The Gastrointestinal System ( digestive system )

The digestive system begins with the oral cavity (mouth and pharynx), which serves as a receptacle for food. Swallowed food enters the gastrointestinal tract (GI tract) consisting of esophagus, stomach, small intestine, and large. The GI tract is a long tube with muscular walls lined by secretory and transporting epithelium. At intervals along the tract, rings of muscle function as sphincters to separate the tube into segments with distinct functions. Food moves through the tract propelled by waves of muscle contraction. The digestive system provides nutrients, water, and electrolytes to the cells of the body from the external environment. At various points along the GI tract, acid, digestive enzymes, and buffers are added to facilitate the breakdown of complex foods into simple molecules (such as amino acids, glucose, and fatty acids). These products are then absorbed into the body.

Functions of the gastrointestinal organs. Transit times shown are the length of time it takes food to reach each indicated point after ingestion

Overview of the main functions of each area of the gastrointestinal system during the passage of food. The following sequence of events results in the efficient assimilation of nutrients from food:

1. Chewing (mastication) of food breaks down the food to create a bolus that is suitable 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. Depending on its composition, food can remain in the stomach for about 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. The pancreas is essential for digestion because it produces numerous enzymes. The pancreas also secretes HCO3 -, which neutralizes acid from the stomach. 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 throughout 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. The functions of the large intestine include fluid and electrolyte transport and fermentation of undigested carbohydrates (e.g., cellulose). 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.

The major functions of the gastrointestinal system are assimilation of nutrients and excretion of waste products:

1. MOTILITY movement of food through the gastrointestinal system as a result of muscle contraction

2. SECRETION movement of fluid and enzyme solutions from cells of GI tract into lumen or ECF

3. DIGESTION Chemical and mechanical breakdown of food macromolecules into absorbable units

4. ABSORPTION the nutrient molecules, which are liberated, can be transported from GI lumen to ECF and than into the circulatory system

The major functions 

of the GI system

STRUCTURAL FEATURES OF THE GASTROINTESTINAL TRACT

There are four major histologic layers in the gastrointestinal tract, starting from the gut lumen and moving outward: 

1. Mucosa 
2. Submucosa 
3. Muscularis externa 
4. Serosa

The mucosa consists of an epithelium. In most areas, the epithelium is highly folded to increase its surface area (for absorption, to be able to stretch) and is frequently invaginated to form the tubular exocrine glands. 

The mucosa epithelium differs in structure along the tract. In mouth, esophagus and rectum epithelium is protective, it’s a low-secretory and low-absorptive. The stomach has a secretory epithelium. The small intestines has an absorptive epithelium.

Exocrine secretions include mucus, electrolytes, water, and digestive enzymes. Numerous endocrine cells are also scattered among epithelial cells. Endocrine cells release gastrointestinal hormones into the blood in response to changes in the luminal environment; for example, the hormone cholecystokinin is released in response to fat and protein in the gut lumen.

The submucosa is a layer of connective tissue that contains the major blood and lymphatic vessels that serve the gastrointestinal tract. This area also contains numerous ganglion cells organized to form the submucosal (Meissner) nerve plexus.

The muscularis externa is responsible for mixing and moving food along the gastrointestinal tract. It contains the two major smooth muscle layers : the inner circular layer (mixing) and the outer longitudinal layer (moving food along the tract. The myenteric (Auerbach) nerve plexus lies between the two layers of muscle.

The serosa is a thin connective tissue layer. Several major structures enter through the serosa, including blood vessels, extrinsic nerves, and the ducts of the large accessory exocrine glands.

The peritoneal organs are covered by a serosa, whereas extraperitoneal or retroperitoneal organs are covered by an adventitia (loose connective tissue). The thoracic esophagus is an extraperitoneal structure and is therefore covered by an adventitia.