BIOH122 Human Biological Science 2 Session 13 Digestive System 1 Mouth to Stomach Bioscience Department Endeavour College of Natural Health endeavour.edu.au
Session Plan o Functions of the digestive system o Layers of the GI tract o Neural innervations of the GI tract o Peritoneum o Mouth o Pharynx o Oesophagus o Deglutition o Stomach anatomy & histology o Stomach mechanical & chemical digestion Endeavour College of Natural Health endeavour.edu.au 2
Functions of the Digestive System Endeavour College of Natural Health endeavour.edu.au 3
The Digestive System o The Digestive System: contributes to homeostasis Breaks down food into forms that can be absorbed and used by body cells. Absorbs water, vitamins, and minerals, and it eliminates wastes from the body. o Digestion: The process of breaking down food into forms that can be absorbed and used by body cells. o Two Basic Process: Mechanical digestion Chemical digestion Endeavour College of Natural Health endeavour.edu.au 4
Organs of the Digestive System Two groups of organs: o The gastrointestinal (GI) tract: a continuous tube that extends from the mouth to the anus through the thoracic and abdominopelvic cavities Mouth, most of the pharynx, oesophagus, stomach, small intestine, and large intestine o The accessory digestive organs: Teeth, tongue, salivary glands, liver, gallbladder, and pancreas Endeavour College of Natural Health endeavour.edu.au 5
Functions of The Digestive System o Functions of the Digestive System: Ingestion: taking food into mouth. Secretion: release of water, acid, buffers, and enzymes into lumen of GI tract. Mixing and propulsion: churning and propulsion of food through GI tract. Digestion: mechanical and chemical breakdown of food. Absorption: passage of digested products from GI tract into blood and lymph. Defecation: elimination of faeces from GI tract. Endeavour College of Natural Health endeavour.edu.au 6
Layers of the GI Tract Endeavour College of Natural Health endeavour.edu.au 7
Layers of the GI Tract o Four-layered arrangement of tissues: in the wall of the GI tract from the lower oesophagus to the anal canal o From deep to superficial: Mucosa Submucosa Muscularis Serosa/Adventitia (visceral peritoneum) Endeavour College of Natural Health endeavour.edu.au 8
Layers of the GI Tract Endeavour College of Natural Health endeavour.edu.au 9
Mucosa o The mucosa: A mucous membrane Inner lining of the GI tract o Consists of: A layer of epithelium: in direct contact with the contents of the GI tract Lamina propria: a layer of connective tissue Muscularis mucosa: a thin layer of smooth muscle Endeavour College of Natural Health endeavour.edu.au 10
Mucosa o The epithelium: consists of: Nonkeratinized stratified squamous: in mouth, oesophagus and anus serves as a protective layer Simple columnar: in the stomach and intestines functions in secretion and absorption Exocrine cells: secrete mucus and fluid into the lumen of the tract, Enteroendocrine cells: secrete hormones Endeavour College of Natural Health endeavour.edu.au 11
Mucosa o The lamina propria: consists of: Loose areolar connective tissue: adheres the epithelium to the lower layers, Blood and lymph vessels: transport absorbed food to other tissues, Mucosa associated lymphatic tissue (MALT): Prominent lymphatic nodules containing immune system cells. Present all along the GI tract, especially in the tonsils, small intestine, appendix, and large intestine. Endeavour College of Natural Health endeavour.edu.au 12
Mucosa o The muscularis mucosa: Throws the mucous membrane of the stomach and small intestine into many small folds Increase the surface area for digestion and absorption. Movements Endeavour College of Natural Health endeavour.edu.au 13
Submucosa o The submucosa: consists of o Blood and lymphatic vessels: receive absorbed food molecules. o Submucous Plexus: An extensive network of neurons from the autonomic nervous system. Innervates mucosa, blood vessels and secretory cells of mucosal glands. o Glands and lymphatic tissue Endeavour College of Natural Health endeavour.edu.au 14
Muscularis o The Muscularis: consists of: o Skeletal muscle: in mouth, pharynx, upper oesophagus and anus voluntary control control over swallowing and defaecation o Smooth muscle: rest of the tract involuntary control mixes, crushes and propels food along by peristalsis o Myenteric plexus Innervate circular and longitudinal smooth muscle layers Endeavour College of Natural Health endeavour.edu.au 15
Serosa o The serosa: a serous membrane Superficial layer to the GI tract organs that are suspended in the abdominopelvic cavity visceral peritoneum o Secretes slippery fluid Endeavour College of Natural Health endeavour.edu.au 16
