Overview of the Digestive System: Functions and Processes

1. Chapter 24 The Digestive System Part A

2. Digestive System • Takes in food • Breaks food down into nutrient molecules • Absorbs these molecules into the blood stream • Rids the body of indigestible remains

3. Digestive System: Overview • The alimentary canal or gastrointestinal (GI) tract digests and absorbs food • Alimentary canal – mouth, pharynx, esophagus, stomach, small intestine, and large intestine: a long tube • Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas; not part of the tube but help with digestion

4. Figure 22.1

5. Digestion • Mechanism for nourishing the body • Most nutrients in food require either degradation ( break apart) or release prior to absorption • Digestion occurs in GI Tract • Digestion • Mechanical • chewing and peristalsis • Chemical • enzymes, HCl

6. The GI tract is a “disassembly” line • Nutrients become more available to the body in each step

8. Digestive Process • There are six essential activities: • Ingestion • Propulsion • Mechanical digestion • Chemical digestion • Absorption • Defecation Figure 22.2

9. Essential Activities of Digestion • Ingestion – taking food in • Propulsion – swallowing and peristalsis • Peristalsis – waves of contraction and relaxation of muscles in organ walls, esophagus and intestine • Mechanical digestion – chewing; mixing; churning food

10. Movement of digestive materials • Visceral (organ) smooth muscle shows rhythmic cycles of activity • Pacemaker cells • Peristalsis • Waves that move a bolus (rounded mass of food that is swallowed) • Segmentation • Churn and fragment a bolus

11. Figure 22.4 Peristalsis Figure 22.4

12. Essential Activities of Digestion bolus Figure 22.3a, b

13. Essential Activities of Digestion • Chemical digestion – catabolic breakdown of food • Absorption – movement of nutrients from GI tract to blood or lymph • Defecation – elimination of indigestible solid wastes

14. Essential Activities of Digestion Figure 22.2 Figure 22.2

15. GI Tract • External environment for the digestive process; open to outside at both ends • Regulation of digestion involves: • Mechanical and chemical stimuli • Extrinsic control by CNS centers • Intrinsic control by local centers

16. Receptors of the GI Tract • Mechano- and chemoreceptors respond to: • Stretch by food in lumen • Osmolarity(solute concentration) and pH • Presence of substrate or • End products of digestion • They initiate reflexes that: • Activate or inhibit digestive glands • Mix lumen contents and move them along

17. Control of the digestive system • Movement of materials along the digestive tract is controlled by: • Neural mechanisms • Parasympathetic and local reflexes • Hormonal mechanisms • Enhance or inhibit smooth muscle contraction • Local mechanisms • Coordinate response to changes in pH or chemical stimuli

18. Nervous Control of the GI Tract • Intrinsic controls • Nerve plexuses near the GI tract initiate short reflexes • Short reflexes are mediated by local enteric plexuses (gut brain) • Extrinsic controls • Long reflexes arising within or outside the GI tract • Involve CNS centers and extrinsic autonomic nerves

19. Nervous Control of the GI Tract Figure 22.4

20. Figure 22.5 The Regulation of Digestive Activities Figure 22.5

21. Digestive System Organs and Peritoneum • Peritoneum – serous membrane of the abdominal cavity • Visceral – covers external surfaceof most digestive organs • Parietal – lines the body wall Figure 22.5a

22. Digestive System Organs and Peritoneum • Peritoneal cavity • Lubricates digestive organs • Allows them to slide across one another Figure 22.5a

23. Digestive System Organs and Peritoneum • Mesentery – double layer of peritoneum • Vascular(blood) and nerve supplies to the viscera • A means to hold digestive organs in place and store fat • Retroperitoneal organs – organs outside the peritoneum • Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum Figure 22.5b

25. The digestive system organs and the peritoneum • Mesenteries • Sheets of serous membranes that support portions of the digestive tract • Greater omentum lies anterior to abdominal viscera • Provides padding, protection, insulation, and energy reserves • Lesser omentum

27. Figure 24.2 Mesenteries Figure 24.2b

28. Figure 24.2 Mesenteries Figure 24.2c

29. Figure 24.2 Mesenteries Figure 24.2d

30. Chapter 24 The Digestive System Part B

31. Blood Supply: Splanchnic Circulation • Arteries that branch off abdominal aorata and the organs they serve include • The hepatic: liver, splenic:spleen, and left gastric: stomach • Inferior and superior mesenteric: small and large intestines • Hepatic portal circulation: • Collects nutrient-rich venous blood from the digestive viscera (organs) • Delivers it to the liver for metabolic processing and storage

