When the temperature drops below freezing, the human body uses these incredible mechanisms to try to stay warm.
Your blood vessels constrict
Your body is built to always maintain a stable core temperature of 37 degrees Celsius (that’s 98.6 degrees Fahrenheit). When the temperature in your environment drops, thermoreceptors in your skin sound the alarm, alerting an area of the brain called the hypothalamus, which acts like a thermostat dedicated to maintaining that 37 degree equilibrium, according to Robert Kenefick, PhD, research physiologist at the U.S. Army Research Institute of Environmental Medicine. One of the first actions the hypothalamus takes: It tightens the blood vessels in your arms, hands, feet, and legs. “Blood delivers heat to the skin,” says Gordon Giesbrecht, PhD, professor of thermophysiology at the University of Manitoba in Canada. “If you decrease blood flow to the skin, you decrease heat loss from the skin.”
You have to urinate
All that vasoconstriction forces fluid to concentrate in your core. This causes volume receptors that talk to your hypothalamus to say, “Hey, maybe you should get rid of some of that fluid—maybe you should pee.” It’s common, say, on the ski slopes for people to use the bathroom right before they head outdoors and then to feel like they need to go again shortly after being outside.
You shiver
Of course you shiver when it’s cold—duh. But the reason you do is utterly interesting. “When vasoconstriction isn’t doing enough to warm you, the hypothalamus tells your muscles to start contracting. One of the byproducts of muscle contraction is heat.” Garden-variety shivering produces about 100 watts of heat, says Giesbrecht. If you get cold enough to enter into mild hypothermia, you can produce 400 to 600 watts of heat through shivering.
You freeze much faster if wet
Why do little kids still shiver when they get out of a heated swimming pool in 80-degree weather? “Water carries heat away from the body 25 times faster than air, so you can lose a great amount of heat very quickly when you’re wet,” says Kenefick. “Shivering is one way your body is trying to raise your core temperature back up.”
You make things worse if you drink alcohol
Think a flask filled with the strong stuff can keep you toasty? You might be dead wrong. The body’s first major reaction to cold is to constrict blood vessels, but “alcohol does the opposite—it causes peripheral vasodilation,” explains Kenefick. “Those blood vessels widen and dump all this heat to the environment.” Your skin will feel warm, but that provides a false sense of security, because this really causes your core temperature to drop, which can lead in extreme cases to hypothermia.
You can become too cold if you dress too warmly
How’s this scenario for ironic? You’re outside in very cold weather shoveling snow, which is a lot of physical work. You’re contracting muscles and generating a lot of heat, so your core temperature actually goes up. “In this situation, your blood vessels dilate instead of constrict, and you start sweating. If that sweat then gets trapped in your clothing, then it can start sucking heat away from your body. That’s a recipe for hypothermia right there,” says Kenefick. The lesson: Don’t overdress if you know you’re going to be exercising vigorously.
You scrunch down and huddle with others
When your body has constricted your blood vessels and made your muscles start shivering, you might also instinctually engage in social behaviors that help conserve heat. When you scrunch down and make yourself smaller, you lose less heat to the environment because you decrease your own surface area.
Over time, your brain stops operating on all cylinders
Once your body goes into a hypothermic state—a core temperature that’s too low—your brain and nervous system have a harder time functioning and maintaining normal thought. “There are stories of hunters or people out in the cold, maybe they fell in water or got lost,” Kenefick says. “The hypothalamus tries all the mechanisms that usually work—and then when those fail, it becomes unclear of what to do next. People have been found in freezing temperatures with all their clothes off, because the hypothalamus said, ‘OK, well, let’s try dumping heat out there—making people feel hot—and see if that will work.” If the hypothermia becomes severe enough, the hypothalamus can get really out of whack.
Your skin turns white and hard
These are signs of frostbite, a condition when your exposed skin gets too cold and freezes. Cheeks, nose, and fingers tend to be especially vulnerable because they are getting less blood flow because of the vasoconstriction. Also, your fingers are cylinders, which gives them more surface area relative to their size and makes them more vulnerable to heat loss. Frostbite means the skin tissue has become damaged. If it’s severe enough, says Kenefick, it can turn black and actually fall off. At first you will feel pain; as your skin gets colder and colder, it will feel numb. “When this happens, the thermoreceptors in your skin have stopped working,” Giesbrecht says.
Your skin gets rashy and red
Some people have an allergic reaction to cold—but not necessarily freezing—weather, a condition known as cold urticaria or less formally, “winter bumps.” It’s not a reaction to very cold weather, like frostbite, but rather just dry, cool conditions, which can cause an allergic-type reaction in certain people with sensitive skin.
You have trouble catching your breath
Normally when you inhale air, your nose helps moisten, humidify, and warm it before it moves down to the lungs. But in some people, like patients with asthma, or in some cases where the weather is just so bitter cold, the air doesn’t warm up enough first, which causes the lungs to spasm and constrict, which makes breathing difficult, says Kenefick.
Your muscles go on the fritz
As muscle tissue cools, it doesn’t work as well, says Giesbrecht. “At first, you lose the ability to do fine motor tasks, like using your cell phone correctly,” he says. “Then eventually [as your bigger muscle groups are affected], you might not, say, be able to hold on to an overturned boat in cold waters.”
