Is it our blood going through the heart, but in that case wouldn’t that mean our heart is a furnace?
In: Biology
The bit missing from other explanations is how heat is generated.
In mammals the nutrients we eat and the oxygen we inhale are used by the mitochondria that are present in every cell of our bodies to generate energy in the form of heat. This is *endothermic* (internal energy) and is unique to modern animals (mammals and, I think, some birds – can’t remember off the top of my head).
All other animals are exothermic (external energy), so can only warm themselves using external sources – eg basking in the sun or staying in warm water.
All of the other mechanisms described in this thread are designed to stop the internal heat becoming excessive (ie becoming a dangerous fever). However, when you are ill allowing the body temperature to rise is one (not terribly reliable) method the body has for combatting infections.
tl;dr: You are less likely to be eaten by a crocodile (exothermic) at the crack of dawn (no overnight heat) than at noon (nice and toasty), but a lion (endothermic) will happily hunt whenever food is around, day or night.
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