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 to freezing cold, 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. Here are the 10 surprising health benefits of cold weather.