Many instances of goosebumps are experienced in the presence of cold temperatures. The stimulus of cold surroundings causes the tiny muscles attached to each hair follicle to contract. This contraction causes the hair strands to literally "stand on end." At the same time, the tiny muscles that are contracting are causing a "bunching" of the skin surrounding the hairs, which results in the "bumps" in goosebumps.
In winter, when people get cold outside, they can experience goosebumps. This is the body's way of preserving its own heat by causing the hairs on the skin to stand up, thus reducing heat loss. Goosebumps are often seen in conjunction with shivering in these instances.
Effect of Heat
Ironically, many people find it hard to understand why people get goosebumps when they are hot, or in the presence of extreme heat. The main reason for this is, sweat. As the perspiration accumulates on the skin, it naturally evaporates. As the sweat evaporates, it cools down the skin surface. As this process occurs, a dramatic temperature difference occurs and the body responds to the "chill" of the evaporation of the sweat and the "goosebump response" kicks in.
Intense Emotion as a Cause of Goosebumps
Ever watch a scary movie and get chills from the suspense, or from a sudden shocking scene? This is the body's response to intense emotion--in this case, fear. People often say when they are scared that they feel their "hair standing on end." Another intense emotional situation that can cause goosebumps is the "fight or flight" response the body can employ in an extremely stressful situation. As the body prepares itself for either fighting or running, it floods the system with adrenalin, the chemical that speeds up heart rate and metabolism in the presence of extreme stress. If you have ever seen a dog that is preparing to fight an intruder or another dog that is invading its "turf," you'll notice the hair on the back of its neck stand up. This is another example of the fight or flight response. These processes also can cause goosebumps as a means of preserving body heat.