The Digestive system

Transcription

1 The Digestive system

2 The GI tract (gastrointestinal tract) Mouth Pharynx Esophagus Stomach Small intestine Large intestine Anus The accessory digestive organs Supply secretions contributing to the breakdown of food Teeth & tongue Salivary glands Gallbladder Liver Pancreas 2

3 Accessory Organs 3

4 Digestive Tract 4

5 The Digestive Process Ingestion Taking in food through the mouth Propulsion (movement of food) Swallowing Peristalsis propulsion by alternate contraction &relaxation Mechanical digestion Chewing Churning in stomach Mixing by segmentation Chemical digestion By secreted enzymes: see later Absorption Transport of digested end products into blood Defecation Elimination of indigestible substances from body as feces 5

6 Histology of alimentary canal wall Same four layers from esophagus to anal canal 1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa from lumen (inside) out 6

7 Inner layer: the mucosa* (mucous membrane) * Three sub-layers 1. Lining epithelium 2. Lamina propria 3. Muscularis mucosae 7

8 More about the mucosa Epithelium: absorbs nutrients, secretes mucus Continuous with ducts and secretory cells of intrinsic digestive glands (those within the wall) Extrinsic (accessory) glands: the larger ones such as liver and pancreas Lamina propria Loose connective tissue with nourishing and absorbing capillaries Contains most of mucosa-associated lymphoid tissue (MALT) Muscularis mucosae Thin layer of muscle producing only local movements 8

9 Second layer: the submucosa* Connective tissue containing major blood and lymphatic vessels and nerves * Many elastic fibers so gut can regain shape after food passes 9

10 The muscularis layer Two layers of smooth muscle responsible for peristalsis and segmentation * Inner circular layer (circumferential) Squeezes In some places forms sphincters (act as valves) Outer longitudinal layer 10

11 Smooth muscle Smooth muscle Muscles are spindle-shaped cells One central nucleus No striations (no sarcomeres) Contractions are slow Does not always require a nervous signal: can be stimulated by stretching or hormones 6 major locations: 1. inside the eye 2. walls of vessels 3. respiratory tubes 4. digestive tubes 5. urinary organs 6. reproductive organs 11

12 Last (outer), the serosa* (the visceral peritoneum) Simple squamous epithelium (mesothelium) Thin layer of areolar connective tissue underneath * Exception: Parts not in peritoneal cavity have adventitia, lack serosa 12

13 Nerves Enteric nervous system: the gut s own Visceral plexuses within gut wall controlling the muscles, glands and having sensory info Myenteric: in muscularis Submucosal 100 million neurons Autonomic input: speeds or slows the system Parasympathetic Stimulates digestive functions Sympathetic Inhibits digestion Largely automatic 13

14 Oral Cavity 14

15 Functions of 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 digestion Of carbohydrates and lipids 15

16 The Tongue Manipulates materials inside mouth Functions of the tongue Mechanical processing by compression, abrasion, and distortion Manipulation to assist in chewing and to prepare material for swallowing Sensory analysis by touch, temperature, and taste receptors Secretion of mucins and the enzyme lingual lipase 16

17 Tongue Stratified squamous epithelium Taste buds contained by filliform, vallate and fungiform papillae 17

18 Tongue pupillae types: Filliform Fungiform Vallate form 18

19 Salivary Glands Oral Cavity Three pairs secrete into oral cavity Each pair has distinctive cellular organization: And produces saliva with different properties Produce 1.0 to 1.5 liters of saliva each day: 70% by sub-mandibular glands 25% by parotids 5% by sub-lingual glands 19

20 Saliva The Salivary Glands 99.4% water 0.6% includes: Electrolytes (Na +, Cl, and HCO 3 ) Buffers Glycoproteins (mucins) Antibodies Enzymes Waste products 20

21 Teeth Two sets Primary or deciduous Baby teeth Start at 6 months 20 are out by about 2 years Fall out between 2-6 years Permanent: 32 total All but 3 rd set of molars by end of adolescence 3 rd set = wisdom teeth Variable 21

22 Teeth are classified according to shape and function Incisors: chisel-shaped for chopping off pieces incisor canine premolar molar Canines: cone shaped to tear and pierce Premolars (bicuspids) Molars - broad crowns with 4-5 rounded cusps for grinding 22

23 Tooth Structure Root: The Teeth Of each tooth sits in a bony socket (alveolus) A layer of cementum covers dentin of the root: providing protection and anchoring periodontal ligament Crown: Exposed portion of tooth Projects beyond soft tissue of gingiva Dentin covered by layer of enamel 23

24 Tooth structure Enamel 99% calcium crystals Hardest substance in body Dentin bulk of the tooth (bonelike but harder than bone, with collagen and mineral) Pulp cavity with vessels and nerves Root canal: the part of the pulp in the root A C B Cementum bone layer of tooth root Attaches tooth to periodontal ligament 24

25 Pharynx Stratified squamous epithelium constrictor muscles Sequentially squeeze bolus of food into esophagus * * Are skeletal muscles Voluntary action Vagus nerve (X) * 25

