The vast majority of organisms function between 10-35°C. There are basically two ways to regulate body temperature and we use these to categorise organisms:
Homoiotherms - These are organisms that that regulate their own body temperature internally. Their internal body temperature is independent of the external temperature. (Don't use the term 'warm-blooded').
Poikilotherms - These are organisms that cannot regulate their own body temperature internally. Their internal temperature fluctuates with the external temperature. (Don't use the term 'cold-blooded').
In your brain is there is an area called the hypothalamus. The hypothalamus has a thermo-regulatory centre and this detects the temperature of your blood. You also have thermo-receptors in your skin and these detect the temperature outside.
A diagram of the process your body goes through to react to changes in external temperature is shown below:
You can also alter your body temperature behaviorally rather than internally. For instance, you can increase your temperature by doing exercise, or putting on extra layers of clothing. You can decrease your temperature by removing clothes, lying in a cool place, etc.
At extreme internal body temperatures your body has to employ other strategies.
Your Low Critical Temperature is about 27° C and at temperatures this low your metabolic rate changes.
At your Lower Lethal Temperature (about 25° C) your system will collapse and you will probably die.
Interesting point: One woman caught under a frozen waterfall whilst skiing survived after her body temperature had fallen to an amazingly low 13.5° C! The cold temperature had slowed down her brain so the amount of oxygen in her blood was sufficient to prevent any brain damage. She was revived after her blood was passed through a heart bypass machine, warmed and then returned to her body.
Your High Critical Temperature is very high, but if it is reached your metabolic rate cannot decrease to lower it. It increases out of control.
If your Upper Lethal Temperature of about 42°C is reached you will die.
Interesting point: Camels could sweat to stay cool in the desert, but they would lose too much water. Instead, unless the temperature is very high (over 40°C), they stop sweating and allow their extremely tolerant tissues to get very hot.
Since these animals cannot control their own body temperature they rely on their behavior to seek out an area at their optimum temperature. In the early morning and evening they are active since it is neither too hot nor too cold. If they need to warm up, they will bask in the sun. At midday they will lie in the shade, and at night they will lie in a crevice or burrow so that their immediate atmosphere is warm.