Goosebumps begin when tiny muscles in your skin are forced to contract, which raises the hair on your skin. This causes little bumps on your skin. It helps to push the blood vessels deeper under your skin so that they aren't exposed to cold air. Sometimes you'll get them when you're nervous or scared, or during stress because it's your body's fight or flight reaction, which generally happens when you get an adrenalin rush. Then it is also closely related to shock.
The term "goose bumps" gets its name from geese. Goose feathers grow from stores in the epidermis which resemble human follicles. When a goose's feathers are plucked, its skin has protrusions where the feathers were, and it is these bumps which the human phenomenon resemble.
answered May 26 '11 at 08:00