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How does body respond in cold environment? List some key strategies body use to gain heat and reduce heat loss.

· Heat production by shivering: Shivering is the alternate contraction and relaxation of small muscle groups and is an involuntary action. When muscles shiver, almost all of the energy of contraction is converted into heat energy.

· Heat produced by metabolism: Neurosecretory cells in the hypothalamus produce thyrotropin-releasing hormone (TRH). This hormone is transported to the anterior pituitary where it stimulates the secretion of thyroid-stimulating hormone (TSH). Thyroid-stimulating hormone is transported in the blood to the thyroid, which in turn increases its output of thyroxine. Thyroxine is a hormone that increases the metabolic rate of all cells of the body, resulting in an increase in heat production.

· Motor nerves from the hypothalamus also cause the medulla of adrenal glands to secrete adrenaline and noradrenaline. These hormones increase the basal metabolic rate

· Constriction of blood flow in skin: When cold is detected, neurons in the hypothalamus send impulses via the sympathetic nervous system to peripheral blood vessels in the skin. The impulses cause arterioles to constrict. This constriction reduces the surface area across which heat transfer can occur and reduces the amount of blood flow close to the skin. Hence heat lost via the skin surface is reduced.

· Piloerection: Piloerection means ‘hair standing on end’. Sympathetic motor neurons convey impulses from the hypothalamus to a muscle at the base of each hair causing the muscle to contract and the hair to become erect. A layer of air is trapped in the erect hair or fur and acts as an insulation layer between the skin of the animal and the external environment.