DIGESTION AND ABSORPTION
Most of the food we eat is in the form of complex molecules that must be broken down until they are small enough to be absorbed through the linings of the intestines and enter the bloodstream. Once in the bloodstream, these molecules are transported to the appropriate organs, particularly liver, muscle, and fat (adipose tissue), for storage until required to provide energy (e.g, during exercise).
Food taken by mouth travels via the esophagus into the stomach (see Exercises 3.1), where the food’s presence stimulates the secretion of various digestive enzymes that begin to break down the complex carbohydrate and protein molecules into simpler constituents, in particular amino acids (proteins) and glucose, galactose, and fructose (carbohydrates).
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When the composition of the food in the stomach fulfills certain criteria, shown in Exercises 3.1, the food is released into the upper reaches of the small intestine. Here digestive secretions from the liver, in the form of bile salts, and from the pancreas break the food down further. The bile secretions have a detergent effect, breaking the fats into an emulsion; the pancreatic and intestinal secretions contain the major portion of the enzymes that digest carbohydrates, fats, and proteins.
When the food enters the lower end of the small intestine, it has been broken down into its basic components and is ready for absorption. The carbohydrates, fats, and proteins are absorbed in the upper and middle portions of the small intestine; minerals, certain vitamins, and iron are absorbed at the lower end of the small intestine. The main function of the large intestine is to absorb the water that escapes absorption in the small intestine.
Blood Transport and Storage of Food Constituents
The fate of the various products of digestion is as follows.