32. Histology of the Alimentary Canal • From esophagus to the anal canal the walls of the GI tract have the same four tunics • From the lumen outward they are the: • mucosa • submucosa • muscularis externa • and serosa • Note: serosa is technically not present on the esophagus • Each tunic has a predominant tissue type and specific digestive function

33. Histology of the Alimentary Canal Figure 24.6

34. Figure 24.3 The Structure of the Digestive Tract Figure 24.3

35. Mucosa • Moist epithelial layer -- lines the lumen of the alimentary canal • Its three major functions are: • Secretion of mucus • Absorption of the end products of digestion • Protection against infectious disease • Consists of three layers: • lining epithelium • lamina propria • muscularis mucosae

36. Mucosa: Epithelial Lining • Consists of simple columnar epithelium and mucus-secreting goblet cells • The mucus secretions: • Protect digestive organs from digesting themselves • Ease food along tract • Stomach and small intestine mucosa contain: • Enzyme-secreting cells • Hormone-secreting cells

37. Mucosa: Lamina Propria and Muscularis Mucosae • Lamina Propria • Loose areolar and reticular connective tissue • Nourishes the epithelium and absorbs nutrients • Contains lymph nodes (part of MALT-Mucosa Associated Lymphatic Tissue) important in defense against bacteria • Muscularis mucosae– smooth muscle cells that produce local movements of mucosa

38. Mucosa: Other Sublayers • Submucosa – dense connective tissue containing: • elastic fibers • blood and lymphatic vessels • lymph nodes • nerves • Muscularis externa – responsible for segmentation and peristalsis • Serosa – the protective visceral peritoneum • Replaced by the fibrous adventitia in the esophagus • Retroperitoneal organs have both an adventitia and serosa

39. Enteric Nervous System • Composed of two major intrinsic nerve plexuses • Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa • Myenteric nerve plexus: • Major nerve supply that controls GI tract mobility • Segmentation and peristalsis • Linked to the CNS via long autonomic reflex arc • Parasympatheitic enhances • Sympathetic inhibits Enteric nevous system or “gut brain” 100 million neurons-more than the spinal cord

40. Chapter 24 The Digestive System Part C

41. Mouth • Oral or buccal cavity: • Is bounded by lips, cheeks, palate, and tongue • Has the oral orifice as its anterior opening • Is continuous with the oropharynx posteriorly • To withstand abrasions: • The mouth is lined with stratified squamous epithelium • The gums, hard palate, and dorsum of the tongue are slightly keratinized

42. Mouth Figure 24.7a

43. Lips and Cheeks • Have a core of skeletal muscles • Lips: orbicularis oris • Cheeks: buccinators • Vestibule – bounded by the lips and cheeks externally and teeth and gums internally • Oral cavity proper– area that lies within the teeth and gums • Labial frenulum – median fold that joins the internal aspect of each lip to the gum Figure 24.7b

44. Lips and Cheeks Figure 24.7b

45. Palate • Hard palate – underlain by palatine bones and palatine processes of the maxillae • Assists the tongue in chewing • Slightly corrugated on either side of the raphe (midline ridge) • Soft palate – mobile fold formed mostly of skeletal muscle • Closes off the nasopharynx during swallowing • Uvula projects downward from its free edge • Palatoglossal and palatopharyngeal arches form the borders of the fauces

46. Cleft palate

47. Tongue • Occupies the floor of the mouth and fills the oral cavity when mouth is closed • Functions include: • Gripping and repositioning food during chewing • Mixing food with saliva and forming the bolus • Initiation of swallowing, and speech • Intrinsic muscles change the shape of the tongue • Extrinsic muscles alter the tongue’s position • Lingual frenulum secures the tongue to the floor of the mouth

48. Homeostatic Imbalance • Ankyloglossia – congenital situation where the lingual frenulum is extremely short • Commonly referred to as being “tongue-tied” • Corrected surgically by cutting the frenulum

49. Tongue • Superior surface bears three types of papillae • Filiform – give the tongue roughness and provide friction • Fungiform – scattered widely over the tongue and give it a reddish hue • Circumvallate – V-shaped row in back of tongue • Sulcus terminalis – groove that separates the tongue into two areas: • Anterior 2/3 residing in the oral cavity • Posterior third residing in the oropharynx

50. Tongue Figure 24.8a