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Apart of the brain, the hypothalamus, acts as the body's thermostat, maintaining this steady temperature no matter how hot or cold the weather. Though the extremities may be chilled, the body's core - and the brain - are protected.
Neurons in the hypothalamus constantly monitor blood flowing through it. When blood temperature rises, the hypothalamus promptly sends out messages through the autonomic nervous system, producing sweat and dilating the blood vessels in the skin.
Low body temperatures, picked up by sensors in the skin, signal the blood vessels in the skin to constrict. The muscles begin to produce heat-generating shivers. Your teeth start to chatter, and goose bumps appear as the fine hairs on the skin stand on end. In addition, the hypothalamus causes the release of the hormones epinephrine and norepinephrine into into the blood. These hormones temporarily increase the body's metabolic rate and produce heat.