By taking up nourishment or nutrients (proteins, fats, carbohydrates,
vitamins, minerals, and trace elements) the body can create energy by
the “combustion” of nutrients to maintain its organization (structure,
catabolic, or energy metabolism) and the functions derived from it
(growth, renewal of cells, body temperature, mechanical and chemical
work). The nutrients are degraded by the enzymes of the digestive
glands in the various divisions of the digestive system, broken down
into chemical combinations that can be absorbed, and taken up (resorbed)
by the gastrointestinal mucosa. These energy-rich combinations
(e. g., fatty acids, amino acids, or glucose) enter the bloodstream
and next reach the liver by way of the portal vein (see Liver, Function
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