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THE DIGESTIVE SYSTEM

INTRODUCTION

1. The digestive system includes the elongate digestive tract and the associated accessory organs. The functions of the digestive system include ingestion, mechanical processing, secretion digestion, absorption, compaction, and defecation.

Histological Organization

Basic Structural Patterns

1. Important histological layers include the epithelium, lamina propria, muscularis mucosae, submucosa, muscularis externa, and adventitia.

2. The digestive tract is lined by a mucous epithelium moistened by the fluid contents and glandular secretions of the epithelial and accessory organs.

3. The lamina propria and epithelium form the mucosa. Proceeding outward you enter the submucosa and then the muscularis externa.

4. The muscularis externa is surrounded by a layer of loose connective tissue called the adventitia, outside of the peritoneal cavity; in the peritoneal cavity this layer is covered by a serous membrane (serosa).

5. This layer, also called the visceral peritoneum, is continuous with the parietal peritoneum that lines the inner surfaces of the body wall. Mesenteries suspend the digestive tract, linking the parietal and visceral peritoneal membranes and providing access routes for blood vessels, nerves, and lymphatics.

6.The peritoneal lining produces and reabsorbs about 7 liters of fluid each day. Inflammation of the lining causes symptoms of peritonitis.

THE FUNCTIONAL ANATOMY OF THE DIGESTIVE SYSTEM

The Oral Cavity

1. The functions of the oral cavity include the analysis of potential foods, mechanical processing, lubrication, and initiating digestion by salivary enzymes.

Gross Anatomy:

1.The cheeks form the lateral walls of the oral cavity. Anteriorly they are continuous with the lips, and a small epithelial fold attaches each lip to the mucosa covering the adjacent gingiva.

2. The hard and soft palates form the roof of the oral cavity. The tongue occupies most of the floor.

3. The posterior margin of the soft palate supports the uvula and two pairs of muscular pharyngeal arches.

4. The oral cavity opens into the pharynx at the fauces.

5. The parotid, sublingual, and submandibular salivary glands discharge their secretions into the oral cavity.

Histological Organization:

1.The lining of the oral cavity consists of a stratified squamous epithelium. The transition between the cutaneous membrane of the skin and the mucous membrane of the digestive tract occurs at the lips, where underlying blood vessels give the area a reddish color.

Special Regional Anatomy:

1. The tongue assists in mechanical processing by manipulating materials in the mouth. Taste buds on the exposed surfaces permit sensory analysis prior to ingestion.

2. A V-shaped line of circumvallate papillae marks the boundary between the body and the root of the tongue.

3. There are extrinsic and intrinsic tongue muscles under the control of the hypoglossal nerve.

4. Lingual tonsils are located on the root of the tongue.

5. Salivary glands produce 1-1.5 liters of saliva per day. Saliva includes water, electrolytes, buffers, waste products, nutrients, immunoglobulins, and salivary (alpha-) amylase.

6. Salivary secretions are controlled by the salivatory nuclei of the medulla. An autonomic salivary reflex can be triggered by stimulating tactile or taste bud receptors on the surface of the tongue. Parasympathetic stimulation accelerates secretion by all of the salivary glands.

7. The teeth permit mastication of materials in the mouth. This breaks down tough connective tissues and saturates the material with salivary lubricants and enzymes.

8. Dentin forms the basic structure of a tooth. The crown is coated with enamel, and the root with cementum. The periodontal ligament anchors the tooth in an alveolar socket.

9. Adult teeth include eight incisors, four cuspids (canines), eight bicuspids (premolars), and twelve molars.

10. The twenty deciduous teeth erupt in childhood and are gradually replaced by the permanent dentition.

11. Important reference terms for orientation along the dental arch include labial, buccal, palatal, lingual, mesial, lateral, and occlusal.

12. Mastication occurs through the contact of opposing occlusal surfaces. Movement of the material across the teeth involves the buccal, labial, and lingual muscles, and after preparation the tongue compacts the mass into a bolus that can be swallowed.

The Pharynx

1. Propulsion of the bolus results from the contractions of the pharyngeal constrictors and the palatal muscles.

The Esophagus

1.Esophageal glands of the submucosa provide additional lubricants that encourage easy passage of the bolus.

