading cause of weather-related deaths in the United States. While the human body is able to withstand high temperatures, unprecedented extreme heat driven by climate change is challenging our ability to adapt. As heat waves become more common, it’s important to know why heat can be so dangerous and who is most at risk for heat-related illnesses. Here’s what happens inside your body when it gets too hot.

How the body handles heat

When you start to heat up from exertion or the temperature outside, your body works to cool itself by moving warmer blood away from your internal organs and cooler blood toward them. It has two main techniques for doing this.

Blood is redistributed from your body’s core to its periphery in order to release heat through your skin. Capillaries at the surface of the skin fill with blood, which is why people often look flushed when they are hot.

You also start to perspire. As sweat evaporates, it cools the skin, lowering the temperature of the blood below. That blood then travels back to your internal organs to cool them down.

The body uses these two techniques all the time to maintain its internal temperature, even when you don’t feel particularly hot.

What happens when it’s too hot

When the air temperature is hotter than skin temperature, which is typically about 90 degrees, the body gains more heat than it can release.

In dry climates, sweat evaporation can continue to cool the body even at high temperatures, but that process becomes less effective as humidity increases. In very humid conditions, sweat doesn’t evaporate; instead, it just drips off the skin without cooling it. That’s why dry heat can feel cooler than humid heat.

As the body works harder to stay cool, the need to increase circulation to the periphery causes your heart rate to go up and your blood pressure to drop. (One cause of injury during heat waves is people fainting when they stand up because their blood pressure is too low.) As you become dehydrated from sweating, your blood volume decreases, lowering blood pressure even further.

Heat can become dangerous starting at temperatures as low as 80 degrees for older adults and people with pre-existing conditions that affect circulation or the sweating response, such as diabetes. People with cardiovascular issues are especially at risk because of the strain on the heart.

If you are young and healthy, temporary exposure to high heat will cause your internal temperature to rise a degree or two and then plateau; this is generally safe, though it may make you feel uncomfortably hot. But in more extreme weather conditions — for instance, 104 degrees and 50 percent humidity — or when you exert yourself in moderate heat and humidity, the body’s internal temperature continues to rise and problems can start to emerge.

The danger of overheating

Heat exhaustion sets in when you have a high internal temperature, generally between 101 and 103 degrees, and usually in combination with dehydration. (A fever can also cause body temperatures this high, especially in children, but it won’t lead to the same cascade of problems.) At that point, you will start to feel fatigued and, if you’re doing an activity, your muscles will tire more quickly. This is thought to be a protective signal from your brain to stop exerting yourself, which can further raise your temperature. You might also experience nausea, headache, a fast heart rate and shallow breathing.

Heat stroke occurs when the body reaches an internal temperature of 104 degrees and above. By this point, blood pressure has often dropped too low for too long, causing internal organs to become deprived of blood and therefore oxygen. The high temperature itself can also cause cell death, leading to organ failure.

Some of the most important organs affected by extreme heat are the kidneys, heart, gut and brain.

When the body is dehydrated, the brain sends a signal to stop circulating as much blood to the kidneys to avoid losing fluid in the form of urine. The kidneys quickly become deprived of oxygen, which damages cells there and can cause kidney failure.

When the heart, which is working in overdrive, can’t keep up with the demand of increased circulation, it also doesn’t receive enough blood and becomes deprived of oxygen. This can cause potentially deadly damage. It’s especially a concern for people with underlying conditions that weaken the heart muscle.

Cells in the gut appear to be particularly susceptible to heat. When they become too hot, the wall of the intestines can start to break down, allowing bacteria to leak into the bloodstream.

The defining characteristic of heat stroke is confusion and delirium as the brain becomes overheated and starved of oxygen. The hypothalamus, which acts as the brain’s internal thermometer, can even start to malfunction and stop sending the signal that the body needs to cool itself. As a result, people with heat stroke might stop sweating while their organs burn up inside.

Exactly how long it takes someone exposed to high temperatures to develop heat stroke and experience permanent organ damage or death differs from person to person. It’s possible to adapt to hot weather to a certain extent — some athletes accustomed to heat are able to function even with internal temperatures above 104 degrees.

When someone does start to experience symptoms of heat stroke, cooling the person down immediately is essential; every minute counts when it comes to minimizing organ damage. The best and fastest way to do this is to immerse the person in an ice bath. If that’s unavailable, wrapping him or her in cold, wet cloths can also help.