26 Esophagus Continuation of pharynx in mid neck Descends through thorax On anterior surface of vertebral column Behind (posterior to) trachea Esophagus * 26

27 Microscopic anatomy of esophagus Contains all 4 layers (see right) Epithelium: nonkeratinized stratified squamous epithelium Mucus glands in wall Muscle (muscularis externa) changes as it goes down Superior 1/3 of esophagus: skeletal muscle (like pharynx) Middle 1/3 mixture of skeletal and smooth muscle Inferior 1/3 smooth muscle (as in stomach and intestines) 27

28 Esophagus histology 28

29 The Stomach Major Functions of the Stomach Storage of ingested food Mechanical breakdown of ingested food Disruption of chemical bonds in food material by acid and enzymes Production of intrinsic factor, a glycoprotein required for absorption of vitamin B 12 in small intestine 29

30 The Stomach Regions of the Stomach Cardiac Fundus Body Pylorus 30

31 The Stomach Anatomy of the Stomach The stomach is shaped like an expanded J: Short lesser curvature forms medial surface Long greater curvature forms lateral surface Anterior and posterior surfaces are smoothly rounded Shape and size vary from individual to individual and from one meal to the next Stomach typically extends between levels of vertebrae T 7 and L 3 31

32 Anatomy of the Stomach Figure 16-8a 32

33 Histology of the Stomach The Gastric Wall Simple columnar epithelium lines all portions of stomach Epithelium is a secretory sheet: Produces mucus that covers interior surface of stomach Gastric pits: shallow depressions that open onto the gastric surface Mucous cells, at the base, or neck of each gastric pit, actively divide, replacing superficial cells 33

34 Gastric Glands The Gastric Wall In fundus and body of stomach: Extend deep into underlying lamina propria Each gastric pit communicates with several gastric glands: Parietal Cells Secrete intrinsic factor and hydrochloric acid (HCl) Chief Cells Are most abundant near base of gastric gland: Secrete pepsinogen (inactive proenzyme): is converted by HCl in the gastric lumen:» to pepsin (active proteolytic enzyme) 34

35 The Gastric Wall 35

36 The Gastric Wall 36

37 Digestion in the Stomach Stomach performs preliminary digestion of proteins by pepsin Some digestion of carbohydrates (by salivary amylase) Lipids (by lingual lipase) Stomach contents Become more fluid ph approaches 2.0 Pepsin activity increases Protein disassembly begins Although digestion occurs in the stomach, nutrients are not absorbed there 37

38 Small intestine Longest part of alimentary canal (2.7-5 m) Most enzymatic digestion occurs here Most enzymes secreted by pancreas, not small intestine Almost all absorption of nutrients 3-6 hour process Runs from pyloric sphincter Small intestine 38

39 The Small Intestine Plays key role in digestion and absorption of nutrients 90% of nutrient absorption occurs in the small intestine Small intestine designed for absorption Huge surface area because of great length Structural modifications also increase absorptive area Circular folds (plicae circulares) Villi (fingerlike projections) 1 mm high simple columnar epithelium Microvilli 39

40 General histology of digestive tract 40

41 The Duodenum The Small Intestine The segment of small intestine closest to the stomach 25 cm (10 in.) long Mixing bowl that receives chyme from stomach and digestive secretions from pancreas and liver Functions of the duodenum: To receive chyme from stomach To neutralize acids before they can damage the absorptive surfaces of the small intestine 41

42 The Jejunum The Small Intestine Is the middle segment of the small intestine 2.5 meters (8.2 ft) long Is the location of most: Chemical digestion Nutrient absorption Small villi 42

43 The Small Intestine The Ileum The final segment of the small intestine 4.5 meters (11.48 ft) long Ends at the ileocecal valve, a sphincter that controls flow of material from the ileum into the large intestine 43

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45 Segments of the Intestine Figure

46 The Intestinal Wall Histology of the Small Intestine Plicae circulares: Transverse folds in intestinal lining Are permanent features: do not disappear when small intestine fills Intestinal villi: A series of fingerlike projections: in mucosa of small intestine Covered by simple columnar epithelium: covered with microvilli 46

47 The Intestinal Wall Histology of the Small Intestine Intestinal glands: Mucous cells between columnar epithelial cells Eject mucins onto intestinal surfaces 47

48 The Intestinal Wall Figure 16-11b 48

49 The Intestinal Wall 49

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51 Large intestine Subdivisions Digested residue reaches it Main function: to absorb water and electrolytes Ceacum Appendix Colon Rectum Anal canal 51

52 Colon has segments: ascending, transverse and descending colon; then sigmoid colon * * Between ileum and cecum 1 st part S-shaped Blind tube 52

53 Histology large intestine No villi Fewer nutrients absorbed Columnar cells in pic = absorptive cells Take in water and electrolytes A lot of goblet cells for mucus Lubricates stool More lymphoid tissue A lot of bacteria in stool 53

