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Lecture 3

WEEK 3 September 23, 25, 26, 30, Oct 1; Anatomy and Physiology of Digestion - Lecture Notes - LIFESCI 2N03

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McMaster University
Life Sciences
Danny M.Pincivero

LIFESCI 2N03 Anatomy and Physiology of Digestion (3) September 23, 2013 Digestive System  Two main groups: 1. Alimentary Canal o Gastrointestinal tract o Continuous digestive tube through whole body o Function a) Digestion – breaks food into smaller fragments so that body can absorb nutrients  CHO  monosaccharide  Fats  fatty acids  Protein  amino acids b) Absorption – moves the digested fragments from the alimentary canal into the blood o Anatomical Structure  Mouth (lips, cheeks, palate, tongue, teeth, salivary glands) – more digestion than absorption  Pharynx  Esophagus  Stomach (cardiac orifice, fundus, body, pyloric canal, pylorus, pyloric sphincter, omentum)  Small intestine (duodenum, jejunum, ileum) – absorb vast majority of macronutrients  Large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon, return, anal canal)  Considered to be “outside” the body (still in external environment) 2. Accessory Digestive Organs – assists alimentary canal in digestion o Teeth, tongue, gallbladder, liver, pancreas o Figure: Digestive System Anatomy 1 LIFESCI 2N03 Digestive System Activities  6 Essential Activities 1. Ingestion – physically taking food into the digestive system 2. Propulsion – moving food through the alimentary canal a. Swallowing – voluntary activity moving food down the esophagus b. Peristalsis  Involuntary movement of food through the esophagus, stomach and intestine o Muscular action; controlled by nervous system  Involves alternative “waves” of contraction and relaxation of smooth muscle o Muscle contracts and puts pressure on substance  substance goes toward the “area of least resistance” which is where the path is relaxed  Alters intestinal vessel (“tube”) diameter 3. Mechanical Digestion – physical preparation of food for absorption; chewing, mixing food with enzymes (tongue), churning of food in stomach a. Segmentation – mixing food in intestine with digestive “juices”; smooth muscle contraction and relaxation; breaks substance into smaller pieces; no net forward movement 4. Chemical Digestion  Involves the secretion of enzymes into the alimentary canal  Chemically breaks down food into “manageable” molecules  Enzymes secreted by glands  Starts in the mouth (salivary glands releasing amylases)  Humans do not have the enzyme required to break down dietary fibre – dietary fiber is not digestible  Dietary fiber good for gastrointestinal health – dietary fiber inhibits some fat and cholesterol absorption; assists in peristalsis 5. Absorption  Movement of digested end products from GI lumen to the bloodstream (or lymph)  Small intestine  Fatty acids, cholesterol and fat soluble vitamins go directly into the lymphatic system; do not go directly to cardiovascular system – transported by lipoproteins 6. Defecation  Elimination of indigestible substances through the rectal canal and anus Alimentary Canal Structure  Esophagus to anal canal  Four Tunics 1. Mucosa - moist epithelial layer that lines the lumen of the alimentary canal; most inner layer  Three Major Functions i. Secretion of mucus ii. Absorption of end products of digestion 2 LIFESCI 2N03 iii. Protection against infectious disease  3 Layers i. Epithelium – direct contact with food o Mucus Secretion  Protect digestive organs from digesting themselves; prevents acid from breaking down cells and creating a hole in organs  Ease food along the tract due to low friction; increases rate of peristalsis o Stomach and small intestine mucosa contain:  Enzyme-secreting cells (eg/ lactase)  Hormone-secreting cells (making them endocrine and digestive organs) ii. Lamina propria – holds structure together o Loose areolar and reticular connective tissue o Nourishes the epithelium and absorbs nutrients o Contains lymph nodes iii. Muscularis mucosae o Smooth muscle cells that produce local movements of mucosa 2. Submucosa – dense connective tissue containing elastic fibres, blood and lymphatic vessels, nodes and nerves 3. Muscularis externa – responsible for segmentation and peristalsis 4. Serosa – the protective visceral peritoneum; most outer layer; lines the abdominal cavity  Replaced by the fibrous adventitia (outer layer of organ) in the esophagus  Retroperitoneal organs have both an adventitia and serosa  Figure 3 LIFESCI 2N03  Small Intestines o Folds (villi); not