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The optimum human body temperature is 36.8°C. A constant body temperature favours efficient enzyme reaction.Temperatures above optimum denature enzymes, while temperature below the optimum range inactivate enzymes. The skin has receptors that detect changes in the temperature of the external environment. When the body temperature is above optimum the following takes place:

1. Sweat: Sweat glands secrete sweat onto the skin surface. As sweat evaporates it takes latent heat from the body, thus lowering the temperature.

2. Vasodilation of Arterioles:The arterioles near the surface become wider in diameter. More blood flows near the surface and more heat is lost to the surrounding by convection and radiation.

3. Relaxation of hair erector muscle: When hair erector muscles relax, the hair lies flat thus allowing heat to escape from the skin surface. When body temperature is below optimum the following takes place:

4. Vasoconstriction of Arterioles: The arterioles near the surface of the skin become narrower.Blood supply to the skin is reduced and less heat is lost to the surroundings.

5. Contraction of hair erector muscles: When hair erector muscles contract, the hair is raised.Air is trapped between the hairs forming an insulating layer.Animals in cold areas have a thick layer of subcutaneous fat, which helps to insulate the body.

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