Peristalsis and Swallowing

1. Peristalsis propels materials along the length of the digestive tract.

2. The buccal phase of deglutition begins with the compaction of a bolus and its movement into the pharynx.

3. The pharyngeal phase begins with the elevation of the larynx, reflection of the epiglottis, and the closure of the glottis.

4. The esophageal phase of deglutition begins with the opening of

the upper esophageal sphincter. Peristalsis then moves the bolus down the esophagus to the lower esophageal sphincter.

5. Once the bolus has entered the pharynx, all voluntary control of its movement has been lost.The pharyngeal muscles are controlled by the swallowing center of the medulla.

6. The peristaltic contractions of the esophageal musculature occur under central and local reflexive controls.

The Stomach

1. The stomach serves as a bulk storage area, and breaks down ingested foods with a combination of mechanical stress, acid secretion, and enzymatic digestion.

Gross Anatomy:

1. Important landmarks in gastric anatomy include the lesser and greater curvatures, the cardia, the fundus, the body, and the pylorus. The pyloric sphincter guards the exit from the stomach.

2. The stomach is highly distensible, and in the relaxed state the lining is thrown into folds called rugae.

3. A pair of mesenteries support the stomach; the greater omentum forms an expansive sheet that covers the anterior surfaces of the viscera. The lesser omentum forms a small pocket between the stomach and the liver.

Histological Organization:

1. Acids and enzymes are secreted by the cells lining the gastric glands.

2. Parietal cells secrete intrinsic factor and hydrochloric acid. These cells generate carbonic acid through the action of carbonic anhydrase. Bicarbonate ions are exchanged for chloride ions in the extracellular fluid, and hydrogen and chloride ions are secreted into the gastric lumen.

3. Chief cells secrete pepsinogen, the inactive precursor of the proteinase pepsin.

4. Enteroendocrine cells of the stomach secrete several compounds, notably the hormone gastrin.

The Regulation of Gastric Function:

1. There are cephalic, gastric, and intestinal phases of gastric secretion. The cephalic phase requires parasympathetic stimulation via the vagus nerve. The gastric phase is mediated by the secretion of gastrin. The intestinal phase involves hormonal feedback from the proximal portions of the small intestine. The Liver, Pancreas, and Small Intestine.

The Liver:

1. The liver, the largest visceral organ, performs metabolic and hematological regulation. It also synthesizes and excretes bile.

2. The shape of the liver conforms to its surroundings. The liver contacts the diaphragm, the anterior abdominal wall, the gall bladder, and the inferior vena cava. Its position is stabilized by its interaction with surrounding structures and the presence of thickened mesenteries.

3. Individual hepatocytes are organized into functional units called lobules. The lobule radiates around a central vein, and each lobule receives blood from the hepatic artery and the hepatic portal vein.The central veins merge to form the hepatic veins that discharge into the inferior vena cava.

4. Canaliculi, found between the opposing faces of adjacent hepatocytes, carry secretions towards the bile duct.

5. The bile ducts from each lobule unite to form the hepatic duct. The hepatic duct meets the cystic duct to form the common bile duct that continues to the duodenum. If bile cannot flow into the duodenum it flows along the cystic duct for storage in the gallbladder.

6. The gallbladder stores and concentrates bile.

7. Bile secretion occurs in response to the release of cholecystokinin, a hormone produced by the small intestine.

8. Bile salts emulsify lipids in chyme and makes them accessible to lipases secreted by the pancreas.

The Pancreas:

1. The exocrine pancreas produces lipases, carbohydrases, and proteolytic enzymes.

2. The elongate pancreas has a head, body, and tail. A single pancreatic duct enters the duodenal wall.

3. The pancreatic ducts begin at the pancreatic acini. The endocrine pancreatic islets are scattered between the acini.

4. The alkaline pancreatic juice contains alpha-amylase, pancreatic lipase, nuclease, trypsin, chymotrypsin, carboxypeptidase, and elastase.

5. The proteolytic enzymes are secreted as proenzymes, and they are activated by an enzyme, enterokinase, in the small intestine.

The Small Intestine:

1. The small intestine includes the duodenum, the jejunum, and the ileum.

2. The intestinal lining in these areas bears permanent transverse folds, called plicae, that increase the surface area for absorption.

