When core temperature rises above the norm, for example during exercise or on a hot day, the hypothalamus triggers responses that increase heat loss and reduce heat production.Sweating. Sweat glands secrete more sweat onto the skin. Water in sweat needs energy to evaporate (its latent heat of vaporisation). This energy is taken from the skin as heat, so the skin and the blood flowing through it are cooled. When the surroundings are hotter than the body, radiation, conduction and convection transfer heat into the body, so evaporation becomes the only way to lose heat. Sweating cools less well in humid air, because less water can evaporate. Some mammals, such as dogs, have few sweat glands and pant instead, losing heat by evaporation from the mouth, tongue and airways.Vasodilation. Arterioles supplying the skin dilate, so more blood flows near the surface and loses heat by radiation (see the Vasodilation lesson).
Hairs lie flat. Hair erector muscles relax, so the hairs lie flat against the skin. Less air is trapped, the insulating layer is thinner and heat is lost more easily.Less heat produced. There is no shivering, and over a longer period the metabolic rate can be lowered by reduced secretion of thyroxine.Behaviour. Resting in the shade, reducing activity and spreading out the body all help.Worked example. Evaporating 1 g of sweat removes about 2.4 kJ of heat. If 500 g of sweat evaporates during a run, the heat removed = 500 × 2.4 = 1200 kJ.
Hairs lie flat. Hair erector muscles relax, so the hairs lie flat against the skin. Less air is trapped, the insulating layer is thinner and heat is lost more easily.Less heat produced. There is no shivering, and over a longer period the metabolic rate can be lowered by reduced secretion of thyroxine.Behaviour. Resting in the shade, reducing activity and spreading out the body all help.Worked example. Evaporating 1 g of sweat removes about 2.4 kJ of heat. If 500 g of sweat evaporates during a run, the heat removed = 500 × 2.4 = 1200 kJ.