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Digestive System

Learn about the anatomy and functions of the digestive system, including the mouth, pharynx, esophagus, stomach, and small intestine. Discover the key roles of each organ in the gastrointestinal tract for food digestion and nutrient absorption.

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Digestive System

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  1. Anatomy Ch. 14 Digestive System

  2. Anatomy of the Digestive System • The alimentary canal is also called the gastrointestinal or GI tract • The GI tract is a continuous, coiled, hollow, muscular tube • The organs of the GI tract: • Mouth • Pharynx • Esophagus • Stomach • Small intestine • Large intestine

  3. Mouth (oral cavity) • The lips or labia protect the anterior opening • The cheeks form the lateral walls • The hard palate forms the anterior roof • The soft palate forms the posterior roof • The uvula is a projection of the soft palate • The tongue occupies the floor of the mouth and attaches to the hyoid bone and styloid process • The lingual frenulum secures the tongue to the floor of the mouth • The palatine and lingual tonsils are found in the posterior portion of the mouth and function in the lymphatic system

  4. Pharynx • Oropharynx is posterior to the oral cavity • Laryngopharynx is continuous with the esophagus • The pharynx contains 2 layers of skeletal muscles • Alternating contractions of these layers propel food through the pharynx into the esophagus (peristalsis)

  5. Esophagus • The gullet or food tube • Runs from the pharynx to the stomach • About 10 inches long • Walls are composed of 4 tissue layers or tunics • All of the remaining organs of the GI tract are composed of the same 4 layers

  6. Mucosa • Inner layer • Lines the lumen • Contains stratified squamous epithelium, connective tissue, and smooth muscle • Submucosa • Beneath the mucosa • Connective tissue • Contains blood vessels, nerve endings, MALT, and lymphatic vessels

  7. Muscularis • 2 layers of smooth muscle • Serosa • Outer layer • Visceral and parietal peritoneum containing serous fluid

  8. The wall of the GI tract contains 2 nerve plexuses • Submucosal • myenteric • These plexuses are part of the ANS • The plexuses regulate the mobility and secretory activity of the GI organs

  9. Stomach • Cardioesophageal sphincter: area of the stomach where food enters from the esophagus • Fundus: expanded lateral part of the stomach • Body: mid portion of the stomach • Pyloric antrum: inferior narrow portion of the stomach • Pyloris: funnel shaped terminal portion which is continuous with the small intestine • Pyloric sphincter: valve between the stomach and small intestine

  10. The stomach varies in length • Diameter and volume depend on how much food it contains • The stomach can hold up to 1 gallon of food • When the stomach is empty it collapses forming folds called rugae • The stomach is a temporary storage area and is the site for food breakdown • The stomach contains 3 layers of muscle: longitudinal, circular, and oblique • Having 3 layers of muscle allows the stomach to move and breakdown food

  11. The chemical breakdown of proteins begins in the stomach • The mucosa of the stomach produces a layer of mucus that protects the wall of the stomach from being damaged by acid or being digested by enzymes

  12. Secretions of the stomach • Intrinsic factor: absorption of vitamin B12 • Pepsin: digestion of proteins • HCl acid: makes the stomach contents acidic and activates pepsinogen • Gastrin: release of gastric juices and stomach emptying • Mucus: protects the stomach lining • Somatostatin: inhibits the activity of the stomach

  13. Most digestive activity occurs in the pyloric region of the stomach • After food has been processed in the stomach it becomes a creamy substance called chyme • The greater omentum covers the abdominal organs • It is filled with fat which helps to insulate, cushion, and protect the abdominal organs

  14. Small Intestine • The body’s major digestive organ • Food is prepared for delivery to the cells of the body • Muscular tube that extends from the stomach to the large intestine • Longest section of the GI tract • Suspended from the abdominal wall by the mesentery

  15. There are 3 divisions of the small intestine: duodenum, jejunum, and ileum (longest) • The ileum meets the large intestine at the ileocecal valve which joins the small and large intestine • Chemical digestion begins in the small intestine • The pyloric sphincter controls the movement of food from the stomach to the small intestine

  16. Secretions of the duodenum of the small intestine • Intestinal gastrin: stimulates gastric secretion and emptying • Somatostatin: inhibits gastric activity • Secretin: increases pancreatic juice and bile output • Cholecystokinin (CCK): increases pancreatic juice output and stimulates the gall bladder which allows the pancreatic juice and bile to enter the duodenum • Gastric Inhibitory Peptide (GIP): inhibits secretion of gastric juice and stimulates insulin release

  17. The pancreatic and bile ducts release enzymes and bile into the duodenum through the hepatopancreatic ampulla • Nearly all food absorption occurs in the small intestine • The wall of the small intestine contains structures called microvilli which release brush border enzymes. • These enzymes allow for digestion of proteins and carbohydrates.

  18. Most of the absorptive structures are found at the beginning of the small intestine • Large numbers of peyer’s patches (lymphatic) are found near the end of the small intestine

  19. Large Intestine • The large intestine is much larger in diameter than the small intestine but is smaller in length • The large intestine extends from the ileocecal valve to the anus • The major functions of the large intestine is to dry out the indigestible food residue by absorbing water and eliminate those residues from the body as feces.