3. The mucosa is thrown into small projections, the intestinal villi, covered with a simple columnar epithelium. Microvilli further increase the available absorptive area. Stem cells, enteroendocrine cells and goblet cells are found in the intestinal crypts that open onto the epithelial surface between the villi.

4. Each villus contains a central lymphatic lacteal as well as a network of capillaries.

5.The duodenum has submucosal glands (Brunner's) that produce an alkaline mucus to protect the small intestine from acid chyme leaving the stomach.

6.The jejunum has large plicae and villi, and no submucosal glands.

7.The ileum has short plicae and villi and aggregations of lymphatic nodules, called Peyer's patches.

8. Distension of the stomach initiates the gastroenteric reflex that increases the rates of peristalsis and secretion along the small intestine. It also triggers the gastroileal reflex that empties the terminal portions of the ileum.

9. Movements of the small intestine are primarily controlled by reflexes mediated by the submucosal and myenteric plexuses.

10. Intestinal juice moistens the chyme, assists in buffering acids, and dissolves digestive enzymes and the products of digestion.

11. Secretions are regulated by a combination of neural and

hormonal factors.

2. Secretin released when acids appear in the duodenum causes an increase in the production of buffers and enzymes in the pancreas, and triggers the secretion of the submucosal glands.

13. Cholecystokinin release follows the appearance of fatty or protein-rich chyme. It accelerates the production of digestive enzymes in the pancreas, and causes ejection of bile from the gallbladder.

14. Glucose-dependent insulinotropic peptide (GIP) release occurs under the same conditions. It causes the release of insulin from the pancreatic islets.

The Large Intestine

1. The large intestine reabsorbs water and some vitamins and nutrients, compacts the feces, and stores fecal materials.

Gross Anatomy:

1. The colon has a larger diameter and a thinner wall than the small intestine. The large intestine is also distinguished by bearing prominent haustra and muscular taenia coli.

2. Important regions of the colon include the cecum, appendix, ascending, transverse, descending, and sigmoid colon, and the rectum.

Histological Organization:

1.The large intestine lacks villi, but has crypts known as the intestinal glands.

Colonic Physiology:

1. Approximately 1500 ml of chyme arrive in the colon each day, but only about 200 ml are eliminated as fecal wastes.

The Rectum and Anus

1. The rectum terminates in the anorectal canal leading to the anus. Muscular sphincters control the passage of fecal material to the anus.

Defecation:

1. Distension of the rectal wall triggers the defecation reflex. Peristalsis moves the fecal wastes towards the anus, along the anorectal canal. Elimination of the fecal materials requires voluntary relaxation of the external anal sphincter.

THE PHYSIOLOGY OF DIGESTION

The Processing and Absorption of Nutrients

1. A typical meal contains carbohydrates, proteins, and lipids. These materials are broken down through hydrolysis.

The Digestion of Carbohydrates:

1. Amylase converts complex polysaccharides into a mixture of disaccharides and trisaccharides. These are broken down to monosaccharides by enzymes at the epithelial surface prior to absorption.

2. Indigestible carbohydrates are metabolized by intestinal bacteria, primarily in the colon.

The Digestion of Lipids:

1. Triglycerides are emulsified into fine droplets that can be attacked by lipases. The resulting fatty acids and monoglycerides interact with bile salts to form micelles. When a micelle nears the intestinal epithelium, the lipids diffuse into the epithelial cells.

2. The epithelial cells synthesize triglycerides and release them into the interstitial fluids as chylomicrons. Most chylomicrons are too large to enter capillaries, and they are transported by the lacteals.

The Digestion of Proteins:

1. Protein digestion involves the gastric enzyme pepsin and the various pancreatic proteases. Peptidases on the epithelial surfaces liberate amino acids that are absorbed and exported to the interstitial fluids.

Water and Electrolyte Absorption

1. Roughly 2-2.5 liters of water are ingested in food or drink. Digestive secretions add another 6-7 liters of fluid to the lumenal contents. All but around 150 ml are reabsorbed.

2. The resorption of water occurs through osmosis, primarily in response to concentration gradients established by the active transport of specific electrolytes and metabolites.

Vitamins

1. The nine water-soluble vitamins are important as cofactors in enzymatic reactions. The fat-soluble vitamins have varied functions. Intestinal bacteria manufacture several water-soluble vitamins and one fat-soluble vitamin, vitamin K.

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