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THE DIGESTIVE SYSTEM MR MULENGA
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LESSON OBJECTIVES By the end of the lesson, you should be able to: Understand the divisions of the digestive system Describe the anatomical structures and functions of the digestive tract.
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OVERVIEW It has the digestive tract (also called the alimentary canal). It runs from the mouth to the anus. The system also contains accessory organs. These are separate organs that aid digestion.
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DIGESTIVE FUNCTION It processes food, extracts nutrients from it, and eliminates the residue. It does this in five stages:
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Ingestion is the process of eating. Propulsion is the movement of food along the digestive tract through peristalsis. Secretion of digestive enzymes and other substances; softens, adjusts the pH of, and chemically breaks down the food. Mechanical digestion is the physical breaking down food into smaller pieces.
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Starts with the chewing of food and continues with the muscular mixing of the stomach and small intestine. Chemical digestion is the chemical breaking down food into simpler molecules using enzymes in the stomach and small intestines.
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Absorption is the entrance of the digested food (now called nutrients) into the body. Defecation is the process of eliminating undigested material through the anus.
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Digestion, the mechanical and chemical breakdown of food into small absorbable particles. Absorption, the uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood or lymph;
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Compaction, absorbing water and consolidating the indigestible residue into feces. Defecation, the elimination of feces.
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COMPONENTS OF THE DIGESTIVE TRACT Mouth Pharynx Oesophagus Stomach Small intestine Large intestine
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MOUTH (ORAL CAVITY) It is adapted to ingest food because it has the teeth and tongue Breaks food into small particles by mastication, Mix it with saliva and swallow it (deglutition). The lips, cheeks, and palate form the boundaries of the mouth.
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The palate is the roof of the oral cavity. The anterior portion, the hard palate, is supported by bone. The posterior portion, the soft palate, is skeletal muscle and connective tissue. It separates the oral cavity from the nasal cavity. Posteriorly, the soft palate ends in a projection called the uvula.
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OESOPHAGUS A muscular tube which passes through an opening in the diaphragm When a bolus enters the oesophagus, it triggers peristalsis. The lower esophageal sphincter controls the movement of food between the oesophagus and the stomach.
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STOMACH Located in the upper left quadrant of the abdomen, below the diaphragm. It’s primary function is to store food. Muscles of the stomach contract and churn to break food. Mix it with gastric juice semi-fluid mixture called chyme.
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STRUCTURE OF THE STOMACH Cardia region, small region around the opening from the esophagus. Fundus the most superior region which balloons above the cardiac region. The body is the main portion of the stomach, which curves to the right and creates two curvatures.
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The lesser curvature is concave and is directed superiorly and to the right. On the opposite side, the convex greater curvature is directed inferiorly and to the left. As the body approaches the exit from the stomach, it narrows into the pyloric region.
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When the stomach is empty its layers show longitudinal folds, called rugae. They allow the stomach to expand
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GASTRIC SECRETIONS The mucosal lining of the stomach has numerous tubular gastric glands. Four different types of cells make up the gastric glands Mucous cells produce two types of mucus in the stomach:
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Thick and alkaline and forms a protective coating for the stomach lining. Thin and watery. It mixes with the food and creates a fluid medium for chemical reactions.
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Parietal cells secrete hydrochloric acid and gastric intrinsic factor. HCL kills bacteria and provides an acidic environment for the action of enzymes in the stomach. Chief cells secrete pepsinogen, an inactive form of the enzyme pepsin.
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HCL converts the inactive pepsinogen into the active enzyme pepsin The endocrine cells secrete the hormone gastrin, which functions in the regulation of gastric activity.
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REGULATION OF GASTRIC SECRETIONS The cephalic phase, begins when an individual thinks pleasant thoughts about food or sees, smells, or tastes food. These impulses cause an increase in the secretion of gastric juice.