  20. The large intestine frames the small intestine and contains subdivisions: cecum, appendix, colon, rectum, and anal canal • The appendix is attached to the cecum • The colon is divided into 4 regions • Ascending: travels up the right side of the abdominal cavity • Transverse: travels across the abdominal cavity • Descending: travels down the left side of the abdominal cavity and enters the pelvis • Sigmoid: found in the pelvis

  21. The anal canal ends at the anus which opens to the exterior • The anal canal has 2 sphincters which control defecation • External: composed of skeletal muscle and is voluntary • Internal: composed of smooth muscle and is involuntary

  22. There are no villi found in the large intestine because most absorption has already taken place • There are a large number of goblet cells that produce mucus which acts as a lubricant

  23. Accessory Organs of Digestion • Teeth • Allow the process of mastication or chewing • Humans have 2 sets of teeth in their lifetimes • Deciduous (baby) teeth • Begin to erupt around 6 months • Full set includes 20 teeth • Permanent teeth • As the permanent teeth enlarge and develop the roots of the baby teeth are reabsorbed • All of the permanent teeth but the 3rd molars (wisdom teeth) have erupted by the end of adolescence • Full set includes 32 teeth

  24. We classify teeth according to shape and function • Incisors: chisel shaped adapted for cutting • Canines: tearing or piercing • Premolars (bicuspids) and molars: have broad crowns with rounded cusps for grinding

  25. A tooth consists of 2 major regions: crown and root • The crown is covered with enamel and is the exposed part of the tooth above the gingiva or gum • Enamel is the hardest substance in the body and is mineralized by calcium salts • The root is embedded in the jaw bone • The root and crown are connected by a region called the neck

  26. The outer surface of the root is covered by a substance called cement which attaches the tooth to the periodontal ligament which holds the tooth in place • Dentin is a bone like material found under the enamel and forms the bulk of the tooth • Dentin surrounds a central pulp cavity which contains blood vessels and nerve fibers • Where the pulp cavity extends into the root it becomes the root canal

  27. Salivary Glands • There are 3 pairs of salivary glands: parotid, submandibular, and sublingual • Saliva is a mixture of mucus and serous fluid • The mucus moistens and helps bind food together into a mass called a bolus • The serous portion contains an enzyme called salivary amylase that begins the process of starch digestion in the mouth • Saliva also contains lysozyme and antibodies that inhibit bacteria growth • Saliva dissolves food chemicals so they can be tasted

  28. Salivary Glands • Parotid Gland • Submandibular Gland • Sublingual Gland

  29. Pancreas • Only the pancreas produces enzymes that break down all categories of digestible foods • The pancreatic enzymes are secreted into the duodenum of the small intestine • These enzymes neutralize the acidic chyme coming from the stomach • The pancreas also produces insulin and glucagon

  30. Liver • Largest gland in the body • The liver has 4 lobes and is suspended from the diaphragm by the falciform ligament • The digestive role of the liver is to produce bile • Bile leaves the liver through the common hepatic duct and enters the duodenum through the bile duct

  31. Bile is a yellow to green watery solution that contains bile salts, bilirubin, cholesterol, phospholipids, and electrolytes • Only the bile salts and phospholipids aid in the digestive process • Bile salts physically breakdown fats

  32. Gallbladder • Found on the inferior surface of the liver • When food digestion is not occurring bile enters the gallbladder to be stored • Bile becomes concentrated while it is in the gallbladder • Bile is released when fatty foods enter the duodenum of the small intestine

  33. Functions of the Digestive System • Overview • The major functions of the GI tract can be summarized in 2 words: digestion and absorption • Processes • Ingestion • Voluntary process of placing food in the mouth • Propulsion • Movement of food from one organ to the next • Peristalsis: altering waves of contraction and relaxation of muscles

  34. Mechanical digestion • Physical breakdown of food into smaller pieces • Mixing, churning, and segmentation • Chemical digestion • Breakdown of large food molecules into their building blocks by enzymes • Occurs by hydrolysis

  35. Carbohydrates (mouth) • Broken down into monosaccharides or simple sugars • Glucose (most important), fructose (fruit), galactose (milk) • Only complex carbohydrates our digestive system breaks down are sucrose, lactose, maltose, and starch • The body is not able to breakdown the polysaccharide cellulose but it does help move food along the GI tract by providing fiber • Broken down by amylase

  36. Proteins (stomach) • Broken down into amino acids • Peptides are the intermediate • Broken down by pepsin and trypsin • Lipids (small intestine) • Broken down into fatty acids and glycerol • Broken down by lipase

  37. Absorption • Transport from the GI tract to the blood or lymph • Food first enters the mucosal cells • Small intestine is the major site • Defecation • Elimination of indigestible residue in the form of feces

  38. Digestive activity is mostly controlled by reflexes of the parasympathetic (rest and digest) division of the ANS. • The most important stimuli for the GI tract is stretch, pH, and products of digestion

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