The Digestive System

1. The Digestive System Chapter 22

2. Introduction • A healthy digestive system is essential to the maintenance of life – homeostasis • Includes the structures of the body that take in food, break it into nutrient molecules, absorb these molecules into the circulatory system, and then eliminate the indigestible wastes

3. Overview of the Digestive System • Various organs are divided into 2 main groups: alimentary canal and accessory digestive organs • Alimentary (aliment = nourishment) canal or gastrointestinal (GI) tract – the muscular digestive tube that winds through the body - mouth, pharynx, and esophagus - stomach, small intestine, and large intestine (end leads to the terminal opening or anus) - in a cadaver its about 9m (30 feet) - food material in the alimentary canal is outside the body (canal is open to the external environment at both ends)

4. Accessory Digestive Organs • Include the teeth and tongue, the gallbladder and various large digestive glands – salivary glands, liver, and pancreas - lie external to and are connected to the alimentary canal by ducts - accessory digestive glands secrete saliva, bile, and digestive enzymes (all help breakdown foodstuffs)

5. Figure 22.1

6. Digestive Processes Perform 6 essential food-processing activities: • Ingestion – taking of food into the mouth 2. Propulsion – movement of food through the alimentary canal - voluntary swallowing, involuntary peristalsis (‘around contraction’) involves alternate waves of contraction and relaxation of musculature in the organ walls • Mechanical digestion – prepares food for chemical digestion - chewing, churning of food in stomach and segmentation: - rhythmic local constrictions of the intestine mix food with digestive juices, increase efficiency of nutrient absorption

7. Chemical digestion – complex molecules are broken down to chemical components - carried out by enzymes secreted by digestive glands into the lumen of the alimentary canal - mouth, stomach, small intestine • Absorption – transport of digested end products from the lumen into the blood and lymphatic capillaries • Defecation – elimination of indigestible substances as feces

8. Schematic summary of digestive processes • Sites at which the 6 steps occur are indicated • The mucosa of almost the entire alimentry canal secretes mucus - protects the canal and lubricates its contents Figure 22.2

9. Peristalsis • Major means of propulsion • Adjacent segments of the alimentary canal alternately contract and relax, moving food distally along the canal Figure 22.3a

10. Segmentation • Non adjacent areas of the intestine alternately contract and relax • The active regions are separated by inactive regions, segmentation moves food onward and then backward to mix the food with digestiv juices Figure 22.3b

11. Abdominal Regions 4 grid lines divide abdominal wall into 9 regions: • Midclavicular lines – vertical lines of grid • Transtubercular plane – inferior horizontal line from the clavicle midpoint - connects tubercles of iliac crests • Subcostal plane - superior line that connects inferior points of the costal margins

12. Figure 22.4a

13. How regions relate to abdominal viscera Figure 22.4b

14. A simpler scheme defining four quadrants Figure 22.4c

15. Histology of the Alimentary Canal Wall Same 4 layers from esophagus to anus: • The mucosa – innermost layer - consists of epithelium, lamina propria, muscularis mucosae • The submucosa – external to the mucosa - contains blood and lymphatic vessels, nerve fibers • The muscularis externa – external to submucosa - 2 layers: circular muscularis (inner) and longitudinal muscularis (outer) - in some areas circular layer thickens to form sphinctrers • The serosa – outermost layer - is the visceral peritoneum

16. Figure 22.5

17. Smooth Muscle • Primarily found in walls of viscera • Fibers elongated – have one centrally located nucleus • Grouped into sheets: - Longitudinal layer is parallel to the long axis of the organ - Circular layer, deeper layer, with fibers that run around the circumference of the organ

18. Smooth Muscle Figure 22.6a, b

19. Smooth Muscle Contraction • Myofilaments operate by interaction with cytoskeleton • Dense bodies – correspond to Z-discs of skeletal muscle • Entry of CA2+ ions into the sacroplasm initiates contraction - caveolae, tiny infoldings • Contraction, slow, sustained, and resistant to fatigue (30X longer to contract and relax) Figure 22.7a–c

20. Innervation of Smooth Muscle • Innervated by ANS – only a few smooth muscle fibers in each sheet - impulse spreads thorugh gap junctions between adjacent fibers - contraction stimulated also by stretching of muscle fibers and hormones - single-unit innervation, whole sheet contracts as a single unit - multiunit innervation exception: iris of the eye and arrector pili muscles of the skin

21. Innervation of Smooth Muscle Figure 22.8 • ANS fibers release their neurotransmitters from varicosities (row of knots on the motor axon) into a wide synaptic cleft

