Digestive System

1. Digestive System Chapter 14

2. The digestive system takes in food (ingests it), breaks it down physically and chemically into nutrient molecules (digests it), and absorbs the nutrients into the bloodstream. Then in rids the body of the indigestible remains (defecates). Overview of the Digestive System

3. Separated into two groups: • Alimentary canal aka gastrointestinal (GI) tract • Performs the whole menu of digestive functions (ingests, digests, absorbs, and defecates) • Ex. Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus • Accessory digestive organs • Assist the process of digestive breakdown in various ways • Ex. Teeth, tongue, several large digestive glands Organs of Digestive System

4. Mouth aka oral cavity • Food enters here • Mucous-membrane lined cavity • Lips for protection of opening • Cheeks form lateral walls • Hard palate forms anterior roof • maxillae and palatine bones • Soft palate forms posterior roof • Uvula is a fleshy fingerlike projection of the soft palate Parts of the mouth

6. Tongue occupies the floor of the mouth • attached to the hyoid bone and styloid process of the skull • maneuvers food, forces food back for swallowing, speech • Lingual frenulum is a fold of mucous membrane that secures the tongue to the floor of the mouth and limits its posterior movements Tongue and parts

7. Deciduous teeth erupt at 6 months of age—(20 teeth) • Replaced with permanent teeth from age 6—(32 teeth) • Incisors- chisel-shaped, cutting into food • Cuspids (canines)- 1 point, tearing and shredding • Premolars- 2 cusps, grinding • Molars- 3+ blunt cusps to crush and grind • Mastication- chewing; mechanical digestion Teeth

8. The pharynx is subdivided into the nasopharynx(part of respiratory system), the oropharynx, and the laryngopharynx (continues to the esophagus) Parts of the pharynx

9. The walls of the pharynx contain two skeletal muscle layers • Inner layer runs longitudinally • Outer layer runs circularly • Alternating contractions of these two muscle layers propel food through the pharynx into the esophagus aka peristalsis Pharynx

10. Runs from pharynx through the diaphragm to the stomach • About 25 cm (10 inches) long • Conducts food to the stomach through peristalsis • The reason why food can get to your stomach even if you are hanging upside down. Esophagus aka gullet

12. The walls of the alimentary organs from the esophagus to the large intestine are made up of the same four basic tissue layers (or tunics) • Mucosa (innermost layer)-lines the cavity • Submucosa • Muscularisexterna-muscle layer • Serosa-(outermost layer) Tissue layers

13. J-shaped, nearly hidden by the liver and diaphragm on the left side of the abdominal cavity Parts of stomach

14. Top opening of stomach where food enters from esophagus is the cardioesophageal sphincter • The bottom opening of stomach where food continues to the small intestine is the pyloric sphincter or valve • Stomach varies from 15-25 cm (6-10 inches long) but diameter varies with amount of food contained • When full, can hold 4L (1 gallon) of food Stomach continued

15. Stomach collapses when empty into large folds called rugae

16. Divided into cardia, fundus, body, pylorus, separated from the duodenum by pyloric sphincter. • Gastric glands secrete acidic gastric juices. • Mixing waves- chyme (processed food that resembles heavy cream) forms and passes to duodenum Parts of stomach

17. Major digestive organ • Muscular tube extending from the pyloric sphincter to the large intestine • Average length of 8-20 feet • Hangs in sausage-like coils in the abdominal cavity, suspended by mesentery • The small intestine has three subdivisions: the duodenum (5%), jejunum (about 40%), and ileum (60%) Parts of the Small intestine

18. Mesentery

19. Ileocecal valve joins the large and small intestines • Some enzymes are produced in the intestinal cells and pancreas • Pancreatic ducts funnel enzymes into the small intestine • Bile is transported through the bile duct from the gallbladder into the duodenum • Small intestine has three structures that increase the absorptive surface—microvilli, villi, circular folds Parts of the small intestine

20. Villi, microvilli, circular folds

21. Larger in diameter than small intestine but shorter in length • About 5 ft long • Extends from ileocecal valve to the anus • Major function is the dry out the indigestible food by absorbing water and to eliminate residues as feces • Frames the small intestines on three sides: appendix, colon, rectum and anal canal Parts of the large intestine

