Digestive System: Key Functions and Mechanisms

1. Chapter 24: The Digestive System

2. Organisms • Need to acquire nutrients from environment Metabolism • Anabolism • Uses raw materials to synthesize essential compounds (ex: cell membrane lipids) • Catabolism • Decomposes organic molecules to provide energy (ATP) that cells need to function • Usually requires oxygen

3. Components of the Digestive System Figure 24–1

4. Digestive Tract • Gastrointestinal (GI) tract or alimentary canal • Is a muscular tube • Extends from oral cavity to anus

5. 6 Functions of the Digestive System • Ingestion: • occurs when materials enter digestive tract via the mouth • Mechanical processing: • crushing and shearing • makes materials easier to propel along digestive tract

6. 6 Functions of the Digestive System • Digestion: • is the chemical breakdown of food into small organic fragments for absorption by digestive epithelium • Secretion: • is the release of water, acids, enzymes, buffers, and salts • by epithelium of digestive tract • by glandular organs

7. 6 Functions of the Digestive System • Absorption: • movement of organic substrates, electrolytes, vitamins, and water across digestive epithelium into interstitial fluid of digestive tract • Excretion: • removal of waste products from body fluids

8. Smooth Muscle • Along digestive tract: • has rhythmic cycles of activity • controlled by pacesetter cells • Cells undergo spontaneous depolarization: • triggering wave of contraction through entire muscular sheet

9. Peristalsis Figure 24–4

10. Peristalsis • Consists of waves of muscular contractions • Circular and longitudinal • Moves a bolus along the length of the digestive tract

11. Peristaltic Motion • Circular muscles contract behind bolus: • while circular muscles ahead of bolus relax • Longitudinal muscles ahead of bolus contract: • shortening adjacent segments • Wave of contraction in circular muscles: • forces bolus forward

12. Segmentation • Cycles of contraction: • Churn and fragment bolus • mix contents with intestinal secretions • Does not follow a set pattern: • does not push materials in any 1 direction

13. The Regulation of Digestive Activities Figure 24–5

14. Neural Mechanisms • Control: • movement of materials along digestive tract • secretory functions • Motor neurons: • control smooth muscle contraction and glandular secretion • located in myenteric plexus

15. Digestive Hormones • At least 18 hormones that affect most aspects of digestive function • Are produced by enteroendocrine cells in digestive tract • Reach target organs after distribution in bloodstream

16. The Oral Cavity Figure 24–6

17. 4 Functions of the Oral Cavity • Sensory analysis: • of material before swallowing • Mechanical processing: • through actions of teeth, tongue, and palatal surfaces • Lubrication: • mixing with mucus and salivary gland secretions • Limited (chemical) digestion: • of carbohydrates and lipids

18. The Epithelial Lining • Of cheeks, lips, and inferior surface of tongue is nonkeratinized, and delicate • Mucosa under tongue and inside cheeks is thin and vascular enough to rapidly absorb lipid-soluble drugs

19. Lingual Papillae • Fine projections on superior surface (dorsum) of tongue • Covered in thick epithelium • Assists in moving materials

20. Lingual Glands • Small glands extend into underlying lamina propria • Secretions flush tongue’s epithelium • Contain water, mucins, and enzyme lingual lipase

21. Salivary Glands • 3 pairs secrete products into oral cavity • Each pair has distinctive cellular organization and produces saliva with different properties • Produce 1.0–1.5 liters of saliva each day

22. Saliva • 99.4% water • 0.6% includes: • electrolytes (Na+, Cl—, and HCO3—) • buffers • glycoproteins (mucins) • antibodies • Enzymes (salivary amylase) • waste products

23. Control of Salivary Secretions • By autonomic nervous system: • parasympathetic and sympathetic innervation • _______________ stimulation accelerates secretion by all salivary glands

24. The Swallowing Process Figure 24–11

25. Swallowing • Also called deglutition (2,400 X a day) • Can be initiated voluntarily • Proceeds automatically • Is divided in 3 phases: • buccal phase • pharyngeal phase • esophageal phase

26. Primary Peristaltic Waves • Movements coordinated by afferent and efferent fibers in glossopharyngeal and vagus nerves • Controlled by swallowing center of medulla oblongata

