Body temperature stays constant when heat gain equals heat loss. Heat is produced inside the body by metabolism, mainly respiration in the liver and muscles, and is exchanged with the surroundings in four ways:• radiation: infrared energy given off or absorbed by the skin
• conduction: transfer to or from anything the body touches, such as air, water or the ground
• convection: moving air or water carries heat away from the skin
• evaporation of water from sweat and from the lungs, which uses latent heat and so cools the bodyWhen the surroundings are hotter than the skin, the body gains heat by radiation, conduction and convection, so evaporation is then the only way to lose heat.
Responses that increase heat loss (when too hot):
• vasodilation: arterioles supplying the skin capillaries dilate, so more blood flows near the surface and more heat is lost by radiation
• sweating: sweat glands secrete more sweat, which evaporates
• hair erector muscles relax, so hairs lie flat and trap less insulating airResponses that reduce heat loss or increase heat gain (when too cold):
• vasoconstriction: skin arterioles constrict, so less blood flows near the surface
• less sweat is produced
• hair erector muscles contract, raising the hairs to trap a layer of air
• shivering: rapid, involuntary contractions of skeletal muscles release heat from respiration
• a higher metabolic rate, raised by the hormones thyroxine and adrenaline
• behaviour, such as putting on clothes or moving into the sunWorked example: evaporating 1 g of sweat removes about 2.4 kJ of heat. Losing 500 g of sweat in an hour removes 500 × 2.4 = 1200 kJ.
• conduction: transfer to or from anything the body touches, such as air, water or the ground
• convection: moving air or water carries heat away from the skin
• evaporation of water from sweat and from the lungs, which uses latent heat and so cools the bodyWhen the surroundings are hotter than the skin, the body gains heat by radiation, conduction and convection, so evaporation is then the only way to lose heat.
Responses that increase heat loss (when too hot):• vasodilation: arterioles supplying the skin capillaries dilate, so more blood flows near the surface and more heat is lost by radiation
• sweating: sweat glands secrete more sweat, which evaporates
• hair erector muscles relax, so hairs lie flat and trap less insulating airResponses that reduce heat loss or increase heat gain (when too cold):
• vasoconstriction: skin arterioles constrict, so less blood flows near the surface
• less sweat is produced
• hair erector muscles contract, raising the hairs to trap a layer of air
• shivering: rapid, involuntary contractions of skeletal muscles release heat from respiration
• a higher metabolic rate, raised by the hormones thyroxine and adrenaline
• behaviour, such as putting on clothes or moving into the sunWorked example: evaporating 1 g of sweat removes about 2.4 kJ of heat. Losing 500 g of sweat in an hour removes 500 × 2.4 = 1200 kJ.