22. Nerve Plexuses • Myenteric nerve plexus – lies between circular and longtiudinal muscularis - controls peristalsis and segmentation • Submucosal nerve plexus – lies in submucosa - signals glands to secrete • Innervation – botyh plexuses contain sympathetic and parasympathetic motor fibers - visceral sensory fibers

23. The Peritoneal Cavity and Peritoneum • Peritoneum – a serous membrane - visceral peritoneum surrounds the digestive organs - parietal peritoneium lines the body wall • Peritoneal cavity – a slit-like potential space

24. The Peritoneal Cavity and Peritoneum • Mesentery – double layer of peritoneum • Holds organs in place • Sites of fat storage • Provides a route for circulatory vessels and nerves Figure 22.9a

25. Peritoneum • Retroperitoneal organs • Behind the peritoneum • Peritoneal organs • Digestive organs that keep their mesentery Figure 22.9b

26. Mesenteries Figure 22.10a

27. Lesser omentum attaches to lesser curvature of stomach Figure 22.10b

28. Greater omentum – a “fatty apron” of peritoneum • Greater omentum and transverse colon reflected Figure 22.10c

29. Sagittal section through the abdominopelvic cavity • Mesenteries attach to posterior abdominal wall Figure 22.10d

30. Secondarily Retroperitoneal Organs • Initially formed within peritoneum • Become retroperitoneal - Fuse to posterior abdominal wall Figure 22.11

31. Table 22.1

32. The Mouth and Associated Organs • The mouth – oral cavity - mucosal layer of stratified squamous epithelium and a lamina propria • Lips and cheeks – formed from orbicularis oris and buccinator muscles (respectively)

33. Oral cavity and pharynx Figure 22.12a

34. The labial frenulum - connects lips to gum • The palate - forms the roof of the mouth Figure 22.12b

35. The Tongue • Interlacing fascicles of skeletal muscle • Grips food and repositions it • Helps form some consonants • Intrinsic muscles – within the tongue • Extrinsic muscle – external to the tongue • Lingual frenulum – secures tongue to floor of month

36. Tongue papillae - filiform papillae has no taste buds - fungiform papillae – taste buds on the tops - circumvallate papillae – marks border between mouth and pharynx; posterior /3 of tongue lies in oropharynx is lined with lingual tonsil

37. Figure 22.13

38. The Teeth • 20 Deciduous teeth – first appear at 5 months of age • 32 Permanent teeth – most erupt by the end of adolescence • Dental formula – shorthand to indicate numbers and positions of the different classes of teeth 2I, 1C, 2P, 3M X 2 2I, 1C, 2P, 3M

39. Figure 22.14

40. Tooth Structure • Longitudinal section of tooth in alveolus Figure 22.15

41. The Salivary Glands • Produce saliva – complex mixture of water, ions, mucus, and enzymes - moistens the mouth, dissolves food chemicals, wets food, binds food into a bolus, enzymes begin the digestion of carbohydrates • Compound tubuloalveolar glands - parotid, largest, lies anterior to the ear with a parotid duct that runs parallel to the zygomatic arch - contains only serous cells

42. Submandibular glands – lie along medial surface of mandible • Sublingual glands – lie in floor of oral cavity - contains primarily mucous cells

43. The Salivary Glands Figure 22.16

44. Sublingual gland – a series of compound tubuloalveolar glands with mostly mucous cells, a few serous cells • Serous demilunes- crescent shaped cap of serous cells at the end of a tubule of mucous cells • Y-shaped structure is a duct

45. The Pharynx • Oropharynx and laryngopharynx - passageways for food, fluids, and inhaled air - lined with stratified squamous epithelium - external muscle layer consists of superior, middle, and inferior pharyngeal constrictors

46. The Esophagus • Gross anatomy – muscular tube that propels swallowed food to the stomach - begins as a continuation of the pharynx - joins the stomach inferior to the diaphragm - cardiac sphincter closes lumen to prevent stomach acid from entering esophagus

47. Microscopic anatomy - epithelium is stratified squamous - when empty mucosa and submucosa are thrown into longitundinal folds - mucous glands are primarily compound tubuloalveolar glands - muscularis externa, skeletal muscle (first 1/3 of length) - adventitia, most external layer

48. Esophagus Figure 22.17a, b

49. The Stomach • J-shaped stomach is the widest part of the alimentary canal - temporary storage tank where food is churned into chyme - breakdown of proteins by secreting pepsin, a protein-digesting enzyme, functions only under acidic condition and hydrochloric acid (destroys harmful bacteria) - while most nutrients are absorbed in the small intestine, some are absorbed through the stomach (water, electrolytes, aspirin and alcohol) - food remains for roughly 4 hours

50. Figure 22.18a