27. The Stomach Lining Figure 24–13

28. 4 Functions of the Stomach • Storage of ingested food • Mechanical breakdown of ingested food • (Preliminary) Disruption of chemical bonds in food material: • by acids and enzymes • Production of intrinsic factor: • glycoprotein required for absorption of vitamin B12 in small intestine

29. Gastric Pits • Are shallow depressions that open onto the gastric surface • Mucous cells: • at base, or neck, of each gastric pit • actively divide, replacing superficial cells

30. Gastric Glands • In fundus and body of stomach: • extend deep into underlying lamina propria • Each gastric pit leads to several gastric glands • 2 main types of cells found in gastric glands: • parietal cells • chief cells

31. Parietal and Chief Cells • Parietal cells • Secrete hydrochloric acid (HCl) • Chief cells • Are most abundant near base of gastric gland (secrete pepsinogen) • Pepsinogen is converted by HCl in the gastric lumen to pepsin

32. Enteroendocrine Cells ofPyloric Glands • Are scattered among mucus-secreting cells of pylorus: • G cells • Produce gastrin (hormone that stimulates both the Chief and Parietal cells) • D cells • Release somatostatin (hormone that inhibits release of gastrin)

33. The Phases of Gastric Secretion Figure 24–15

34. The Cephalic Phase • Begins when you see, smell, taste, or think of food: • directed by CNS • prepares stomach to receive food

35. The Gastric Phase • Begins with arrival of food in stomach: • builds on stimulation from cephalic phase • lasts 3-4 hours

36. The Intestinal Phase • Begins when chyme first enters small intestine: • After several hours of mixing contractions: • when waves of contraction sweep down length of stomach

37. Digestion in the Stomach • Stomach performs preliminary digestion of proteins by pepsin: • some digestion of carbohydrates (by salivary amylase) • lipids (by lingual lipase) • Chyme • become more fluid • pH approaches 2.0 • pepsin activity increases (proteins)

38. Absorption in the Stomach • Although some digestion occurs in the stomach, nutrients are not absorbed there • Only small lipid-soluble particles can cross stomach lining • Alcohol • Drugs (aspirin)

39. The Small Intestine • Plays key role in digestion and absorption of nutrients • 90% of nutrient absorption (lipids, proteins, carbohydrates) occurs in the small intestine

40. The Intestinal Wall Figure 24–17

41. The Duodenum • The segment of small intestine closest to stomach • 25 cm (10 in.) long • “Mixing bowl” that receives: • chyme from stomach • digestive secretions from pancreas and liver

42. The Jejunum • Is the middle segment of small intestine • 2.5 meters (8.2 ft) long • Is the location of most: • chemical digestion • nutrient absorption

43. The Ileum • The final segment of small intestine • 3.5 meters (11.48 ft) long

44. Brush Border Enzymes • Integral membrane proteins • On surfaces of intestinal microvilli • Break down materials in contact with brush border • Ex: Enterokinase • A brush border enzyme • Activates pancreatic proenzyme Trypsinogen

45. Enteroendocrine Cells • In intestinal glands • Produce intestinal hormones: • Gastrin • Cholecystokinin (CCK) • Secretin

46. Functions of the Duodenum • Has few plicae and Small villi • receives chyme from stomach • neutralizes acids before they can damage the absorptive surfaces of the small intestine

47. Intestinal Secretions • Watery intestinal juice • 1.8 liters per day enter intestinal lumen • Moistens chyme • Assists in buffering acids • Keeps digestive enzymes and products of digestion in solution

48. Intestinal Movements • Chyme arrives in duodenum • Weak peristaltic contractions move it slowly toward jejunum • Segmentation will periodically “mix” everything up

49. The Gastric Reflexes • The Gastroenteric Reflex • Stimulates motility and secretion along entire small intestines • The Gastroileal Reflex • Triggers relaxation of ileocecal valve: • Allows materials to pass from small intestine into large intestines

50. Functions of the Pancreas • Endocrine cells: • of pancreatic islets • secrete insulin and glucagon into bloodstream • Exocrine cells: • acinar cells • Produce pancreatic juice (alkaline mixture of digestive enzymes, water, ions)