Most organisms operate within a relatively narrow range of body temperatures. The body temperature of humans can vary only a few degrees from 37°C without serious consequences. The main problem is that at too high a temperature, proteins change conformation and begin to denature. At too low a temperature, chemical reactions slow down too much. At 0°C and below, ice crystals form within cells and destroy them. However, the diverse animals of the world have numerous adaptations for surviving changes in body temperatures and for living in a wide range of environmental temperatures.
Humans and other mammals have a regulatory system for maintaining their body temperatures very close to their set points. Found in a region of the brain called the hypothalamus, this regulatory system acts as a kind of thermostat, receiving temperature information from sensory receptors and sending commands to controlled systems—the tissues and organs that respond by heating or cooling the body.
The vertebrate thermoregulatory system integrates many sources of information. The hypothalamus, itself, senses temperature and thereby acts as a major resource for temperature information. Temperature sensors in the skin send the hypothalamus information about environmental temperature. The hypothalamus also integrates information about the time of day, whether you are asleep or awake, whether you are exercising, and whether you have an infection, and it uses this information to create set points for thermoregulatory responses. By comparing the hypothalamic temperature with the set points, the hypothalamus generates error signals. These signals drive the body's thermoregulatory responses that keep the body temperature within an optimal range.
Experiments in which the hypothalamus is heated and cooled have shown that the hypothalamus is a major temperature sensor for the body and that an animal has separate set points for activating different warming and cooling responses. For example, as an animal's body temperature cools, blood vessels in the skin are among the first controlled systems to respond, and they do so by constricting. If the temperature drops more, shivering begins. When the animal's body becomes too warm, the blood vessels are again among the first to respond, this time by dilating. If the body temperature increases, panting may begin.
Other experiments have shown that these different set points can be adjusted. For example, when the hypothalamus is cooled while the animal is located in a cold environment, the animal initiates its heating responses before the hypothalamus itself has cooled significantly. Compare this to an animal in which the hypothalamus is cooled while the animal is in a warm environment. In this case, the animal's warming responses begin at relatively cooler hypothalamic temperatures. Sensors in the skin provide the hypothalamus with feedforward information, which triggers the hypothalamus to change its temperature set point. In the case of the cold environment, the change allows the hypothalamus to activate heating responses before the animal's body has cooled off significantly.
From these experiments, it is clear that the body's thermostat is a complicated regulatory system that allows the body to maintain temperature homeostasis within a wide range of environmental temperatures.
Textbook Reference: Concept 29.6 Animal Function Requires Control Mechanisms