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Chapter 3

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Biomedical Physio & Kines
BPK 110
Diana Bedoya

Chapter 3: Digestion Part of nervous/cardiovascular/endocrine system Atoms: smallest units of an element that retains the properties of the element Molecules: units of two or more atoms of the same or different elements bonded together Cells: the basic structural and functional units of living things Organs: discrete structures composed of more than one tissue that performs a specialized function Hormones: chemical messengers produced in one part of body, released into blood, travel to other part of body where they elicit responses Digestion: Process of food broken down into components small enough to e absorbed into blood Absorption: Process of taking substances from GI tract into interior of the body Feces: body waste  unabsorbed food residue, bacteria, mucus, dead cells Organs of the Digestive System Gastrointestinal tract + 4 accessory organs (salivary glands, pancreas, liver, gall bladder) GI tract- 9m (30ft long) from mouth to anus - food in lumen not inside body because not absorbed yet Mucosa/mucosa cells: line the lumen of gut, contributes to secretion and absorption - live only 2-5 days (in contact with acid). Sloughed off and eliminated, new mucosal cells generated Transit time: time food takes to travel length of GI tract (mouth to anus) - 24 – 72 hours (1-3 days), depending on diet, physical activity, emotional state, health status, medication **4 types of tissues** Epithelium: lines cavities and surfaces, forms glands Connective tissue: connects, supports, or separates different types of tissue and organs Muscle tissue: smooth, skeletal, cardiac Nervous tissue: made up of different kinds of nerve cells. Function: sensory input, control of muscles and glands, homeostasis, mental activity Digestive System Secretions Mucus: a viscous fluid secreted by digestive glands. Lubricates, moistens, nod protects cells of GI tract from harsh environment. Enzymes: Proteins that accelerate rate of specific chemical reactions without being changed themselves - Hydrolysis: reaction using input of water to break down larger molecules into their structural units - Condensation: reaction in which two structural units combine to create a larger molecule, resulting in loss of a water molecule Digestion and Absorption of Nutrients Sensory input alone (smell/thought of food) activates nervous system to secrete saliva and stomach digestive enzymes Mouth - beginning of chemical and mechanical digestion Saliva: watery fluid produced and secreted into mouth by salivary glands; contains lubricants, enzymes, other substances (mostly water) Salivary amylase: breaks starch molecules into shorter carbohydrate chains Lingual lipase: initiates digestion of fat = chemical digestion - Saliva protects against tooth decay by washing away food particles - Saliva contains antibacterial enzymes, such as lysozyme, which helps prevent tooth decay Chewing = mechanical digestion - 32 teeth (adults) - breaking food into smaller pieces to be swallowed; increasing surface area to come in contact with digestive juices - tongue mixes food and saliva and aids in chewing Pharynx (throat) - responsible for swallowing - also part of respiratory tract and brings air into lungs - Swallowing: bolus (chewed food + saliva) directed to back of mouth by tongue, air passage blocked by epiglottis, food goes down esophagus Epiglottis: piece of elastic connective tissue that covers opening to lungs during swallowing Esophagus - no digestion here; just passageway - bolus moved down by peristalsis Peristalsis: coordinated muscular contraction that move material through GI tract - food leaves esophagus and passes through lower esophageal sphincter to enter stomach Sphincter: layer of muscle that encircles the tube of the digestive tract and act as a valve - Sphincter contracted – valve closed. Sphincter relax – valve open - sphincter prevents contents of stomach from going up esophagus, otherwise, heartburn Stomach - temporary storage site for food - Beginning of protein digestion - bolus mashes and mixed with acidic stomach secretions to form chyme (semiliquid food mass) - Some digestion occurs, but very little absorption (with exception of water, alcohol, some drugs like Aspirin and Tylenol) - 3 layers of smooth muscle: horizontal, longitudinal, diagonal, for powerful contraction that churns and mixes food Gastric juice - water, mucus, HCl, pepsinogen (inactive to not damage gastric glands that produce it) - released by gastric glands in pits that are in stomach lining - HCl begins protein digestion by unfolding proteins and converting pepsinogen  pepsin - HCl also kills bacteria in food and stops salivary amylase - Mucus lines stomach wall so acidic gastric juices (HCl and pepsin) do not digest the proteins found in cells of stomach wall Regulation of stomach activity - Signals from nerves and hormones coming from brain, stomach, and small intestine control stomach activity; how much stomach churns, how much gastric juices released how fast materials empty out of the stomach - Site, smell, thought of food stimulates brain, tell stomach to increase gastric secretions - Stomach distension  brain stimulates gastrin secretion from stomach  increases gastric secretion and stomach mixing - Food in small intestine causes SI to release hormones, which increases secretion of enzymes and substances into SI - Chyme in SI also signals to stomach to delay its emptying so SI has time to digest that chyme before receiving more - Chyme in stomach takes 2-6 hours to empty into SI, but rate depends on size and composition of meal; large meal takes longer to leave stomach; liquids leave quickly, solids take longer b/c need to become liquefied via mixing with gastric juices - Meal rich in starches and sugar << fibre, protein << high-fat meals stay in stomach longest, feel full Small Intestine - 6m (20 feet) - Most digestion in duodenum - SI huge surface area via long lengths and folds - Peristalsis propels chyme forward - Segmentation divides up chyme, mix it with enzymes, helps chyme encounter walls of SI so nutrients absorbed into blood Segmentation: Coordinated, periodic muscular contractions that aid in digestion and absorption, but do not significantly propel chyme forward Secretions (secretions from pancreas, gallbladder, and SI itself aid in digestion in SI) Pancreas  pancreatic juice (digestive enzymes + bicarbonate) - SI requires neutral environment - Acidic chyme in SI causes intestinal cells to release a hormone secretin, which stimulates pancreas to secrete bicarbonate into SI - Chyme with amino acids + fats in SI  CCK (from SI)  1. delays stomach emptying 2. Stimulates gall bladder to contract and release bile 3. Stimulates pancreas to release pancreatic amylase, pancreatic proteases, and lipases Pancreatic amylase (starch  double sugars) Pancreatic proteases = Trypsin, Chymotrypsin (protein  short chains of amino acids) Lipase (fat  fatty acids, glycerol monoglycerides) Bile: a digestive fluid made in liver and store in gallbladder that’s released into the SI where it aids in fat digestion and absorption (mechanically breaks down fat into smaller globules to increase SA so lipases can digest fats efficiently) Intestinal digestive enzymes in cells lining SI digests - double sugars/disaccharides ▯ simple sugar - chains of amino acids ▯ single amino acids Absorption Nutrients must pass from lumen of gut  through mucosal cells lining tract  blood or lymph Most digestion in duodenum, absorption occurs along entire length of SI Nutrients must pass from lumen of GI tract  mucosal cells lining the tract (single layer)  blood or lymph Simple diffusion: Unassisted movement of substances across cell membrane (from high to low concentration) - Eg. Vitamin E, fatty acids absorbed into mucosal cells by diffusion Osmosis: Unassisted diffusion of water across cell membrane Facilitated diffusion: Assisted diffusion of a substance across the cell membrane with the help of a protein carrier (passive diffusion, high to low) - Eg. Fructose (all sugars), proteins Active transport: Transport of substances across cell membrane with the aid of a protein carrier and the expenditure of energy (against concentration gradient) - Eg. Glucose, amino acids Large Intestine - Sphincter between small and large intestine control movement - Caecum, colon, rectum, anus - Caecum = attachment point for appendix,
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