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Homeostasis: the maintenance of a balanced internal environment
Homeostatic control of body temperature in a bird or mammal is controlled by the nervous system. Among other things, this nervous system contains many sets of internal "thermostats".
These thermostats are prone to "click" if the temperature in their local area becomes too high or too low. By this, I mean that if the temperature in a particular area goes outside boundaries that the body has set then the thermostat changes its position. This, in turn, sends a signal to the brain alerting of this fact, and thus making it aware that the temperature has changed unacceptably in that area.
To counter the change, the brain then responds by commanding the hypothalamus to emit hormones. These hormones will cause several different effects, depending on the severity of the change. One effect is shivering (cold), another panting and sweating (hot) and "goosebumps", or the contracting of the skin (cold). The hormones do not do the task themselves - the hormone causes a chemical change in the area to which it is sent which then causes the effect itself.
Take the case of a person, soaked in water and then put in a cold wind. This person will have lost a lot of surface heat in the initial soaking, and then the wind comes and takes away the heat that was meant to warm the skin again.
When the person was first wet, there would have been a drop in body temperature. The brain would have been informed of this through its system of thermostats and nerves, and would have responded by telling the brainstem to tell the heart to pump blood quicker (to restore heat).
When this heat was drawn off from the wind, goosebumps would have appeared, because the brain had realised that the blood treatment was not living up to its expectations and informed the hypothalamus (?) to tighten the skin to keep heat in. This would have worked to a degree, but the wind is still hard at it and the body is still losing heat.
The brain then instructs the reflex centre of the body, the spinal cord, to set the whole body shivering. This produces motion (which uses energy) but also conserves heat.
Unfortunately, these procedures carried out by the brain are often not enough to bring body temperature back up, and unless other methods are taken, such as changing into dry clothes, the person will suffer from over-exposure and may, if left long enough, even die.
What I have been trying to show over the past few paragraphs is the way in which the body constantly monitors and acts on the changing conditions in it. If the temperature drops, elaborate steps are taken to attempt to bring it back up. If it is short of energy and there is no food in the stomach to consume, then we will feel hunger. In short, there is a "delicate interplay between the hormonal system and the nervous system" which helps keep our bodies in a stable condition.