smooth – increases surface area; increase ability to absorb o Villus = fingerlike projection  covered in microvilli o Untreated Celiac Disease – lose ability to absorb nutrients because of a decrease in surface area; microvilli become shortened Nervous System Control  Enteric Nervous System o Composed of two major intrinsic nerve plexus i. Submucosal Nerve Plexus (Meissner’s Plexus) – regulates glands and smooth muscle in the mucosa; present in small and large intestine ii. Myenteric Nerve Plexus (Auerbach’s Plexus) – major nerve supply that controls GI tract mobility (contracting smooth muscle) and motility (movement of food within digestive system in lumen); present throughout alimentary canal  Segmentation and peristalsis are automatic involving local reflex arcs o Reflex – don’t think about continually, spinal cord control of muscle o Presence of food with activate motor neurons  Linked to the CNS via long autonomic reflex arc o Splanchnic Circulation 4 LIFESCI 2N03  Arteries – branch off abdominal aorta  Superior mesenteric –small intestine  Inferior mesenteric – large intestine  Serves gut, spleen, pancreas and liver  ¼ of cardiac output is dedicated to the digestive system (resting???)  Cardiac Output - total volume of blood leaving the left ventricle per minute  Left ventricle delivers oxygenated blood to the heart itself and to every cell that uses oxygen to support its metabolism  Increases after a meal  Reflexes in place that are transmitted by NS, that causes a shift in blood flow – signals digestive system to break down food and move nutrients into the cardiovascular system  Reduction when physically active – shift blood flow to muscles o Function  Metabolic support to intestinal cells  Secretion of chemicals into GI lumen  Cells of small intestine will make enzymes, enzymes will go to the lumen to break down molecules, such as lactose  Absorption of macro- and micro- nutrients  Macronutrients o Only water soluble nutrients into the splanchnic circulation (monosaccharide’s and amino acids) o Fats (lipids) and fat soluble nutrients (dietary lipids, cholesterol) go directly into lymphatic system  Micronutrients o ADEK – moves with dietary lipids into lymphatic system (fat soluble micronutrients) o Water soluble vitamins A, B, C go into splanchnic circulation o Hepatic Portal Vein  Blood flow into liver (from digestive organs)  Liver to process and store whatever we have absorbed (nutrients)  Non-fat, water soluble  ~ 3 inches long  ~75% of blood flow into the liver through hepatic portal vein  ~ 25% of blood flow into the liver through hepatic artery  Intestinal cells are aerobic – need oxygen from arterial blood o Hepatic Vein  Blood flow away from liver (into vena cava, eventually into the right atrium) Digestive Anatomical Structure 1. Mouth (lips, cheeks, palate, tongue, teeth, salivary glands) – oral cavity o Palate – forms the roof of mouth i. Hard Palate – palatine bones and palatine processes of maxilla; forms a rigid structure for food  Tongue pushes food toward hard palate, then toward posterior aspect of oral cavity ii. Soft Palate – formed mostly of skeletal muscle; closes the nasopharynx during swallowing  Prevents food from going into nasal cavity o Tongue – composed of bundles of muscle fibers; sense of taste (chemical structure of what was taken into oral cavity) from sensory organs  Function a) Grips food during chewing b) Repositions the teeth c) Forms a bolus and initiates swallowing o Salivary Glands – produces saliva  Function a) Cleanses the mouth b) Dissolves food chemicals for tasting c) Moistens food; reduces friction d) Initiates breakdown of food (enzymes) o CHO and fats o Enzymes  Salivary Amylase – breakdown of polysaccharides (≥8 monosaccharide chain)  Lingual Lipase – initiates breakdown of dietary lipids (not large extent; begins process) 5 LIFESCI 2N03 o Intrinsic Gland – buccal glands (most common); interspersed throughout oral cavity o Extrinsic Glands – parotid, submandibular and sublingual glands; clear separation from other tissues that makeup the oral cavity; use ducts o Exocrine Glands – contains ducts that open into the oral cavity to release secretions into oral cavity o Mastication – chewing; mechanical breakdown of food; mixing with saliva; voluntary and reflexive muscle activity (similar to walking) 2. Pharynx – swallowing o 1 Part; Oropharynx – connection between oral cavity and top part of throat nd o 2 Part; Laryngopharynx – connection between larynx and lower part of throat; continuous with esophagus i. Pathway for food, fluids and air