What occurs when the arrector pili muscles contract?

When the arrector pili muscles contract, the primary effect is that hairs become erect, leading to the phenomenon known as goosebumps. These small muscles are attached to hair follicles and are responsible for the hair standing up when stimulated, which is often a response to cold temperatures or strong emotional states such as fear or excitement. This reaction is part of the body's fight-or-flight response and serves to create an insulating layer of air for warmth or to make an individual appear larger to potential threats.

This involuntary muscle action results in a noticeable texture on the skin, known as goosebumps, as the skin around the hair follicles pulls inward and the hair shafts are caused to rise.