What is the noticeable effect of the arrector pili muscle in humans Vasodialation of blood vessels in the dermis causing raised bumps on the skin

Cooler temperatures cause body hairs to flatten against the skin creating noticeable bumps.

Vasoconstriction of blood vessels in the dermis causing noticeable bumps on the skin.

Muscle contraction that causes body hair to stand on end creating "goose bumps