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Welcome to "Anatomy and Physiology of the Gastrointestinal System".
Before beginning, please take a moment to read the course details..
Course Length: Approximately 3 hours. Your
time may vary based on modem speed, prerequisite knowledge and
other factors.
Prerequisites: None.
Date Published: November 2003
Valid Until: November, 2006
The digestive system, also referred to as the gastrointestinal or alimentary tract, contains the organs involved in the ingestion and processing of food. The digestive system plays a role in four major functions: ingestion, digestion, absorption, and elimination. This course describes the anatomy and physiology of the gastrointestinal system, and explains its involvement in acid related disorders and inflammatory diseases.
The first section, ANATOMY, introduces the organs that make up the digestive tract including the mouth, pharynx, oesophagus, stomach, small intestine, and large intestine, and a number of accessory organs that assist in the processing of foods and the extraction of nutrients.
The second section, PHYSIOLOGY, describes the sub-structure of the alimentary canal and its relationship to function, especially acid secretion. The production and regulation of gastric acid secretion is also covered.
The third section, GI TRACT AND DISEASE, gives an overview of how the gastrointestinal tract is involved in disease, particularly acid related disorders and inflammatory diseases. These areas are covered in detail in separate courses that are part of the gastrointestinal therapeutic area curriculum.
Anatomically, the digestive system consists of a 30-foot long, mucous membrane-lined tube beginning at the mouth, where food enters the body, and ending at the anus, where solid waste is excreted. This section introduces each of the key organs of the digestive system and a number of accessory organs that assist in the processing of foods and the extraction of nutrients.
These are the objectives for this section.
After you finish this section, you should be able to:
Food is ingested through the mouth, or oral cavity. This lesson describes the structure of the oral cavity, and explains how the salivary glands begin the digestion of food, and how the pharynx directs food to the next step in the alimentary canal.
The boundaries of the oral cavity include the lips, the cheeks, the palate, and the tongue. The back of the mouth is continuous with the pharynx, or throat.
The digestion of food begins in the mouth with the mechanical breakdown by chewing. Chewing helps break down food and mixes saliva with food. Saliva is secreted from the salivary glands and contains an enzyme, salivary amylase, which breaks complex carbohydrates such as starch and glycogen down into smaller fragments. Saliva also lubricates food, making it easier to swallow.
The mixture of chewed food and saliva is called a bolus. The tongue compacts the bolus of food and initiates swallowing. During swallowing, the bolus of food first passes through the pharynx, a transition area between the mouth and the oesophagus. The pharynx is a passageway for food, fluids, and air, but food is prevented from entering the respiratory pathways during swallowing by a flap of tissue known as the epiglottis.
Once a chewed bolus of food leaves the oral cavity, it enters the oesophagus. This lesson describes the structure of the oesophagus and the role of two sphincters found at either end of the oesophagus. It also introduces the process of peristalsis.
The oesophagus is a hollow tube that leads from the throat to the stomach. Although most of this 10-inch long tube is located in the thorax, the lower end of the oesophagus passes through the diaphragm and enters the abdomen at the oesophageal hiatus. Within the abdomen, the oesophagus joins the stomach at the cardiac orifice.
The oesophagus is separated from the pharynx and stomach by valve-like rings of circular muscle tissue known as sphincters. The sphincter that separates the upper end of the oesophagus from the throat is called the upper oesophageal sphincter (UOS), and the sphincter that separates the lower end of the oesophagus from the stomach is known variously as the lower oesophageal sphincter (LOS), the cardiac sphincter, or the gastroesophageal sphincter.
Under resting conditions, when there is no food to be delivered from the mouth to the stomach, the oesophagus is collapsed and both sphincters are closed. When food is moved to the back of the mouth by contractions of the pharyngeal muscles, the upper oesophageal sphincter relaxes and the bolus of food is propelled into the oesophagus by the tongue. Food is moved through the oesophagus by successive waves of muscular contraction known as peristalsis. The time required for a bolus of food to pass through the length of the oesophagus is normally less than 10 seconds. At the lower end of the oesophagus, the combined effect of the peristaltic wave and the weight of the bolus causes the lower oesophageal sphincter to open, allowing the swallowed food to enter the stomach.
In addition to regulating the passage of food into the stomach, the lower oesophageal sphincter plays an important role in protecting the oesophagus from the damage that would otherwise result from exposure to the highly acidic gastric fluids of the stomach.
Once a bolus of food leaves the oesophagus, it enters the stomach. This lesson describes the structure of the stomach and its dual role in the digestive process.
The stomach is a sac-like organ found just under the diaphragm. It is typically “J-shaped” and is located high in the abdomen, usually on the left side. The inner, concave surface of the stomach is referred to as the lesser curvature, while the outer, convex surface is referred to as the greater curvature.
Anatomically, the stomach is divided into four regions. The small area surrounding the lower oesophageal sphincter is called the cardia. The rounded, dome-like area at the top of the stomach is known as the fundus, and the narrow, funnel-like area at the bottom of the stomach is known as the antrum. The area between the fundus and the antrum is known as the corpus, or body, of the stomach. The pyloric sphincter separates the stomach from the duodenum.
The stomach plays an important role in the digestion of food by: secreting gastric acid and pepsin, and physically by peristaltic contractions.
As the stomach fills, digestive glands in the corpus and fundus release hydrochloric acid, a strong acid that helps digest food, facilitates the conversion of digestive enzymes to their active form, and kills many bacteria.
The strong, muscular walls of the stomach can relax to accommodate large amounts of food. The peristaltic contractions of muscles within the stomach wall mix digestive juices and food to produce a semi-fluid substance known as chyme. Chyme leaves the stomach and enters the small intestine through the pyloric sphincter, a muscular, ring-like structure.
An important aspect of the stomach's contribution to digestion is the slowing of gastric emptying, leading to a metering of gastric content into the duodenum in small amounts, so as not to overwhelm the digestive and absorptive capabilities of the small intestine. The rate of gastric emptying is variable, depending on dietary content. For example, it is much slower with high fat foods.