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Module 15 - Digestive System .docx

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Western University
Physiology 2130
Paul Gillespie

Digestive System Main function • Break down organic nutrients so that they can be absorbed into the body • Organic Nutrients can be: o Carbohydrates o Proteins o Fats o Water o Vitamins o Minerals Name the basic processes of the digestive system • Secretion o Release of digestive fluids into the lumen of the digestive tract o Fluid may include:  Water  Mucus  Acid  Electrolytes  Enzymes  Bile salts  Digestive enzymes • Digestion o Process whereby food is broken down into smaller molecules by the digestive enzymes so they can be taken up by the body • Absorption o Process whereby these small molecules are taken by the circulatory system and are distributed throughout the body • Motility o Movement of the “food” through the digestive system by the contraction of smooth muscle that lines the walls of the tract • Excretion o Removal of the unwanted waste products from the body after almost all of the wanted material is absorbed Draw and label a picture of the digestive system 1. In the mouth a. Food is broken up by chewing (mastication) b. Food is mixed with saliva c. Salivary glands produce saliva to moisten and begin digesting some food particles 2. Esophagus a. Straight muscular tube that connects the mouth and pharynx to the stomach Digestive System 3. Stomach a. Stores, mixes and digests some food and delivers food to the small intestine 4. Liver a. Many functions, but for digestion it produces and secretes bile 5. Gallbladder a. Stores and concentrates bile 6. Pancreas a. Secretes digestive enzymes, hormones, and bicarbonate 7. Large Intestine a. Stores and concentrates undigested material and absorbs salt and water 8. Rectum a. Site where the defecation reflex is triggered List the names of the salivary glands and describe their functions (mouth) • Digestion of your meal begins in the mouth, where it is broken down by chewing (or mastication) and mixed with saliva to form a bolus • Saliva o Water (99.5%) o Ions and proteins (0.5%) o Secreted by:  Parotid  Submandibular  Sublingual o Glands can produce up to 2 liters of saliva each day o Helps lubricate the bolus of food an begins digesting carbohydrates  Due to the enzyme amyla o Production and secretion of saliva under the control of the autonomic nervous system Describe the sequence of events in swallowing • Once the bolus of the food has been formed and sufficiently lubricated with the saliva, it will be pushed to the back of the mouth by the tongue • Then the swallowing reflex will be initiated • Uvula of the soft palate closes over the nasopharynx • Larynx is lifted by muscles in the neck, and the epiglottis bends over the glottis, covering the larynx • Bolus moves the esophagus through the cardiac orifice (also called the lower esophageal sphincter) and into the stomach by a wave of smooth muscle contraction called peristalsis Digestive System Draw and label a diagram of the stomach and list the function and secretion from each part • Stomach is divided into three general areas: o Upper dome-shaped: fundus o Middle body:  Largest part of the stomach o Lower antrum (pyloric region) • Pyloric sphincter, located at the distal portion, regulates the emptying of the stomach into the first part of the small intestine, the duodenum • Stomach (when empty) o Thrown into folds, or rugae, which increase the surface area and allow for expansion of the stomach as it fills with food • Function o Stomach liquefies, mixes, and stores each bolus of food from the meal o Chime  Mixture that is slowly released into the small intestine  Mixing of the chime is achieved by peristaltic contraction of the stomach walls that begin at the fundus and end at the antrum  These contractions also help move the chime through the pyloric sphincter into the small intestine  Motility • Muscle activity that causes the movement of substances through the digestive tract o Regulates the amount of food entering the small intestine so that it can be fully digested then absorbed o Some digestion does take place in the stomach  Proteins begin to be digested in the stomach by the enzyme pepsin o Very little absorption takes place  Only certain substances like alcohol and aspirin can cross the lining of the stomach to be absorbed into the bloodstream • Stomach secretes a variety of substances o Mucus  Secreted by cells in the body  Antrum of the stomach  Protects lining of the stomach from the extremely acid environment that is produced by HCl o HydrochloricAcid (HCl)  Secreted by cells in the stomach body  Begins to denature (unfold) the complex protein and connective tissue molecules found in the meat of the hamburger Digestive System  Kills bacteria and converts the inactive enzyme pepsinogen to its active form called pepsin o Pepsinogen  Produced by cells in the body of the stomach  When converted to its active form pepsin by HCl, it begins to digest large protein molecules o Gastrin  Secreted by cells in the atrum of the stomach  Stimulates secretion of HCl o Intrinsic factor  Helps with the absorption of vitamin 12 Describe the function of the Pancreas • Lies just below the stomach • Has both digestive (or exocrine) and endocrine functions • Exocrine products of the pancreas are secreted into a long pancreatic duct o Duct merges with the common bile duct (from the liver and gallbladder) just before entering the duodenum • Produces and secretes the o Carbohydrate-digesting enzyme amylase o The protein-digesting enzyme trypsin, chymotrypsin and proteases o Fat-digesting enzyme lipase o Sodium bicarbonate into the duodenum to neutralize the acid from the stomach Draw and label a diagram of the small intestine and the intestinal villi • The stomach contents empty through the pyloric sphincter into the small intestine • Longest section of the digestive tract (9 meters) • Ends at the ileocecal sphincter where it empties into the ascending colon of the large intestine • Three segments: o Duodenum o Jejunum o Ileum • Large surface area o Inner wall of the small intestine is thrown into folds o Folds contain fingerlike projections called Villi (singular form=villus) • Villi contain a capillary network and lymphatic lacteal to absorb the digested material • Alayer of epithelial cells, which secrete digestive enzymes, covers the villi o These cells have microvilli that fact out into the lumen of the intestine, forming a brush border Digestive System • Absorption and digestion takes place in the first two