Why do the small hairs on the arms stand up when one gets into a very hot bath just as they do when exposed to cold weather?

Answer 1

Goose bumps are a vestige from the days when humans were covered with hair. When it's hot and you need to cool down, little muscles at the base of each hair relax. Your hair becomes relaxed. Your sweat glands pump out body heat in sweat. Your blood vessels get big to take more heat to the skin to get rid of it. When it's cold, the arrector muscle pulls the hair up. The duct to the sweat glands gets small to conserve heat. Our blood vessels also get small to save heat. Hair standing up doesn't make very good insulation - we don't have enough fur for that. Humans don't have very much hair on their bodies anymore. Millions of years ago, humans probably did. And that hair standing on end helped keep people warmer. Those little muscles we have on the end of each hair still work. They still make goose bumps. Cold is not the only thing that can cause our hair to stand on end. Fear or anger can cause the same reflex. The same is true for other mammals. You'll notice that on a cat or dog. Their fur gets bigger when they're angry or afraid.

Answer 2

When the body is exposed to extreme temperatures, whether hot or cold, the small hairs on the arms may stand up in response to the temperature change. This is a natural physiological reaction called piloerection, where the hairs are raised to help regulate body temperature by trapping air near the skin to either insulate and retain heat or aid in cooling the body.