Neural innervations of the GI tract Endeavour College of Natural Health endeavour.edu.au 17
Neural innervations of the GI tract o The gastrointestinal tract: regulated by Enteric Nervous System (ENS): an intrinsic set of nerves Autonomic Nervous System (ANS): an extrinsic set of nerves Gastrointestinal Reflex Pathways Endeavour College of Natural Health endeavour.edu.au 18
Enteric Nervous System o Enteric Nervous System (ENS): the brain of the gut, consists of 100 million neurons that extend from the oesophagus to the gut motor neurons, interneurons, and sensory neurons o Two plexuses: Myenteric plexus: control gastric motility Submucosal plexus: control the secretory cells o Can function independently of the CNS Endeavour College of Natural Health endeavour.edu.au 19
Enteric Nervous System Endeavour College of Natural Health endeavour.edu.au 20
Autonomic Nervous System (ANS) o Autonomic Nervous System (ANS): Forms neural connections with ENS and regulates functions of ENS o Parasympathetic nerves: increase GI secretion and motility by increasing the activity of ENS neurons. Vagus (X) nerves: most parts of the GI tract, except the last half of the large intestine, Sacral spinal cord: last half of the large intestine o Sympathetic nerves: decrease GI secretion and motility by inhibiting the neurons of the ENS. arise from the thoracic and upper lumber regions of the spinal cord Endeavour College of Natural Health endeavour.edu.au 21
Gastrointestinal Reflex Pathways o Gastrointestinal Reflex Pathways: chemoreceptors and stretch receptors are associated with the sensory neurons of the ENS synapse with other neurons located in the ENS, CNS, or ANS. o Regulate secretions and motility in response to stimuli present in the lumen. Endeavour College of Natural Health endeavour.edu.au 22
Neural innervations of the GI tract: Summary Marieb, and Hoehn, 2011 Endeavour College of Natural Health endeavour.edu.au 23
Peritoneum Endeavour College of Natural Health endeavour.edu.au 24
Peritoneum o The peritoneum: The largest serous membrane of the body. o Two divisions: The parietal peritoneum: lines the wall of the abdominal cavity. The visceral peritoneum: covers some of the organs and constitutes their serosa. o Peritoneal cavity: The potential space between the parietal and visceral portions of the peritoneum contains serous fluid. Endeavour College of Natural Health endeavour.edu.au 25
Peritoneum o Peritoneal folds: Weave between the viscera. Bind the organs to one another and to the walls of the abdominal cavity. Contain blood vessels, lymphatic vessels, and nerves of the abdominal organs. o Five major peritoneal folds: Greater omentum Falciform ligament Lesser omentum Mesentery Mesocolon Endeavour College of Natural Health endeavour.edu.au 26
Parts of the Peritoneum Endeavour College of Natural Health endeavour.edu.au 27
Greater Omentum and Mesentery Endeavour College of Natural Health endeavour.edu.au 28
The Mouth Endeavour College of Natural Health endeavour.edu.au 29
Mouth o Mouth (oral or buccal cavity): o Formed by: Cheeks, Hard and soft palate Lips Tongue Endeavour College of Natural Health endeavour.edu.au 30
Salivary Glands o Salivary Glands: secrete saliva into the oral cavity o Small salivary glands: In mucous membrane of the mouth and tongue labial buccal palatal lingual o Major salivary glands: lie outside the oral mucosa Parotid Submandibular Sublingual glands Endeavour College of Natural Health endeavour.edu.au 31
Salivary Glands o Parotid: inferior and anterior to the ears, between the skin and the masseter muscle o Submandibular: medial and partly inferior to the body of the mandible o Sublingual glands: beneath the tongue and superior to the submandibular glands. Endeavour College of Natural Health endeavour.edu.au 32
o Saliva: Composition and Functions of Saliva o Composition: 99.5% water and 0.5% solutes such as salts, dissolved gases, various organic substances, and enzymes o ph: 6.35 6.85, only slightly acidic Endeavour College of Natural Health endeavour.edu.au 33
Composition and Functions of Saliva o Water: dissolve foods to be tasted by gustatory receptors and begin digestive reactions. o Mucus: lubricates food to facilitate bolus formation and swallowing o Bicarbonate and phosphate ions: buffer acidic foods that enter the mouth o Chloride ions: activate salivary amylase o Salivary amylase: starts the breakdown of starch in the mouth Endeavour College of Natural Health endeavour.edu.au 34
