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Lecture 2

Lecture 2 - Digestion.doc

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Queen's University
Health Studies
HLTH 230
Jeffery Lalonde

Lecture 2 - Digestion Obstacles of digestion 1. Eat, drink, breathe through our mouths -food -> stomach -air -> lungs 2. Diaphragm lies below the lungs and separates upper from lower half -food must pass the diaphragm 3. Materials in GI tract should move slowly forward at a pace that allows all reactions to occur -cant be happening too slow or too fast 4. Food must be lubricated to allow for transport -too much lubrication -> too fast -too little lubrication -> too slow 5. Some water must be passed to eliminate waste, but most retained to prevent dehydration 6. Enzymes digest protein, fat, and carbohydrates while the body is made of protein, fat, and carbs -need to have a system that breaks down food, but not us! 7. Waste is excreted periodically and voluntarily digestion: the process by which food is broken down into absorbable units When does digestion begin? -begins when food enters gastrointestinal tract (the mouth) What is the GI tract? -gastrointestinal tract: flexible muscular tube (mouth->anus) where digestion + absorption occur What is the Lumen? -inner space within the GI tract -like a tube throughout the body (like a hole through the body) Figure 3.1 in textbook!!!! - brief overview Mouth’s Role in Digestion • where digestion begins • food is masticated into smaller pieces bolus is formed -> foods blend with fluids • • fluids start to dissolve food particles to allow for tasting • some enzyme action begins -ex. amylase - carbohydrate digestion Mouth’s Role in Tasting • mouth contains 9000 - 10,000 tastebuds that are replaced every 10 days • 4 basic tastes 1) sweet - alcohol hydroxyl groupings (ex. sugar) 2) sour - organic acids (ex. vinegar) 3) salty - cations like sodium (ex. salt) 4) bitter - no specific chemical structure (ex. caffeine, tonic water) ← *a 5th taste is often included: umami/savory - taste associated with MSG Basics about taste buds • children have the most discriminating tastes Lecture 2 - Digestion -# of taste buds decline with age • taste buds not specialized for any taste qualities -taste buds have a variety of receptor sites that respond to different taste sensations -sweet/sour perceived on tip of the tongue, salt on hard palate taste vs. flavour -taste: 4/5 basic tastes (sweet, salty, sour, bitter, savory) -flavour: total sensory impression when food is eaten -combination of taste, odour, and mouthfeel -ex. pureed turkey dinner have the same taste! but their flavour is different because one is pureed and one is solid. Flavour (texture) is different, but tastes the same tongue • plays part in tasting food • also moves food around for chewing and swallowing Dysphagia • difficulty swallowing • sometimes seen in individuals with ALS or Parkinson’s disease or after a stroke saliva: name given to secretion of salivary glands -contains water, salts, mucus, and enzymes -protects teeth and linings of mouth and esophagus -keeps mouth clean by helping flush away food residues -ex. when eating nachos, saliva moistens and breaks down to help swallow components of saliva salivary amylase: enzyme that breaks polysaccharides into disaccharides mucus: holds bolus of chewed foods together + lubricates (thick and slippery) lysozyme: destroys bacteria (has antibacterial action in mouth) *xerostomia = decrease in saliva production -> difficulties chewing + swallowing Swallowing • when bolus of food is swallowed, it passes through the pharynx and into the esophagus pharynx: short tube that is shared by the respiratory system and digestive system epiglottis: cartilage (flap) in the throat that helps protect airway from fluids and food during swallowing -makes sure food goes into esophagus and not into the trachea (into the lungs) Esophagus - where food passes through • muscular tube that extends between pharynx and stomach (passes through diaphragm) • has sphincters at both ends -Upper Esophageal Sphincter (UES) opens to allow food into esophagus Lecture 2 - Digestion -Lower Esophageal Sphincter (LES/cardiac sphincter) relaxes + allows food into stomach peristalsis: ringlike contractions of the circular smooth muscles pushing the bolus towards the stomach -occurs throughout entire GI tract to push contents along Digestive Problems 1) Heartburn/Gastroesophageal Reflux -backglow of gastric contents into esophagus because of failure of LES to contract properly (GERDisease) 2) Achalasia -failure of LES to relax during swallow -> causes esophagus to become swollen due to build up of food in esophagus -LES doesn’t open as it should and food builds up -people should be eating more liquids/more watery foods/less acidic foods Stomach • J-shaped sac between esophagus and small intestine • 3 main sections -fundus: part of stomach that lies above esophageal opening -body: middle of stomach -antrum: lower part of stomach • approx. volume of 50ml when empty + can increase to 1L after a meal • has the thickest walls and strongest muscles of all the GI tract • 3 types of muscles in the stomach -longitudinal -circular -diagonal • bolus passes from esophagus into stomach • lower esophageal sphincter closes to prevent stomach contents from returning to esophagus • stomach retains bolus in upper portion • food is slowly transferred into lower portion where gastric juice is added -Gastric secretions include: -mucus: slippery substance secreted by cells of GI lining that protects gastric mucosa against mechanical and acidic damage -pepsinogen: gastric enzyme cleaved into pepsin by HCl -pepsin begins protein digestion by splitting a.a. chains -HCl: hydrochloric acid -activates pepsinogen, aids in breakdown of food, kills many microorganisms (helps protect from food borne illness) -intrinsic factor: glycoprotein in stomach that joins with B12 in small intestine -gastrin: hormone secreted by stomach wall that acts on glands of stomach -leads to increased secretion of components of hydrochloric acid -when pH 1.5 is reached, gastrin producing cells are “turned off” -produced gastrin to reach 1.5 but when it is reached it is turned off Ex. ON good in stomach causes cells of stomach wall to start releasing gastrin Lecture 2 - Digestion -> gastrin stimulates stomach to secrete HCl -> stomach pH reaches 1.5 pH -> negative feedback -> acidity in stomach causes cells of to stop releasing gastrin • stomach contents are ground into semi-liquid texture called chyme • the 3 sets of muscles force chyme downward but pyloric sphincter remains closed while stomach releases gastric juices • when chyme is completely liquefied, pyloric sphincter opens about 3x/min to let chyme into small intestine *no significant amount of food/water is absorbed from stomach into blood • small % of alcohol is absorbed • Acetylsalicyclic acid (aspirin) is absorbed in the stomach Small Intestine • chyme passes through pyloric sphincter into top of small intestine • chyme bypasses an opening from common bile duct • small intestine has 3 sections -duodenum -jejunum -ilium • approximately 10 ft coiled in abdomen • segmentation: periodic squeezing/partitioning of intestine along its length by circular muscles -these contractions mix chyme and promote close contact with digestive juices and absorbing cells of intestinal walls before letting contents move slowly along secretin: chyme in the small intestine stimulates the duodenum wall to release secretin into blood -secretin stimulates pancreas to release bicarbonate rich juices, neutralizing acidic chyme’ *secretin helps pH to get where it needs to be so enzymes can work Liver • performs variety of functions • stores glycogen, fats, iron, copper, many vitamins role in activation of Vitamin D • *main contribution to digestion = secretion of bile (emulsifier) which enters into duodenal lumen -bile is stored in the gall bladder and aids in fat digestion Gall Bladder • opening of bile duct (into duodenum) is guarded by the sphincter of Oddi • Sphincter of Oddi prevents bile from entering duodenum except during meal digestion • when sphincter is closed, most of the bile is diverted up into gall bladder where it is stored Pancreas • elongated gland that lies behind stomach (above first loop of duodenum) Lecture 2 - Digestion • exocrine function: secretes pancreatic juice -proteolytic enzymes (breakdown of protein) -pancreatic amylase (breakdown of carbs) -pancreatic lipase (breakdown of fat) mix of enzymes is tailored to type of diet • -ex. if someone eats a lot of protein and suddenly switches to carbs, body will adapt pancreatic juice -excretes NaCHO3 (sodium bicarbonate) rich fluid -neutralizes acidic chyme -allows pancreatic enzymes to function -enzymes require a neutral/slightly alkaline environment Absorption • carbohydrates, proteins, and lipids have been disassembled in the stomach and small intestine • vitamins and minerals generally don’t need to be broken down -undergo little or no change prior to absorption • undigested residues (like fibre) continue through GI tract, carrying out some waste Small Intestine • where most absorption takes place • surface area equivalent to a tennis court • circulating blood below surface transports absorbed molecules • nutrients are absorbed across membrane of small intestine into circulating blood by: 1) simple diffusion -some nutrients (water and small lipids) cross intestinal cells freely 2) facilitated diffusion Lecture 2 - Digestion -specific transport carrier (ex. B12) is required to transport nutrients from one side of membrane to the other -or the carrier may change the membrane -> allowing nutrient to pass through 3) active transport -glucose and a.a. move against concentration gradient (requires energy) • small intestine is covered with projections called villi, with each villi covered in microvilli • goblet cells are found on and around villi and between the villi are crypts crypts: tubular glands that secrete intestinal juices into the small intestine goblet cells: secrete mucus • enzymes within the brush border further digest some of the energy yielding nutrients into smaller particles • brush border: microvilli lining digestive tract • cells absorb nutrients into blood and lymph Preparing Nutrients for Transport • after a nutrient has crossed the villus, it enters either the blood stream/lymphatic system • water soluble nutrients and smaller products of fat digestion are released directly into blood stream and then onto liver • larger fats/fat soluble vitamins are assembled into chylomicrons and move into lymph and then into blood stream after bypassing liver *larger fats do not directly go into blood stream Large Intestine • large intestine absorbs salt and water • undigested residues, including fibre, pass through ileocecal valve (valve connecting the end of the small intestine to beginning of large intestine) • upon entering, contents pass the opening into the appendix and travel along the large intestine • when intestinal contents pass the rectum, water is absorbed by the colon • fibre retains some water by creating the pasty consistency of stool • stool carries some bile acids, additives and contaminants out of the body haustral contractions: primary method of movement in large intestine -occur every 30 minutes • because of slow colonic movement, bacteria have time to grow and accumulate • 3/4 times a day there is a mass movement which drives feces 1/3 to 3/4 the length of the colon in a few seconds • when fecal material is moved into rectum, distension of rectum stimulates defecation reflex Gastrointestinal Bacteria • 10 trillion bacteria made up of 400 or more different species • many bacterial have positive benefits - including preventing infectious bacteria from thriving Food and Bacteria prebiotics: foods that are not digested (ex. fibres) but promote bacterial growth by acting as food for bacteria probiotics: living microorganisms found in foods that when consumed in sufficient quantities are beneficial to health Lecture 2 - Digestion Specialization of GI tract • different nutrients are absorbed in different places throughout the GI tract • nutrients that are ready for absorption are absorbed early, while those requiring more digestion are absorbed later Food Combining *myth: body can not handle different foods at the same time *truth: some foods are actually handled better in combination -ex. vitamin C with iron absorption (keeps Fe++ in ferrous form) -absorption may be limited by some combinations (ex. phytates in legumes can bind to minerals and limit absorption) ...therefor eat rounded, well-balanced meals! The Vascular System • circulatory system is a closed system of ves
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