segments of the small intestine (duodenum and jejunum) • Digestion of chime occurs by digestive enzymes from the pancreas and epithelial cells • Bile, from the liver, helps digest fat although it is not a digestive enzyme • Small intestine is also the site where most of the digested material gets absorbed into the circulatory and lymphatic systems • Note: specific digestive enzymes for each part of the meal • The process of absorption is greatly enhanced by: o Large surface area produced by the villi lining the walls of the intestine o Large blood supply to the digestive tract o Motility or movement of the food into the intestine List the building blocks for carbohydrates • Carbohydrates o Made of a single building block called monosaccharides  Example: Glucose, Fructose, etc. o Two monosaccharides are called disaccharides  Example: Maltose, lactose and sucrose o Polysaccharides  More than two monosaccharides  Example: Starch and Glycogen Describe the sequence of events in carbohydrate digestion and absorption • In order to absorb the larger carbohydrates, polysaccharides must all be broken down into monosaccharides • Monosaccharides can be absorbed by transport systems in the walls of the small intestine • Digestion of carbohydrates begins in the mouth by the salivary enzyme amylase o Amylase breaks up the large polysaccharides into smaller polysaccharides and maltose • In the stomach o Digestion stops due to the acidic environment which denatures amylase • Small Intestine o Begins again because the pancreas secretes amylase into the duodenum o Pancreas also secretes bicarbonate ions which neutralize the acid from the stomach o The neutralized environment allows for amylase to work o Pancreatic amylase digests the polysaccharide starch to the disaccharide maltose  It is not absorbed because it is not a monosaccharide o Each disaccharide requires a specific enzyme to break it down Digestive System o Enzymes are located in the brush border of the microvilli located on the intestinal epithelial cells  Enzyme lactase: lactose  galactose and glucose  Enzyme maltase: maltose (starch)  two glucose  Enzyme sucrose: sucrose (table sugar)  glucose and fructose • Lactose Intolerance o Can’t eat food containing milk o Inability to produce enzyme lactase o Can’t digest lactose o If not absorbed into the body, it will stay in the digestive tract  diarrhea o If present in the large intestine, bacteria will feed on it and produce gas, causing bloating and cramps o Synthetic lactase is available though for these people • CarbohydrateAbsorption  Identical to glucose reabsorption in the kidneys o Intestinal epithelial cells contain Na /K pumps on their basal side that + establish a [] gradient for +a (high on outside and low on inside) o This gradient powers the Na /glucose co-transporter  Atype of secondary active transport mechanism  Located on the luminal side of the cell + o This transporter moves glucose into the cell as Na move in, down their [] gradient o Once glucose is inside the cell it will then diffuse out through the basal side by facilitated diffusion List the building blocks for proteins • Long chains of amino acids linked together • 20 amino acids o 11 nonessential that can be produced in body o 9 essential that must come from diet • Different groups of amino acids require different enzymes to break them apart • Proteins must be broken down into the amino acid building blocks o To be absorbed by transport systems in small intestine Describe the sequence of events in protein digestion and absorption Digestive System • Digestion of proteins begins in the stomach • HCl converts the inactive pepsinogen to the active enzyme pepsin • HCl also helps uncoil the long, twisted strands of proteins • This unfolding of the protein gives the pepsin access to the long protein chains in order to digest them into smaller chains called polypeptides • These polypeptides then pass through the pyloric sphincter into the small intestine where they continue to be digested and absorbed into the body • Pancreatic Enzymes o Continue protein digestion o Must have environment with a neutral pH  Chyme from the acidic stomach must be neutralized  Achieved by bicarbonate • Secreted from the pancreas  Neutral pH environment • Pepsin which was secreted by the stomach to begin the process of digestion is inactivated • Enzymes trypsin and chymotrypsin from the pancreas continue to the job in the small intestine that was begun by pepsin in the stomach • Proteases o Class of enzymes o Continue digesting proteins into amino acids o Enzymes are produced in the pancreas and secreted into the small intestine o Some found along the brush border of the intestinal epithelial cells o Two different classes:  Each responsible for breaking apart amino acids located in different parts of the protein  Endopeptidases • Break the bonds between amino acids in the inner part of the protein  Exopeptidases • Break the bonds between amino acids at of the protein • AminoAcids Present with few small peptides o Absorption takes place o Secondary active transport +  Requiring the presence of a Na [] gradient  They power a co-transporter that also moves the amino acids into the cell  The remaining small peptides are absorbed by endocytosis List the building blocks for lipids • Fats (also called lipids) o Triglycerides Digestive System o Phospholipids o Cholesterol o Long-chain fatty acids • Not water soluble • Complex set of steps necessary to digest fats and then absorb them into the “watery environment” Describe the sequence of events in lipid digestion and absorption • Emulsification o Churning action of the stomach o Helps break up the large drops of fat into smaller ones o Important  Lipid-digesting enzymes work better with small droplets than very large ones o Is continuous throughout the whole digestion process  Avoid small droplets forming into big droplets Describe the site where bile is produced and list its function • Bile o Produced in liver o Transported to the gallbladder  Stored  Concentrated o Not a digestive enzyme o Contains  Water  Bile salts • Keeps the lipid droplets emulsified  Cholesterol  FattyAcids  Many ions o Gallbladder releases bile into the duodenum of the small intestine during a meal Describe the process of Fat Digestion and Absorption • Fat Digestion o Colipase  Secreted by the pancreas  Not a digestive enzyme  Allows lipase to get access to the lipid interior of the fat droplet since its coated in bile salts and the enzyme cannot reach and digest the lipids  With colipase, pancre
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