Cold water triggers goosebumps to trap heat, while warm water relaxes the skin to release it.

The goosebump reflex is scientifically known as piloerection. It is controlled by the arrector pili muscles, which are tiny smooth muscle fibers connected to hair follicles. When the sympathetic nervous system detects a drop in temperature, it releases adrenaline to make these muscles contract. This reaction is a vestigial trait from our ancestors who had thicker body hair to trap heat.In contrast, warm water or high ambient temperatures activate the parasympathetic nervous system. This triggers the release of acetylcholine, a neurotransmitter that signals the arrector pili muscles to relax. At the same time, the body undergoes vasodilation, where blood vessels near the skin surface widen. This process increases blood flow to the skin so heat can be radiated away into the environment.Research published in the journal 'Cell' in 2020 by Harvard University scientists found that these muscle contractions also play a long-term role. The researchers discovered that the same muscles responsible for goosebumps are linked to stem cells that regulate hair growth. This suggests that the body uses cold signals not just for immediate warmth, but to stimulate thicker hair growth over time. This complex biological thermostat ensures the human core temperature stays around 98.6 degrees Fahrenheit.