Most body processes have obvious origins, but what keeps us at ye olde 98.6? I know that if we need to cool down, we generate sweat, but what warms us up? How is a fever created?
The only thing I can think of is friction in the bloodstream, but that seems kind of silly.
HennaDancer
Body heat come from “burning” food iside your cells. A fever occurs when your body, in response to an infection, retards the processes that normally cool you, allowing excess heat to build up. Vastly simplified, of course.
Also note that 98.6 degrees is a myth. Body temperature varies considerably from person to person and over time. Some people have higher metabolisms and have higher body temperatures. If you sleep your body temperature goes down, if you are active it goes up.
Also, 98.6 is one possible healthy temperature within a range of healthy body temperatures. Everyone’s natural temperature is somewhat different, but we’re all above about 97 and below 100. The hypothalamus (which is the regulatory center of the brain that concerns itself with body heat) isn’t calibrated to a tenth of a degree. 
Did you ever hear of the term “calories”? One kilocalorie (which we call one “calorie” or better - but not quite correct - one “Calorie”) is the amount of heat necessary to raise one cc of water one degree Centigrade at atmospheric pressure.
Most of the calories we ingest go to keeping our furnace active. Only a small percentage actually goes to metabolism, moving muscles, being active, etc.
I was surprised to read in Guns, Germs, and Steel that a fever is not a “byproduct” of another process so much as a means to raise the body temperature to kill off bacteria.
Swing and a miss! One calorie is the amount of heat needed to raise the temperature of 1 cc of water by 1 degree C. A food Calorie, or kilocalorie, is the amount of heat needed to raise the temperature of one liter 1 (1000 cc) or water by 1 degree C. I think you know that, but got a case of “fumble fingers”. However, pressure doesn’t enter the equation so much. It can be largely ignored here, as it really only affects phase change. I’ve never seen it included in the definition.
Leaving aside calories and kilocalories for the moment, I think the OP could use a more complete answer. IANABiochemist but here goes:
As I understand it, nutritious food, mostly complex carbon compounds, passes through the walls of blood vessals in the intestines and thus enters the blood stream in molecular form. These molecules are then distributed to the various cells throughout the body.
The cells ingest the food molecules and, in a chemical process carefully controlled by enzymes, uses oxygen (also delivered by the bloodstream) to oxidize (“burn”) the food. This process is exothermic (heat-producing) and this heat is then used by the cells to stimulate motion and, of course, to keep us warm.
Our body temperature is thus controlled by regulating the rate at which cells throughout the body burn nutrients.
This contributes nothing to the answer. I would just like to give HennaDancer props for an excellent forum question. We seem to see fewer of these types of questions of late.
There are a few other ways the body regulates heat, including sweating when the body is too hot, and goose bumps and shivering when the body is too cold. Sweating reduces temperature as the water on the skin evaporates; goose bumps would increase insulation if you have a lot of hair on your skin (but of course most people don’t), and shivering warms you up because the muscle movements burn up the calories (you would get the same effect by doing exercises or running around).
The body. 
I was going to add, though, that I thought that the blood vessels had something to do with it as well. The hot blood going through the arteries, etc?
Maybe I’m misunderstanding the question. Are we discussing how the body produces heat, or what makes us radiate that heat?
Well said, Satyagrahi.
Now, the next logical question is: why aren’t all animals who burn food warm blooded?
How do birds and mammals control their body temperature but reptiles and fish cannot?
As I understand it… reptiles and fish don’t need to control their body temperatures. Their systems are designed to operate at lower temperatures and over wider range of temperatures, so they burn only as much food as they need, and as efficiently as possible.
Mammals and birds, on the other hand, have developed systems that require slightly warmer operating conditions to function appropriately… thus the metabolism is altered to adjust the consumption of energy to sustain that temperature… allowing a greater proportion of the energy ‘burned’ to dissipate as internal heat, and sometimes burning energy for no other reason than to maintain temperature. They also tend to use more energy for other purposes than the ‘cold-blooded’ life forms, which requires them to eat more.
Actually, it does enter the equation so much. Check this out for a brief explanation. For more detail, you can look in a pchem textbook or something that deals with statistical thermo.
I don’t feel like actually calculating how the heat capacity of the water in a human would change under the range of physiological conditions found in a human. I doubt it is very large, so the approximation given previously is probably accurate enough.
It’s been ages since I took physiology, but it is my understanding that the actual heat-producing mechanisim is muscle tone. Even a relaxed muscle has individual fibers that tense up a few seconds, then relax, expending energy and making things warm. I would suppose cold-blooded critters’ muscles don’t do that.
DD