ii. Consists of 2 layers of skeletal muscle controlling peristalsis iii.Coordinated contraction moves food into the esophagus o Deglutition (swallowing) st  Buccal Phase (1 Phase of Swallowing) – tongue forces food up and back against palate; voluntary activity  Pharyngeal-Esophageal Phase (2 ndPhase of Swallowing) – reflexive; controlled by “swallowing center” in lower pons and medulla, via cranial nerves  Center = cluster of motor neurons that controls some physiological activity o Eg/ centers that control breathing, strength of heart muscle contraction etc  Steps i. Tongue blocks off mouth ii. Soft palate rises to close nasopharynx iii. Larynx rises and is covered by epiglottis o Do not want food to go down trachea 3. Esophagus o Hollow tube surrounded by muscle o Moves bolus toward the stomach o Approximately 10 inches long o Pierces the diaphragm muscle through the esophageal hiatus (“opening”)  Dome shaped diaphragm muscle separates thoracic cavity from abdominal cavity 4. Stomach (cardiac orifice, fundus, body, pyloric canal, pylorus, pyloric sphincter, omentum) o Cardiac Orifice – opening into the stomach; connection between the esophagus and the stomach; surrounded by the cardiac (gastroesophageal) sphincter (a ring smooth muscle that when contracts closes off an opening)  Relaxes cardiac sphincter when bolus is on its way to the stomach – allows access to stomach  Cardiac sphincter prevents gastric reflux into esophagus o Stomach – “storage tank”; chemical breakdown of proteins; converted to chyme; 6-10 inches long; empty stomach volume = 50 mL; “full” stomach volume = ≤4L o Rugae – folds in the stomach lining; increase surface area o Cardiac Region – “near the heart”; opening of the stomach (coming from esophagus); proximate region of stomach o Fundus – dome-shaped part (bulge), lateral to cardiac region o Body – mid-portion of the stomach o Distal part of stomach o Pyloric Region – funnel-shaped, distal to body o Pyloric Antrum – superior and wider portion of pyloric region o Pyloric Canal – narrows form the pyloric antrum o Pylorus – terminal portion of pyloric region; opening into the first part of the small intestine (duodenum) regulated by the pyloric sphincter o Pyloric Sphincter – controls stomach emptying  Allows more time for mixing in stomach and time for small intestine to work so that maximum nutrients can be absorbed o Greater curvature – convex lateral surface of stomach o Lesser curvature = concave medial surface of stomach o Omenta – mesenteries that connect the stomach to other organs and peritoneum 6 LIFESCI 2N03 o Peritoneum – connective tissue that houses nerves, blood vessels that supply abdominal organs; provide structural support and keep organs in place o Not on diagram o Gastric Pits – secretes gastric juice and alkaline mucus (bicarbonate) o Alkaline mucus – protective o HCl – created by parietal cells o Pepsinogen – activated into pepsin; created by chief cells o Pepsin – accelerates breakdown of dietary proteins; chemical protein digestion o Bicarbonate – raises the pH; offsets acidic HCl o Parasympathetic nervous system – vagus nerve, increases secretion of gastric glands following ingestion of food o Vagus nerve – major nerve through which parasympathetic NS operates o Autonomic nervous system that is more active when resting (as compared to sympathetic NS) o Local Reflexes – physical presence of bolus will stretch stomach; sensory receptors in stomach walls will send off action potentials synapse with P NS; signal this system to activate glands and begin secretion o Stimuli – head (brain; thoughts related to food; electrical impulses can begin stomach secretions to prime the stomach for food), stomach, small intestine  o Muscularis Externa – made up of muscle cells; muscles contract and change shape (shorten) and generate and transmit forces to other anatomical structures  Longitudinal Layer  Circular Layer – when circular muscle contracts – reduce interior volume; applies pressure on chime  Proximal contraction – push food toward pyloric region  Contraction in pyloric region – push food toward cardiac region  Non-uniform contraction of stomach with gastric juices allows for mechanical digestion  Oblique Layer o Vomiting (emesis)  Causes  Extreme stomach stretching (overeating)  Irritants (Alcohol, bacteria, spicy foods)  Syprus of ipecac o Ipecac – dried root from Brazil  Emetic – agent that induces vomiting (acting locally or centrally) o Central nervous system – psychological factors o Exercise – body tries
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