system
Given the following post, respond as if you're commenting on Reddit:
human
Where does body heat come from?

I understand that blood is warm and the cardiovascular system distributes blood/oxygen/heat throughout the body, but how does the blood get hot in the first place?
gpt
The catabolism of sugar, fat, and protein with oxygen to make ATP (basically transferring energy from chemical bonds to a different type of bond that is used by the body's cells as fuel) is not 100% efficient. This produces heat and thus warms our blood. The blood circulates to your core and warms your body. 

Heat and sound are types of energy leaving a system. An engine, for example, is not efficient and loses a lot of energy from combustion of fuel to heat and sound.