Swelling of blood vessels increases the amount of heat carried by the blood to the skin, allowing the excess heat to be expelled from the body. Blood vessels also constrict and conserve heat to raise body temperature.
Humans are homeothermic and maintain a relatively constant body temperature through the use of several heat regulating mechanisms, while poikilotherms' body temperature frequently changes with the environment. Vasodilation is one of the ways humans regulate body temperature. Humans also have sweat glands that allow cooling from the evaporation of sweat. Hormones can stimulate the metabolic rate and produce more or less heat.
Goosebumps are the body's attempt to raise hair follicles on the skin to adjust temperature. The body's tendency to shiver when cold is due to an increase in the contraction of muscles to generate heat. The hypothalamus is the organ responsible for controlling responses to heat and cold.
Exposure to a prolonged body temperature higher than normal is called hyperthermia. Fever is a hyperthermic condition created by the body in an attempt to eliminate foreign invaders. Hypothermia exists when the body temperature is lower than normal. Prolonged exposure to cold can slow down the metabolic rate and is a life-threatening event.