The term digest means to break down or disintegrate. Therefore, digestion serves to break down the food we eat into smaller substances that are suitable for absorption into our body. All of the activities of digestion take place in our digestive or gastrointestinal tract. The digestive tract is a tube 22 to 28 feet long that actually passes through our body as shown in Figure 2.11. As food moves through the length of the digestive tract, it is really on the outside of the body. Only when a substance crosses the cell lining of the digestive tract and enters into our circulation is it actually inside our body, which is called absorption.
Digestion requires both physical and chemical operations. The teeth, along with the musculature of the mouth, stomach, and small intestine, work to physically grind, knead, and mix food with digestive juices. At the same time, the muscular lining of our digestive tract serves to propel the digestive mixture forward.
Chemical digestion involves the activities of digestive enzymes that will break down large complex food molecules into smaller substances appropriate for absorption. Proteins, carbohydrates, and lipids must be split into simpler molecules for absorption. Furthermore, the vitamins and minerals found in foods must be liberated from other food molecules
and complexes in order to be absorbed as well. Bile is also involved in chemical digestion; however, it functions not as an enzyme but more as a detergent. Bile is pivotal in the digestion and absorption of lipid substances.
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