Re: How does the human body produce heat ?

Hi Erron, 
This is a very involved question you have asked if I answer it at an advanced level.  
You don’t say what school you are in or what your present grade level is,
so I will assume you are fairly young.
The first thing that happens to the food that we eat is that the digestive system 
physically and chemically breaks it down into smaller and smaller particles until it 
can be absorbed through the intestinal walls as individual molecules.  These molecules
 then enter the blood capillaries and are distributed throughout the body to be taken
up by cells that need the 
	(1) carbohydrate molecules for the energy , 
	(2) amino acid molecules for building proteins, or 
	(3) lipid molecules (fat) for building membranes, certain of the hormones, 
and other things, or just to store this concentrated form of potential energy 
(the fat molecules) for a rainy day.
Those cells that start using the carbohydrate molecules for energy is where the bulk of 
the heat is produced.  What happens first is that the carbohydrate has to be absorbed 
as glucose or another closely related monosaccharide molecule that the cell can change 
to glucose.  The glucose is then metabolized (broken down) to carbon dioxide and water 
through a large number of chemical steps that builds ATP molecules (the energy storage 
“cellular bullets”) that can be use for almost all the cell’s needs for reactions that require
energy in an easily usable from.  Building the ATP and using the ATP is like you burning a 
twig out in the yard (in a safe place, of course) to heat a small cup of soup.  The wood, 
as it burns, combines with oxygen and is changed into carbon dioxide and water, plus a 
little smoke which is a leftover product of incomplete burning of the wood.  Now the body 
is much more efficient than this and doesn’t leave any smoke as it consumes the glucose 
and produces the ATP molecules.  Cells do produce heat while “burning” the glucose and 
while using the ATP just like the fire heating the soup.
For a more detailed description of these biochemical events you can go to any good 
biochemistry textbook and it will give you the actual molecular reactions.  I have one such 
book “Biochemistry” 3rd edition, by Lubert Stryer, published by W. H. Freeman and Company, 
New York.  There are two reference in this book on heat production; page 421 at the bottom, 
and page 443-4.  The last section gives a fascinating description of how molecular “burning” 
produces enough heat in the bumblebee’s thorax to allow him to fly even when it is cold outside.

Now I haven’t even talked about the control of heat production, but the cells can burn 
oxygen with glucose to produce the ATP and heat in some of their reactions or sometimes 
the cells do not use oxygen while “burning” a different molecule to produce the heat.  
But whatever the metabolic pathways involved in producing heat, this activity is controlled 
by a hormone from the thyroid gland.  The term basal metabolic rate (BMR) is used to talk 
about this level of activity and thyroxin, or really triiodothyronine, another thyroid hormone, 
is responsible for maintaining the appropriate level of metabolism that will keep your 
temperature at approximately 37 degrees centigrade throughout your life when you are healthy.  
You can find a very good detailed explanation of this system in any endocrinology textbook.  
I happen to like “Basic and Clinical Endocrinology” 5th edition, by Francis Greenspan and Gordon  
Strewler, published by Appleton & Lange, Stamford, Connecticut.  Pages 214-215 deal with this 
topic in their book.
Go to your local library and take a look around for these types of books and you will have many 
hours of enjoyable reading.

My best to you, Dr. Swanson