The Digestive System

1. The Digestive System

2. Introduction • The digestive system consists of the muscular digestive tract and various accessory organs. • Digestive functions include ingestion, mechanical processing, digestion, secretion, absorption, compaction, and excretion.

3. Overview of the digestive Tract • The digestive tract includes the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus.

5. Histological Organization • The epithelium and underlying connective tissue,the lamina propria, form the mucosa of the digestive tract. • Next, outward, are the submucosa, the muscularis externa, and the adventitia, a layer of loose connective tissue. • In the peritoneal cavity, the muscularis externa is covered by the serosa, a serous membrane

6. Double sheets of peritoneal membrane called mesenteries suspend the digestive tract.

7. Movement of Digestive Materials • The neurons that innervate the smooth muscle of the muscularis external are not under voluntary control. • The muscularis externa propels materials through the digestive tract by means of the contractions of peristalsis. • Segmentation movements in areas of the small intestine churn digestive materials.

8. Peristalsis

9. The Oral Cavity • The functions of the oral cavity are: • Analysis of potential foods • Mechanical processing using the teeth, tongue and palatal surfaces • Lubrication by mixing with mucus and salivary secretions • Digestion by salivary enzymes

10. The oral cavity, or buccal cavity, is lined by oral mucosa • The hard palate and soft palate form its roof and the tongue forms its floor.

12. The Tongue • The primary functions of the tongue include: • Mechanical processing • Manipulation to assist in chewing and swallowing • Sensory analysis.

13. The Salivary Glands • The salivary glands discharge their secretions into the oral cavity • Saliva lubricates the mouth, dissolves chemicals, flushes the oral surfaces and helps to control bacteria. • Salivation is usually controlled by the ANS.

15. Teeth • Mastication occurs through the contact of the opposing surfaces of the teeth • The periodontal ligament anchors the tooth ins a bony socket. • Dentin forms the basic structure of a tooth • The crown is coated with enamel, and the root is covered with cementum

17. The 20 primary teeth or deciduous teeth are replaced the the 32 teeth of the secondary dentition during development.

18. The Pharynx and Esophagus • The pharynx is the passageway between the oral cavity and the esophagus • The esophagus is a muscular tube about 10 in. long and ¾ inches in diam. • It is lined with stratified squamous epithelium that resists abrasion, hot or cold temp. and chemical attack. Mucus glands lubricate surface and keep material from sticking.

19. Swallowing Called deglutition The mass of food being swallowed is called a bolus

20. The Stomach • The stomach has four major functions: • Temporary storage of ingested food • The mechanical breakdown of food. • The disruption of chemical bonds by acids enzymes • The production of intrinsic factor a compound necessary for the absorption of vitamin B12

21. Chyme forms in the stomach as gastric and salivary secretions are mixed with food. • The four regions of the stomach are the cardia, fundus, body and pylorus • The pyloric sphincter guards the exit from the stomach. • In a relaxed state the stomach lining contains numerous rugae (ridges and folds)

23. The Gastric Wall • Within the gastric glands, parietal cells secrete intrinsic factor and hydrochloric acid. • Chief cells secrete pepsinogen which acids in the gastric lumen convert to the enzyme pepsin (digests proteins) • Gastric gland endocrine cells secrete the hormone gastrin.

24. The Regulation of Gastric Activity • Gastric secretion includes: • The cephalic phase which prepares the stomach to receive ingested materials • The gastric phase which begins with the arrival of food in the stomach • The intestinal phase which controls the rate of gastric emptying

28. The Small Intestine • The small intestine includes the duodenum, the jejunum, and the ileum. • The ileocecal valve, a sphincter, marks the transition between the small and large intestines.

30. The Intestinal Wall • The intestinal mucosa bears transverse folds called plicae, or plicaecirculares, • and small projections called intestinal villi. • They increase the surface area for absorption. • Each villus contains a terminal lymphatic called a lactealwhich absorbs lipids.

32. Intestinal Secretions • Intestinal glands secrete intestinal juice, mucus,and hormones. • Intestinal juice moistens the chyme, helps buffer acids,and dissolves digestive enzymes and the products of digestion. • Intestinal hormones include secretin, cholecystokinin(CCK) and gastric inhibitory peptide.

33. Digestion and the Small intestine • The most important digestive and absorptive functions occur in the small intestine. • Digestive enzymes and buffers are provided by the pancreas, liver and gallbladder.

34. The Pancreas • The pancreatic duct penetrates the wall of the duodenum, where it delivers the secretions of the pancreas.

36. The pancreas has two functions • Endocrine (secreting insulin and glucagon into the blood) • Exocrine (secreting water, ions, and digestive enzymes into the small intestine) • Pancreatic enzymes include lipases, carbohydreases and proteases and nucleases.

37. The Control of Pancreataic Secretion When chyme arrives in the small intestine secretin triggers the pancreatic production of a fluid containing buffers, primarily sodium bicarbonate, what help bring the pH of the chyme under control. Also CCK is released which stimulates the pancreas to secrete pancreatic amylase, pancreatic lipase, nucleases and protease enzymes.

38. The Liver • Largest visceral organ in the body, with over 200 known functions.

40. Liver Functions • Metabolic regulation: controls the composition of circulating blood. Filter out toxins. • Hematological regulation: liver is the largest blood reservoir in the body. Removes aged or damaged red blood cells, debris, and pathogens. • The gallbladder stores and concentrates bile for release into the duodenum.

41. Bile salts emulsify lipids for digestion. • The liver produces about 1 liter of bile/day but it is only released into the intestine when CCK is released.

43. The large Intestine • The main functions of the large intestine are: • Reabsorb water and compact the feces • Absorb vitamins liberated by bacteria • Store fecal material prior to defecation.

45. Three Parts of Large Intestine • Cecum: collects and stores material from the ileum. Vermiform appendix is attached to the cecum. • Colon: larger diameter and thinner wall than sm. Intestine/ • Rectum: terminates in the anal canal leading to the anus.

46. Physiology of the Large Intestine • The large intestine reabsorbs water and other substances such as vitamins, bilirubin, bile salts, and toxins; • Bacteria residing in the large intestine are responsible for intestinal gas or flatus. They also produce vitamin K (needed for clotting), Biotin (glucose metabolism), and vitamin B5(steroid hormones and neurotransmitters)

47. Digestion and Absorption • The digestive system breaks down the physical structure of the ingested material and then disassembles the component molecules into smaller fragments through hydrolysis.

48. Amylase breaks down complex carbohydrates. These are broken down into monosaccharides by other enzymes and are absorbed by the intestinal epithelium through facilitated diffusion and co-transport.

49. Triglycerides are emulsified into large lipid drops. The results interact with bile salts to form micelles which diffuse across the intestinal epitheliums. • Protein digestion involves the gastric enzyme pepsin and various pancreatic proteases.

50. Water and Electrolyte Absorption • About 2-2.5 liters of water are ingested each day and digestive secretion provide 6-7 liters • Nearly all is reabsorbed by osmosis.