Final answer:
The arrector pili muscle attaches to the hair follicle, causing hair to stand up (goosebumps) when contracted, often due to cold or emotional stress, as part of the body's response to conserve heat.
Step-by-step explanation:
The arrector pili muscle is a tiny muscle that attaches to the base of a hair follicle. When this muscle contracts in response to external stimuli, often cold or emotional stress, it causes the hair to stand up, a phenomenon known as goosebumps.
This reaction is part of the body's sympathetic nervous system response and is designed to help insulate the body by trapping more air, which can help conserve heat. Additionally, the muscle's contraction can help to express sebum from the sebaceous gland.