What is the purpose of the arrector pili muscle?

The primary purpose of the arrector pili muscle is to raise hair follicles, which results in "goosebumps." This muscle is a tiny smooth muscle that attaches to hair follicles, and when it contracts, it pulls the hair upright. This reflexive action occurs in response to cold temperatures or emotional states such as fear or excitement. The formation of goosebumps also serves as an evolutionary response; in ancestors with more body hair, raised hairs would add insulation by trapping more air or make the animal appear larger to potential threats.

Other functions listed in the options do not identify the role of the arrector pili muscle specifically. For instance, while sweating is facilitated by sweat glands, it is not directly influenced by the contraction of the arrector pili muscle. Similarly, blood circulation is managed primarily through the vascular system rather than by this specific muscle, and though the skin plays a role in UV protection—with its various layers—the arrector pili muscle does not contribute to this protective mechanism. Thus, the correct answer highlights its unique function related to hair follicle movement.