At 20-degree centigrade atmospheric temperature, body reacts by

The core temperature of a human is regulated and stabilized primarily by the hypothalamus, a region of the brain linking the endocrine system to the nervous system, and more specifically by the anterior hypothalamic nucleus and the adjacent preoptic area regions of the hypothalamus. As core temperature varies from the set point, endocrine production initiates control mechanisms to increase or decrease energy production/dissipation as needed to return the temperature toward the set point.Cutaneous vasoconstriction -Aerioles carrying blood to superficial capillaries under the surface of the skin can shrink (constrict), thereby rerouting blood away from the skin and towards the warmer core of the body. This prevents blood from losing heat to the surroundings and also prevents the core temperature dropping fuher. This process is called vasoconstriction. Shivering - Muscles can also receive messages from the thermoregulatory center of the brain (the hypothalamus) to cause shivering. This increases heat production as respiration is an exothermic reaction in muscle cells. Shivering is more effective than exercise at producing heat because the animal (includes humans) remains still.BMR - basal metabolic rate - rate of metabolism; the minimal amount of energy expended by our body during rest. Ref: Ganong&;s review of medical physiology;24th edition; page no-31