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The gastric phase, food in the stomach and the distention of the stomach wall stimulate gastrin secretion. Gastrin, stimulates the secretion of gastric juice, which contains hydrochloric acid and pepsinogen. HCL acidifies the stomach contents and activates the pepsinogen into pepsin, which breaks down proteins into peptides
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Intestinal phase, this phase is triggered by passage of chyme through the pyloric sphincter into the duodenum. Distention and the presence of acid chyme in the duodenum stimulate the secretion of intestinal hormones, which in turn inhibit gastric secretions.
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These hormonal and neural inhibitory responses help prevent excess acid chyme from entering the small intestine. When the chyme is neutralized and moves away from the duodenum, the inhibitory responses stop and gastric secretion is again stimulated. The intestinal phase regulates the entry of chyme into the small intestine.
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STOMACH EMPTYING Peristalsis in the stomach pushes chyme toward the pyloric region. As the chyme accumulates, the pyloric sphincter relaxes and a small amount of chyme is pumped into the small intestine.
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If the stomach empties too slowly, the rate at which nutrients are digested and absorbed is diminished. On the other hand, if the stomach empties too quickly, before particles are thoroughly mixed with the gastric juice, the efficiency of digestion is reduced and the acid chyme may damage the intestinal mucosa.
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SMALL INTESTINE It extends from the pyloric sphincter to the ileocecal valve. Finishes the process of digestion. Absorbs the nutrients, and passes the residue on to the large intestine.
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STRUCTURE AND FEATURES OF THE SMALL INTESTINE It has circular folds which increase the surface area for absorption. Villi project from the circular folds, and further increases the surface area. Each villus surrounds a blood capillary network and a lymph capillary, or lacteal.
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These function in the absorption of nutrients. The length of the small intestine is divided into three regions: duodenum, jejunum, and ileum.
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THE DUODENUM It begins at the pyloric sphincter and continues to the jejunum. Receives the chyme from the stomach and secretions from the liver and pancreas.
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THE JEJUNUM AND ILEUM Jejunum is continuous with the third portion, the ileum. There is a gradual decrease in the number and length of the villi from the beginning of the jejunum to the terminal portion of the ileum.
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SECRETIONS OF THE SMALL INTESTINE Intestinal glands secrete large amounts of watery fluid that is neutral or slightly alkaline. It keeps the chyme in a liquid form. The fluid is readily reabsorbed by the capillaries in the microvilli. Submucosa of the duodenum secrete mucus
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The alkaline mucus protects the intestinal wall from the acid chyme and digestive enzymes. These enzymes, include the following: Peptidase, acts on peptides. Maltase, sucrase, and lactase, act on disaccharides (double sugars). Intestinal lipase, acts on fats.
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Small intestine cells also produce, Secretin which stimulates the pancreas to secrete a fluid which neutralizes chyme. It also secretes Cholecystokinin which stimulates the release of bile from the gallbladder. CCK also inhibits gastric motility and secretions.
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LARGE INTESTINE It begins at the ileocecal junction, where the ileum enters the large intestine, and ends at the anus. The junction has an ileocecal sphincter.
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STRUCTURE AND FEATURES OF THE LARGE INTESTINE The type of tissue is the same as other parts of GIT but there are some distinguishing characteristics. The mucosa has large numbers of goblet cells but does not have any villi.
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REGIONS OF THE LARGE INTESTINE The cecum is the proximal portion of the large intestine. It extends inferiorly from the ileocecal junction. The vermiform appendix is attached to the cecum.
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The colon is the longest portion of the large intestine. The ascending colon begins at the ileocecal junction and travels upward on the right side. Then it turns to the left to become the transverse colon. Then it turns downward as the descending colon.
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Later the descending colon makes an S-shaped curve, called the sigmoid colon, and then becomes the rectum. The rectum continues from the sigmoid colon to the anal canal and has a thick muscular layer. The anal canal, which continues from the rectum and opens to the outside at the anus.
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At the superior end of the anal canal there is an internal anal sphincter. This sphincter is under involuntary control. There is an external anal sphincter at the inferior end of the anal canal. This is under voluntary control.
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FUNCTIONS OF THE LARGE INTESTINE It absorbs fluid and electrolytes It also eliminates of waste products. The large intestines only secrete mucus which hold the fecal matter together.