What does it mean when the hairs on your arm stand up?

The phenomenon of hairs standing up on your arm is called piloerection. It occurs when the tiny muscles at the base of each hair follicle contract, causing the hair to stand up straight. This is usually a response to cold temperatures, fear, or excitement.

When you are cold, your body diverts blood flow away from your extremities and towards your core in an effort to conserve heat. This can cause the skin on your arms to become cold and goosebumps to appear. Goosebumps are caused by piloerection, which traps a layer of air near the skin's surface, providing some insulation.

Fear and excitement can also cause piloerection, even if you are not cold. This is because these emotions trigger the release of adrenaline, which can stimulate the sympathetic nervous system, which is responsible for controlling piloerection.

Piloerection is a normal and harmless response to certain environmental and emotional stimuli. It is not a cause for concern unless it is accompanied by other symptoms, such as fever or pain.