Why does is the human body temperature 98.6 degrees?

I was just wondering what makes all that heat in our bodies. I have a theory that it is caused by the friction of our blood running against the walls of our veins but I’m not sure. So if you could just verify this or tell me the real reason I would appreciate it.

It’s the burning of food. One calorie of food provides enough energy to raise 1 kg of water by 1 degree Celsius (2.2 pounds by 1.8 degrees F). If you eat 2000 calories per day, much of that is turned into heat for your body by digestion. The rest of the energy goes into moving muscles (to walk, pump blood, etc.) and powering other internal activies.


If your body temp is raised by the burning of calories; what keeps it at 98.6 degrees? Something else has to regulate this because we all don’t eat the same thing everyday.

But it seems no matter how much you eat, 1000 or 6000 calories a day, … your body temperature remains at 98.6F!
Unless you are sick and have a fever ( another question ). Why? Is it that you will store the excess calories as fat in order to maintain the body’s temperature at 98.6F? I agree that calories are expended (“burned”) as needed to maintain the body’s temperature.

You breathe out some of it. Natural radiation takes care of some of it. Sweating helps. There are lots of ways you get rid of it. Plus, as DeutschFox says, not all of the calories you eat will be used. Some are stored as fats, others as large carbohydrates like glycogen, etc.

First of all, the calories listed on your food packages are actually kilocalories. 1 kilocalorie raises the temperature of 1 kg of water 1 degree C, as Arjuna stated.

Temperature regulation involves a lot of complex and sophisticated processes in the body - you can throw the “friction of the blood” idea right out.

A few examples: when you’re cold, you shiver. The shivering muscles burn stored energy, releasing heat. Also when cold, blood flow to the extremities decreases. Your ears, fingers, toes, etc actually drop in temperature and the warm blood is kept in the torso to keep the vital organs warm.
When you’re hot, you sweat. The evaporating water removes excess heat from the body.
There are lots of these kinds of pathways that regulate temperature.

Some more information here

Good site, android209. I was going to mention that 98.6 is an average. Roommate is about 97.1 and I run a usual 99.2.

98.6F is not average anymore or so they said.

The human body is around 98.6C because your hypothalamus tries very hard to keep it that way regardless of what you eat or how you sweat. True that each gram of fat gives you 9kcal/g, alcohol 7kcal/g, carbohydrates and protein about 4kcal/g. These are converted through the usual boring biochemical pathways (Kreb’s cycle, fatty acid oxidation, etc.) into sugars. The real action takes place in the mitochondria where the oxidation chain generates lots of ATP energy and some heat too. Babies have “brown fat” at their shoulder blades allowing htem to make more heat and less energy by uncoupling the final oxidation cycle. The amount you sweat, pant and bring blood close to the skin to cool off depends on how far from your hypothalamus setpoint you are (which depends on what you are doding). The setpoint can be affected by certain infections. Temperatures change by a few degrees depending on if you measure the oral, rectal, axillary or ear drum temperature. It also changes throughout the day, chances of you being exactly 98.6F are low, this is what happens to any specific number on a standard deviation curve.

Who is this “they” you speak of?

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Your body is one big chemical reaction. My guess would be that that is the optimal temperature for all of these chemical reactions to take place.

98.6 never was average. When human body temperature was first measured, by (I believe) Celcius, he recorded it as 37 degrees Celcius, knowing that his instruments weren’t good enough to give any more than one degree of precision. When that’s converted to Farenheit, you get 98.6, but that’s saying that there’s a lot more precision than there actually is in that measurement.

Whether or not Dr_P is an M.D., he is closer to the truth.
The essential mistake of all is epitomized by WiredGuy: your body is not “one big chemical reaction”. It’s a view of an educated plumber (uneducated one thinks that it’s blood friction; why big rivers are not boiling then? the friction of gallons of water is greater).
If Andres Celsius measured the body temp at 37deg in 1742 or 1743, he made a mistake. It still was 36.6deg (actually, it’s closer to 37deg in the liver). It’s ~39deg in many mammals. There is a several degree range at which biochemical reactions occur; many are faster at 56degC, at which temperature many thermal cabinets are kept in the labs. I don’t know why it’s 37deg or so in humans. I think that it’s some “compromise” between an optimum for biochemical reactions and heat loss, which would be greater in our environment at higher body temperatures.
In all mammals (and birds) the range is quite narrow, 35-40C. The stability of the temperature in a healthy individual is remarkable. Whether it’s maintained by the hypothalamus alone, it’s never off by more then one or two decimal points. Any “big chemical reaction”, on the Sun or in the DuPont reactor, can only “envy” that precision. Incidentally, it’s very common for human body to maintain many things in the very narrow range simultaneosly! For many years they are trying to simulate only one function - blood sugar concentration - with dismal results. Despite all the successes, like in chess playing, space exploration, etc. One function only!

My pal, there is a wonderful study of the human body temperature throughout the day at:

As you can see, its not really 98.6, but close…

Well, this is not exactly true. Your body temperature will acutally fluctuate by several degress depending on your activity. If you are exercizing heavily, your body temperature will be warmer, if you are resting it will be cold. But the main point is true. There are several ways to generate heat, and several ways to lose heat. Your body will choose to maintain itself to within a few degrees of the optimum.

When you have a fever, the virus isn’t making your body hot, your immune system decides to increase your body temperature. Many viruses and bacteria like things a little cooler and turning up the body temperature is a way to slow them down. Sometimes these mechanisms break down and you get a runaway fever, but parental panic over fevers is almost always unwaranted.

Anyway, the bottom line is that most of the heat generated by your body comes from the burning of fuels from your food. Mammals burn lots of food to keep their bodies at a high temperature, and generally cannot tolerate much fluctuation. But reptiles burn much less food and are quite cold unless they have external heat. But most reptiles prefer to keep themselves warm. An active lizard probably has a body temperature as high as a mammal’s, it’s just that they got hot from basking in the sun rather than burning fuel. So mammals need somewhere around ten times as much food as reptiles of the same body weight.

When Farenheit made his little marks on the tube of mercury, he used his lab assistant, who was running a slight fever, as the benchmark.

If we were rational, our temperature would be 100 degrees.

But we’re not rational…which would explain 98 Degrees.

Ready? If the human body is 98.6 deg, why then are we hot when the temperature is 98.6 in the room? ah, fun. I think Cecil answered this?

I’m not a doctor, but I can answer that one, handy. Our body is continually producing heat though various processes, so if we want to maintain a constant temperature, we need to lose heat at the same rate. One of the main ways we lose heat is via conduction to our environment. However, this only works if our environment is cooler than us, and if it’s hotter, we’ll actually gain even more heat this way. When the ambient temperature is equal to body temperature, we can’t cool down as easily.

As to Celcius’ measurement,

Npo, peace, he didn’t make a mistake. Had he said it was 37.0, that would be a mistake, as that implies that it’s somewhere between 36.95 and 37.05 . As it is, he just said 37, which implies that it’s between 36.5 and 37.5, which is correct. The problem is that when that number was converted to farenheit, it wasn’t recognized that it wasn’t precise, so too many digits were given in the converted form.