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.
When the body's temperature is warmer than the set point, the hypothalamus sends instructions to various organs to cool the body down. When the body gets too cool, the hypothalamus commands the body to do the opposite and perform a warming response.
Blood vessels are some of the targets under the control of the hypothalamus. For example, when the body is cooler than its set point, the hypothalamus triggers blood vessels in the skin to constrict. The constriction prevents blood from circulating close to the body's surface and thereby reduces heat loss to the environment.
When the body is too cool, the hypothalamus also stimulates shivering. The repeated contractions of muscle fibers generate heat in the body.
The hypothalamus controls an endocrine organ called the thyroid gland. When the body is exposed to cold for a prolonged period of time, the hypothalamus triggers this gland to release hormones that, in turn, trigger cells of the body to increase their metabolic rates. A higher metabolic rate results in a greater production of body heat.
When the body gets too warm, the hypothalamus triggers cooling responses. These responses include dilating blood vessels near the body's surface, increasing perspiration for evaporative cooling, and accelerating the rate of breathing, which also cools the body.
We can describe these actions of the hypothalamus in a diagram. The hypothalamus receives temperature information from receptors in the skin, in the viscera, and in the hypothalamus itself. With this information, the hypothalamus compares the body's temperature to its set point.
Differences between the actual body temperature and the set point constitute error signals. The hypothalamus operates through a negative feedback loop. For example, if the body is too cool, the hypothalamus triggers a warming response. In a negative feedback loop the response counteracts, rather than enhances, the deviation from the set point and returns the body's temperature to its set point.
A key sensor for temperature information appears to be the hypothalamus itself, although the hypothalamus also integrates information from other sensors in the body. An experiment on a ground squirrel, in which the hypothalamus was either warmed or cooled, provides evidence that the hypothalamus acts as a temperature sensor.
When the hypothalamus was cooled, the animal increased its metabolic rate and thereby elevated its body temperature. In contrast, when the hypothalamus was heated, the squirrel's body temperature dropped. This experiment demonstrates that the hypothalamus acts as a temperature sensor in addition to being the body's thermostat.
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.