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produced by the parietal cells. As the parietal cells become less efficient, the production of both hydrochloric acid and intrinsic factor falls.
    The most common problems associated with the stomach are upset stomach, gastric ulcers, and underproduction of hydrochloric acid.
The GI Mucosa
    The GI mucosa, depicted in Figure 2.2 , is the inner lining of the digestive tract. It consists of the lumen, which is the space inside the digestive tube; the epithelial layer; the lamina propria; and the muscularis mucosae, or smooth muscles. The entire mucosa is a large mucous membrane, not unlike the tissues inside of your nose. It is here that our food makes contact with us and is eventually absorbed in the intestines. It is home to trillions of bacteria and fungi and is the center of our body’s immune system. It is your body’s first line of defense against infections and other invaders.
    The primary layers of the gut mucosa (also called gut-associated lymphoid tissue, or GALT) include the epithelium, the lamina propria, and the muscularis mucosae. The epithelium is comprised of a single layer of cells that come into direct contact with your food. This layer of cells is held together in tight junctions (desmosomes) that prevent leakage of molecules between the cells. (See Chapter 4 .) This epithelial layer replaces itself every five to seven days and uses glutamine as its primary energy source. There are exocrine cells among the epithelial cells that secrete mucus and fluid as well as endocrine cells that release hormones.

     

    Figure 2.2 The GI mucosa.
(The McGraw-Hill Companies, Inc./Al Telser photographer)
     

     

    Figure 2.3 Lamina propria and epithelial tissue.
(The McGraw-Hill Companies, Inc.)
     
    The lamina propria connects the digestive system to the lymphatic system for digestion of fats and to the blood for absorption of nutrients ( Figure 2.3 ). The lamina propria is where lymphatic nodules, lymphocytes, plasma cells, and macrophages form the first line of defense against infections. These are lymphatic nodes (like the ones that swell in your throat when you have a bad cold) that run throughout the digestive system but are most prominent in the tonsils, small intestine, appendix, and large intestine. The lymphatic system circulates fluids throughout your body, drains excess fluids from the fluid between cells (interstitial fluid), initiates immune response against infection and allergy, transports the fat-soluble vitamins A, D, E, and K, and brings digested fats into the bloodstream, among other things.
    The lamina propria is also where cytokines, such as IL-6, IL-10, TNF-alfa, and others, are produced. Some cytokines are inflammatory, while others are healing.
    The final layer of the GALT is the muscularis mucosae, which is a thin layer of smooth muscle that runs from the stomach through the small intestine. (See Chapter 9 .)
Small Intestine
    The small intestine is hardly small. If this coiled-up garden hose were stretched out, it would average 15 to 20 feet long. If spread flat, it would cover a surface the size of a tennis court. Here food is completely digested and absorbed. Nutrients are absorbed through hundreds of small, fingerlike folds called villi in the epithelium, which are covered, in turn, by millions of microvilli. (Think of them as small loops on a towel that then have smaller threads projecting from them.) The villi and microvilli are only one cell layer thick, but they perform multiple functions of producing digestive enzymes, absorbing nutrients, and blocking absorption of substances that aren’t useful to the body.
    The intestinal wall has a paradoxical function: it allows nutrients to pass into the bloodstream while blocking the absorption of foreign substances found in chemicals, bacterial products, and other large molecules found in food. Some foods we eat and medications we use irritate the intestinal wall, and it can lose the ability to discern between nutrients and foreign substances.