Composition and Functions of o Lysozyme: kills bacteria Saliva o Immunoglobulin A (IgA): prevents attachment of microbes to the epithelium, o Help remove waste molecules from the body, which accounts for the presence of urea and uric acid in saliva o Protects mouth from infection with its rinsing action - 1 to 1.5 liters/day o Keep the mucous membranes of the mouth and pharynx moist Endeavour College of Natural Health endeavour.edu.au 35
Salivation o Salivation is entirely under autonomic nervous control. o Increase salivation sight, smell, sounds, memory of food, tongue stimulation - rock in mouth cerebral cortex signals the salivatory nuclei in brainstem - (CN 7 & 9) parasympathetic nerves (CN 7 & 9) o Stop salivation dry mouth when you are afraid sympathetic nerves Endeavour College of Natural Health endeavour.edu.au 36
Digestion in the Mouth o Mechanical digestion (mastication or chewing) breaks food into pieces mixes food with saliva so it forms a bolus o Chemical digestion Salivary amylase Begins starch digestion at ph of 6.5 or 7.0 found in mouth When bolus and enzyme hit the ph 2.5 gastric juices hydrolysis ceases Lingual lipase Secreted by glands in tongue Activated in stomach and breakdown triglycerides into fatty acids and diglycerides Endeavour College of Natural Health endeavour.edu.au 37
Pharynx and Oesophagus Endeavour College of Natural Health endeavour.edu.au 38
Pharynx o The pharynx: a funnel-shaped tube that extends from the internal nares to the oesophagus posteriorly and the larynx anteriorly. o Composed of: skeletal muscle and lined by mucous membrane. o The nasopharynx functions in respiration only, whereas the oropharynx and laryngopharynx have digestive as well as respiratory functions. o Function: Propel food into the oesophagus and then into the stomach. Endeavour College of Natural Health endeavour.edu.au 39
Oesophagus o The oesophagus: a collapsible, muscular tube that lies behind the trachea and connects the pharynx to the stomach. It pierces the diaphragm through an opening called the oesophageal hiatus, and ends in the superior portion of the stomach. o Function: Secrete mucus and transport food to the stomach. Endeavour College of Natural Health endeavour.edu.au 40
Histology of the Oesophagus Mucosa (stratified squamous) Submucosa (mucous glands) Muscularis (upper 1/3 skeletal) (lower 1/3 smooth) Adventitia Endeavour College of Natural Health endeavour.edu.au 41
Deglutition (Swallowing) Endeavour College of Natural Health endeavour.edu.au 42
Swallowing o Deglutition/swallowing: Moves a bolus from the mouth to the stomach. o Facilitated by saliva and mucus o Organs involved: The mouth, pharynx, and tongue o Three stages: Voluntary stage Pharyngeal stage (involuntary) Oesophageal stage (involuntary) Endeavour College of Natural Health endeavour.edu.au 43
Swallowing o Voluntary stage: Begins when the bolus is forced into the oropharynx by tongue movement upward and backward against the palate o Pharyngeal stage: The bolus in the oropharynx stimulates receptors, which send impulses to the deglutition centre in the medulla oblongata and lower pons of the brain stem. The soft palate and uvula move upward to close off the nasopharynx, The epiglottis closes off the opening to the larynx, The bolus moves through the oropharynx and the laryngopharynx. The upper oesophageal sphincter relaxes and the bolus moves into the oesophagus. Endeavour College of Natural Health endeavour.edu.au 44
Swallowing o The oesophageal stage: begins when the bolus enters the oesophagus. o Peristalsis: A progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis, pushes the bolus further down the tract o Lower oesophageal sphincter: Relaxes as the bolus approaches the end of the oesophagus, and the bolus moves into the stomach. o Oesophageal glands: Secret mucus to lubricate the bolus and reduce friction. Endeavour College of Natural Health endeavour.edu.au 45
Swallowing The passage of solid or semisolid food from the mouth to the stomach takes 4 to 8 seconds; very soft foods and liquids pass through in about 1 second. Endeavour College of Natural Health endeavour.edu.au 46
Swallowing Endeavour College of Natural Health endeavour.edu.au 47
Stomach Anatomy and Histology Endeavour College of Natural Health endeavour.edu.au 48
Stomach o The stomach: a J-shaped structure that begins at the bottom of the oesophagus and ends at the pyloric sphincter. o Functions: Mixes saliva, food, and gastric juice to form chyme. Serves as reservoir for food before release into small intestine. Secretes gastric juice. Secretes gastrin into blood. Endeavour College of Natural Health endeavour.edu.au 49
Anatomy of the Stomach o Gross anatomical subdivisions: Cardia Fundus Body Pylorus Endeavour College of Natural Health endeavour.edu.au 50
