Why is mammalian blood warm?

[Tranquility]

Biology is by far my weakest science.... And it appears the kids in my class have realized this.

 

I have been asked the following question and have no idea. Any help is greatly appreciated

 

Why is mammalian blood warm? how is it warmed? and where is it warmed?

[Mr Skeptic]

Any use of energy will produce waste heat -- this is what warms us. As I understand it, there is something called brown fat that is responsible for some of this.

 

Chemical reactions, in general, happen faster when they are warmer. Likewise, enzymes need to function throughout the temperature ranges of our body -- which for us warm blooded creatures is very small range. So now our enzymes don't need to function at colder temperatures. For these reasons, cold blooded creatures also like warmth, which is why many of them spend time basking in the sun, and why you won't see reptiles in the arctic.

 

The cost of this, is that we use tremendous amounts of energy just to stay alive, at least compared to cold blooded creatures -- who can easily go months or years without eating. Someone has suggested that it is not actually such a bad cost -- it also allowed mammals to live on a high-carbon low-protein diet (ie, plants), in which case we would still need to eat a lot to get protein and we actually make use of the excess carbon by turning it into heat. Likewise, our brain is not such a huge cost because we need to use that energy anyhow to keep warm.