54 Rectum In pelvis Strong longitudinal muscle layer Has valves Anal canal Sphincters (close opening) Internal* smooth muscle involuntary * External* skeletal muscle voluntary * * 54

55 The Pancreas Lies posterior to the stomach Is wrapped in thin, connective tissue capsule 55

56 The Pancreas Histological Organization Lobules of the pancreas: Are separated by connective tissue partitions (septa) Contain blood vessels and tributaries of pancreatic ducts In each lobule: ducts branch repeatedly end in blind pockets (pancreatic acini) 56

57 The Pancreas Pancreatic Acini Are lined with simple cuboidal epithelium Contain scattered pancreatic islets Pancreatic Islets Endocrine tissues of pancreas Scattered (1% of pancreatic cells) 57

58 one acinus Pancreatic exocrine function Compound acinar (sac-like) glands opening into large ducts (therefore exocrine) Acinar cells make 22 kinds of enzymes Stored in zymogen granules Grape-like arrangement Enzymes to duodenum, where activated 58

59 Pancreatic endocrine function (hormones released into blood) Islets of Langerhans are the hormone secreting cells Insulin (from beta cells) Lowers blood glucose (sugar) Glucagon (from from alpha cells) Raises blood glucose (sugar) 59

60 The Pancreas Pancreatic Secretions 1000 ml (1 qt) pancreatic juice per day Controlled by hormones from duodenum Contain pancreatic enzymes 60

61 Pancreatic Enzymes The Pancreas Pancreatic alpha-amylase: A carbohydrase Breaks down starches Similar to salivary amylase Pancreatic lipase: Breaks down complex lipids Releases products (e.g., fatty acids) that are easily absorbed 61

62 Pancreatic Enzymes Nucleases: The Pancreas Break down nucleic acids Proteolytic enzymes: Break certain proteins apart Proteases break large protein complexes Peptidases break small peptides into amino acids 70% of all pancreatic enzyme production Secreted as inactive proenzymes Activated after reaching small intestine 62

63 The Liver Anatomy of the Liver Is the largest visceral organ (1.5 kg; 3.3 lb) Is wrapped in tough fibrous capsule Is covered by visceral peritoneum Is divided into lobes 63

64 The Surface Anatomy of the Liver 64

65 Hepatocytes Are liver cells Adjust circulating levels of nutrients: The Liver Through selective absorption and secretion In a liver lobule form a series of irregular plates arranged like wheel spokes Many Kupffer cells (stellate reticuloendothelial cells) are located in sinusoidal lining As blood flows through sinusoids: Hepatocytes secrete materials such as plasma proteins 65

66 Hepatocytes (liver cells) Many organelles Rough ER manufactures blood proteins Smooth ER help produce bile salts and detoxifies blood-borne poisons Peroxisomes detoxify other poisons, including alcohol Golgi apparatus packages Mitochondria a lot of energy needed for all this Glycosomes - role in storing sugar and regulation of blood glucose (sugar) levels Produce ml bile each day Secrete into bile canaliculi (little channels) then ducts Regeneration capacity through liver stem cells 66

67 A Portal Area Contains three structures: The Liver Branch of hepatic portal vein Branch of hepatic artery proper Small branch of bile duct 67

68 Liver Histology 68

69 Figure Copyright McGraw-Hill Companies

70 Figure Copyright McGraw-Hill Companies

71 Just some of the liver s repertoire Produces bile Picks up glucose from blood Stores glucose as glycogen Processes fats and amino acids (gluconeogenesis) Stores some vitamins Detoxifies poisons and drugs Makes the blood proteins 71

72 The Liver The Physiology of the Liver 1. Metabolic regulation 2. Hematological regulation 3. Bile production 72

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76 Bile Ducts Figure 16-16a 76

77 Bile Ducts Figure 16-16b 77

78 The Gallbladder Is a pear-shaped, muscular sac Stores and concentrates bile prior to excretion into small intestine Is located in the fossa on the posterior surface of the liver s right lobe 78

79 The Gallbladder Functions of the Gallbladder Stores bile Releases bile into duodenum, but only under stimulation of hormone cholecystokinin (CCK) CCK: Hepatopancreatic sphincter remains closed Bile exiting liver in common hepatic duct cannot flow through common bile duct into duodenum Bile enters cystic duct and is stored in gallbladder 79

80 The Gallbladder Physiology of the Gallbladder Full gallbladder contains ml bile Bile composition gradually changes in gallbladder: Water is absorbed Bile salts and solutes become concentrated 80

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82 Liver histology Liver lobules (about one million of them) Hexagonal solid made of sheets of hepatocytes (liver cells) around a central vein Corners of lobules have portal triads (see next pic) 82

83 Portal triad Portal arteriole Portal venule Branch of hepatic portal vein Delivers substances from intestines for processing by hepatocytes Bile duct Carries bile away Liver sinusoids Large capillaries between plates of hepatocytes Contribute to central vein and ultimately to hepatic veins and IVC Kupffer cells Liver macrophages Old blood cells and microorganisms removed 83

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