You know, what with being warm-blooded and all. Can it be said how more or less clearly for my lame brain?; i.e., I “know” that fat and glucose and the hypothalamus all come into play, but then I get lost in the trees.
Somewhere there has to be – one? many different? – exothermic chemical reactions that you can point to and say: the temperature has just gone up 0.00x°, thus (re-)warming up this spot, and then the bloodstream (only?) picks up the heat from all these spots is passes through – is this so broad as to be wrong? – and makes sure entropy doesn’t win for a while.
[If it sounds like this OP started out as wordplay, it’s because it did. I then realized it was true Ithe-less-symbolic-version-ofRL but didn’t know why.]
Here’s some reading to get you started until a real bio guru stops by. The article is short on details about the thermodynamics, but it’ll give you a host of new terms to chase down.
Entropy always wins. Always. Just about every bio-chemical reaction is exothermic. The few biochemical reaction that are endothermic are things like photosynthesis. You’re not a plant. Well, I’m assuming you’re not a plant, this is the internet, you could be an especially intelligent geranium named “Leo” wondering why “Leo Bloom” for all I know.
Anyway, animals are an exothermic system. Digestion itself is exothermic. There’s always heat loss. There’s always entropy. Shoot, there’s even heat loss in turning excess calories into fat. There’s heat lost in building muscle. There’s heat loss in a baby growing. Increased order in a small system does not mean entropy was cheated. Building that order is never 100% efficient. The perfection in efficiency is lost as heat. That’s why entropy always wins.
However, how the body manages this heat may be the question you’re trying to ask. That gets into thermoregulation. That’s not about endothermic reactions, but rather dumping the thermal energy created by exothermic reactions when the temperature is too hot and speeding up some exothermic reactions when it’s too cold.
Let me add to that. Diabetes does fuck up some of the thermoregulation. In a nut shell, insulin is the probably the most important hormone in metabolism, and metabolism is probably the chief component of thermoregulation. If either the production of insulin (Type I diabetes) or its receptors (Type II) is wonky, then the thermoregulation is wonky, too. Those reactions are all exothermic, though.
Liver most constant source and active and muscles most involved in regulating heat production.
Sources include the oxidation of food itself, especially protein, muscles (during strenuous exercise 75% of the energy goes towards heat production and only 25% to work), and then other organs.
So 50% efficiency is pretty good for glycolosis and that means the other half if going to heat.
Finally this article contains some tables that might allow an answer:
Average masses, energy use per kg, and thus total energy used at rest by organs in the human body:
Liver 1.39 kg; 200 Cal/kg/d = 278 Cal/d
Brain 1.33 kg; 240 Cal/kg/d = 319 Cal/d
Heart 0.31 kg; 440 Cal/kg/d = 136 Cal/d
Kidneys 0.29 kg; 440 Cal/kg/d = 128 Cal/d
Skeletal muscle @26kg; 13 Cal/kg/d = 338 Cal/d
And on average 19 kg adipose and 24.7 kg “residual” with lesser contributions but still adds up to BMR.
Muscles at work have less efficiency but at rest all are likely roughly 50% to heat, again with control being partly by how much muscle activity, and various cooling mechanisms (such as sweating and vasodilation)