Control of Body Temperature

Mammals and birds keep their core body temperature (the temperature of the blood and the organs deep inside the body) within narrow limits. In humans this is about 37 °C. Controlling it is an example of homeostasis: keeping the internal environment within set limits despite changes outside.The main reason is enzyme activity. Metabolic reactions are catalysed by enzymes, and every enzyme has an optimum temperature. If the core temperature falls, molecules have less kinetic energy, there are fewer successful enzyme-substrate collisions and metabolism slows. If it rises too far above the optimum, hydrogen bonds and other bonds holding the tertiary structure break, the active site changes shape and the enzyme is denatured. Denaturation is not reversed by cooling, so overheating is more dangerous than cooling.Temperature affects other things too: membranes become more fluid when warm, and nerve impulses travel more slowly when cold.Heat enters and leaves the body in two ways. It is generated internally by metabolism, mainly respiration in the liver and muscles, and it is exchanged with the environment by radiation, conduction, convection and evaporation. Body temperature stays constant only when heat gain equals heat loss.Control works by negative feedback: a change away from the norm is detected and responses are triggered that return the temperature towards the norm. Animals differ in how they do this. Endotherms rely mainly on heat from their own metabolism, while ectotherms rely mainly on heat from their surroundings. These two strategies are covered in the next lessons.