Anatomy of the Stomach o Rugae: The mucosal folds, when the stomach is empty o Pyloric sphincter: A smooth muscle sphincter through which the pylorus communicates with the duodenum of the small intestine o Lesser curvature: The concave medial border of the stomach o Greater curvature: The convex lateral border Endeavour College of Natural Health endeavour.edu.au 51
Histology of the Stomach o Layers of stomach wall: Mucosa Submucosa Muscularis Serosa Endeavour College of Natural Health endeavour.edu.au 52
Histology of the Stomach o Mucosa: contains o Surface mucous cells/epithelial layer: A layer of simple columnar epithelial cells on the surface of the mucosa Secrete mucus o Muscularis mucosae: smooth muscle o Gastric glands: columns of secretory cells formed by extension of epithelial cells extend down into the lamina propria Form Gastric pits Endeavour College of Natural Health endeavour.edu.au 53
Histology of the Stomach Cells of gastric glands: o Three types of exocrine glands cells: Mucous neck cells Chief or zymogenic cells Parietal or oxyntic cells o Enteroendocrine cells: G cells o Gastric juice: The secretions of the mucous, parietal, and chief cells 2000 3000 ml per day Endeavour College of Natural Health endeavour.edu.au 54
Histology of the Stomach o The muscularis: has three layers of smooth muscle: longitudinal, circular, and an additional inner oblique layer. Permits greater churning and mixing of food with gastric juice o The serosa: a part of the visceral peritoneum. At the lesser curvature the visceral peritoneum extends upward to the liver as the lesser omentum. At the greater curvature the visceral peritoneum continues downward as the greater omentum and drapes over the intestines. Endeavour College of Natural Health endeavour.edu.au 55
Stomach mechanical and chemical digestion Endeavour College of Natural Health endeavour.edu.au 56
Stomach: Mechanical Digestion o Mixing waves: peristaltic movements pass over the stomach every 15 to 25 seconds. macerate food, mix it with secretions of the gastric glands, and reduce it to a soupy liquid called chyme. o Intense waves near the pylorus o Vigorous waves in body of stomach o Gentle waves in fundus o Gastric emptying: As food reaches the pylorus, each mixing wave periodically forces about 3 ml of chyme into the duodenum through the pyloric sphincter, Endeavour College of Natural Health endeavour.edu.au 57
Stomach: Chemical Digestion o Salivary amylase: Active for about an hour in fundus while food is not mixed with gastric juice o Gastric Juice: churning mixes chyme with acidic gastric juice Salivary amylase: Inactivated Lingual lipase: Activated and digest triglycerides into fatty acids and diglycerides Endeavour College of Natural Health endeavour.edu.au 58
Stomach: Chemical Digestion o o o o o Hydrochloric acid Kills microbes in food Denatures (unfolds) protein molecules Activates pepsinogen into pepsin Intrinsic factor: Absorption of vitamin B12 for RBC production Pepsin: Breaks down proteins into peptides Effective acidic environment. Gastric lipase: Splits short chain triglycerides of milk into fatty acids and monoglycerides most effective at a ph of 5 to 6 (as in an infants stomach) has a limited role in the adult stomach Mucus: Forms protective barrier that prevents digestion of stomach wall. Endeavour College of Natural Health endeavour.edu.au 59
Stomach: Chemical Digestion o Gastrin hormone: Acts as a local endocrine hormone. Stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen Contracts lower oesophageal sphincter, Increases motility of stomach, Relaxes pyloric sphincter Endeavour College of Natural Health endeavour.edu.au 60
Stomach: Absorption Only a small amount of nutrients are absorbed in the stomach o Epithelial cells: Impermeable to most materials. o Mucous cells: Absorb some water, ions, and short-chain fatty acids, as well as certain drugs (especially aspirin) and alcohol. Endeavour College of Natural Health endeavour.edu.au 61
Readings and Resources o Tortora, GJ & Derrickson, B 2014. Principles of Anatomy and Physiology, 14th edn, Wiley. o Harris, P, Nagy, S & Vardaxis, N 2010, Mosby s Dictionary of Medicine, Nursing and Health Professions, 2nd edn, Mosby Elsevier. o Guyton, AC & Hall, JE 2011, Textbook of Medical Physiology, 12th edn, Saunders Elsevier. o Marieb, EN & Hoehn, K 2010, Human Anatomy and Physiology, 8th edn, Benjamin Cummings Pearson. o Moore, KL, Dalley, AF & Agur, AMR 2010, Clinically Orientated Anatomy, 6th edn, Lippincott, Williams & Wilkins. Endeavour College of Natural Health endeavour.edu.au 62
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