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It is obviously very onerous to generate heat at all, although it has advantages. We don't have to lie on the sun like a crocodile to get warmer. And we avoid the freezing of our body water by out own means. But wouldn't just a little of heat be enough?

Why do humans go as far as 37 C? Couldn't we just have a temperature of 10 C to avoid freezing but stay at it?

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Read this article ncbi.nlm.nih.gov/pubmed/15217694. It says that a lower temperature may have anti-aging effects, but it seems the human body is not 'designed' for other temperature en.wikipedia.org/wiki/… –  Cornelius May 8 at 11:35
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It's simply not possible for humans to live with an absolute body temperature below 20 C for the simple biophysical reason that it would prevent flux of protein through the Golgi and therefore biosynthetic and secretory pathways, as well as endocytosis. The lipid membranes become too "stiff" to correctly function.

My own sense is that the nominal 37 C body temperature is a result of all of the ATP hydrolysis reactions that happen all the time. The chemical reaction is not efficient, and some of that energy is absorbed, heating the tissue.

There are various grades of hypothermia, from mild (33–35 °C) to severe (< 28 °C). Death from hypothermia results only from prolonged exposure, for example, being in ice-cold water results in death after 50-100 minutes. The basic mechanism is that the body dies from a combination of decreased electrical conductance of pacemaker nerves in the heart, and a slowed heart rate incompatible with sustaining life. Ultimately, hypothermic death is a dysregulation of the body's thermoregulatory mechanism, rather than an immediate biochemical problem as I've pointed out above. For a detailed review, there is an open access article detailing hypothermic etiology and pathophysiology.

M.L. Mallet. Pathophysiology of accidental hypothermia. QJM (2002) 95 (12): 775-785.

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And why are temperatures of 35 C or less already life threatening? What happens (or probably doesn't happen) when the temperature drops 3-4 degrees? –  Quora Feans May 8 at 12:13
    
it isn't out of the question for eukarotes to flourish at low temperatures - see en.wikipedia.org/wiki/Psychrophile , membrane fluidity could be adjusted via the content of unsaturated fatty acids in phospholipids. What would be challenging would be frequently undergoing large temperature shifts. –  Alan Boyd May 8 at 13:11
    
@Alan - that's true. I should clarify I was talking about humans only since that was the focus. I've edited my answer. –  leonardo May 8 at 15:05
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