Chapter 7: The Gastrointestinal System

OVERVIEW

1. The gastrointestinal (GI) system consists of the GI tract and the accessory exocrine glands.

   1. The GI tract includes the mouth, the esophagus, the stomach, the small intestine, and the large intestine.

   2. The major accessory glands are the salivary glands, the liver, the gallbladder, and the pancreas.

2. The GI system is traditionally two functions:

   1. Assimilation of nutrients.

   2. Excretion of waste products via the biliary system.

3. Mucosal immunology is a third major function of the GI system.

   1. The GI system is the largest immunological compartment in the body reflecting the need to protect against microbial pathogens but also allow immunological tolerance to antigenic substances in the diet.

   2. The mucosa-associated lymphoid tissue (MALT) are lymphoid aggregates in the intestine known as Peyer's patches:

      1. Specialized cells in the overlying epithelium called microfold or M-cells take up antigens from the lumen.

      2. Peyer's patches contain T and B lymphocytes.

      3. CD4 T-cells recognize extracellular antigens (e.g., pathogenic microorganisms).

      4. CD8 T-cells recognize intracellular antigens (e.g., tumors and viruses).

      5. B-cells in MALT mostly secrete immunoglobulin A.

4. Assimilation of nutrients from food occurs in the following sequence (Figure 7-1):

   1. Chewing (mastication) breaks food down to create a bolus for swallowing. Saliva lubricates food and provides enzymes for digestion. It takes about 10 seconds for swallowed food to travel down the esophagus to the stomach.

   2. Food remains in the stomach for 1–4 hours. Stomach motility mixes and grinds food into small particles suitable for delivery to the small intestine via the pyloric sphincter. Exocrine secretions from the stomach mucosa help to dilute and dissolve food; gastric acid assists in dissolving and denaturing the components of food.

   3. Entry of food into the small intestine is coordinated with the delivery of major exocrine secretions from the biliary system and the pancreas.

      1. Pancreatic enzymes are essential for digestion. The pancreas also secretes HCO₃⁻, which neutralizes acid from the stomach.

      2. Contractions of the gallbladder deliver stored bile to the intestine. Bile acids are the major organic component of bile and are important for lipid assimilation.

   4. Food moves through the small intestine within 7–10 hours. Motility patterns in the fed state mix food with digestive enzymes and distribute nutrients over the absorptive surface. All significant absorption of nutrients occurs in the small intestine.

   5. Transit through the large intestine, from the cecum to the sigmoid colon, usually occurs over a period of 12–24 hours.

      1. Functions include fluid and electrolyte transport and fermentation of undigested carbohydrates (e.g., cellulose).

      2. Storage of fecal waste occurs in the distal large intestine; elimination of fecal waste typically occurs within 1–3 days after ingestion of a meal.

STRUCTURAL FEATURES OF THE GI TRACT

1. There are four major histologic layers in the GI ...