22. Colon

23. The anal canal has an external voluntary sphincter (external anal sphincter) and an internal involuntary sphincter (internal involuntary sphincter) • Act like purse strings to open and close the anus • No villi present in the large intestine, no absorption at this point Parts of the large intestine

26. Ingestion-placing food in mouth • Propulsion-swallowing, peristalsis • Mechanical digestion-mixing in mouth, churning in stomach, segmentation in small intestine; physically breaking down into smaller particles • Chemical digestion-food broken down into building blocks by enzymes; water is necessary as dissolving medium and softening agent Functions of Digestive System

27. Absorption-digested food must enter the mucosal cells by active or passive transport processes; mostly in small intestine • Defecation-elimination of indigestible residues from the GI tract vis the anus in the form of feces Functions of Digestive System

28. Food physically broken down by chewing • Salivary amylase (enzyme) mixes with food and chemically breaks starch into maltose • More saliva is produced when food enters the mouth • The pharynx and esophagus simply provide a passageway to the stomach • No digestion occurs here Activities of Mouth, Pharynx, Esophagus

29. Involves the tongue, soft palate, pharynx, and esophagus • Buccal phase is voluntary and occurs in the mouth • Pharyngealesophageal phase is when the bolus moves through the pharynx and esophagus by peristalsis • Epiglottis is closed over the larynx to avoid going through the windpipe Deglutition aka Swallowing

31. The presence of food and a rising pH in the stomach stimulate the stomach cells to release the hormone gastrin. • Gastrin prods the stomach glands to produce still more of the protein-digesting enzymes (pepsinogens), mucus, and hydrochloric acid. • Under normal conditions, 2-3 liters of gastric juice are produced every day. • Hydrochloric acid and pepsinogens are acidic enough to burn the stomach however, mucus protects the stomach lining. • If your stomach didn't produce a new layer of mucous every two weeks, it would digest itself. Activities of the Stomach

32. Pepsin and rennin are the two main protein-digesting enzymes in the stomach • Little chemical digestion occurs in the stomach • Only aspirin and alcohol are absorbed in the stomach, very little else • As food enters and fills the stomach, the walls stretch and the muscle layers compress and pummel the food breaking it apart physically and mixing the food with gastric juice to make chyme Activities of the stomach

33. Peristaltic waves move toward the pylorus • The pyloric end of the stomach acts as a pump that delivers small amounts of chyme into the duodenum while forcing most of the contained material backward where it undergoes further mixing • It takes about 4 hours for the stomach to empty completely and 6 hours if the meal has a high fat content Activities of the Stomach

34. Carbohydrates and proteins have started digestion in the stomach but fat digestions hasn’t occurred. • By the end of the small intestine, digestion is complete and nearly all food absorption has occurred • Brush border enzymes in the microvilli break down double sugars into simple sugars and complete protein digestion Activities of the Small intestine

35. Contains bicarbonate to neutralize the acidic chyme • Contains enzymes that: • Complete digestion of starch (pancreatic amylase) • Carry out about half of protein digestion (trypsin, chymotrypsin, carboxypeptidase, etc…) • Totally responsible for fat digestion • Pancreas is the only source of lipase • Digest nucleic acids (nucleases) Pancreatic juice

36. Digestive Enzymes in the Small intestines

37. When chyme enters the small intestine, it stimulates the mucosa cells to produce several hormones, secretin and cholecystokinin which influence the release of pancreatic juice and bile • Bile and pancreatic juice enter the small intestine together to emulsify fat for absorption and digestion • Most substances are absorbed through the intestinal cell plasma membranes by active transport then into the capillary beds in the villi to be transported through the blood to the liver Activities of the Small intestine

38. Microvilli

39. All that remains from the small intestine is some water, indigestible food (plant fiber), and large amounts of bacteria. • Residue spends 12-24 hours in the large intestine • Bacteria metabolize remaining nutrients and release methane and hydrogen sulfide gas -> flatus • About 500 mL of gas is produced each day, more when certain foods are eaten Activities of Large intestine

40. Bacteria also make some vitamins in the large intestine • Ex. E. coli makes vitamin K • Vitamins, some ions, and most of the remaining water are absorbed here • Feces are delivered to the rectum containing undigested food residues, mucus, millions of bacteria, and just enough water for smooth passage Activities of the Large intestine