Digestive System PDF

Digestive System Digestion: The process by which large complex nutrient molecules are broken down into simpler molecules capable of being used by the organism for food. Basic Organization n Mouth n Esophagus n Stomach n Small intestine n Large intestine n Anus Associated Structures n...

Digestive System Digestion: The process by which large complex nutrient molecules are broken down into simpler molecules capable of being used by the organism for food. Basic Organization n Mouth n Esophagus n Stomach n Small intestine n Large intestine n Anus Associated Structures n Pancreas Contribute to small n Liver intestinal digestion n Gallbladder n Salivary glands Structures in Mouth n Lips n Teeth n Tongue n Salivary glands  Monogastric Teeth n Function: n Mechanically reduce particle size n Increase surface area Four types: n Incisors are used for cutting n Canine (fangs, eye teeth, tusks) are tearing teeth n Premolars and molars (cheek teeth) grind the food Monogastric Tongue Function: n Comprised of three muscles n Maneuvers food in the mouth n Moves feed to teeth for grinding and to the back of the mouth for swallowing n Can distinguish between feed and toxins by papillae or taste buds  Monogastric Salivary Glands Types of Glands: Zygomatic Parotid Sublingual Mandibular  Salivary Glands Gland Type of secretion Main constituents Parotid Serous Water, enzymes, ions Submaxillary Mucous or mixed Mucin (mucous), mucin plus enzymes (mixed), water Sublingual Mucous or mixed Mucin (mucous), mucin plus enzymes (mixed), water Functions of Saliva n Moisten feed (salt and water) n Lubrication (aids swallowing) n Starch and(or) lipid digestion (amylase and(or) lipase)  Monogastric Esophagus n Transport of food from mouth to stomach n Uses peristaltic contractions (wave contractions) n Horse/Pig: n Striated muscles for first 2/3 n Smooth muscles for last 1/3 n In horse, esophagus joins stomach at an oblique angle and cardiac sphincter (the valve between the stomach and esophagus) only allows one-way flow n MOST horses cannot belch out gas or vomit n Dog: n Striated muscles throughout allow GREAT control of digesta movement both directions Deglutition (Swallowing) n Reflex initiated by presence of food in pharnyx n Propulsion of food to stomach by esophageal peristalsis Gastric Digestion n Functions n Reservoir for controlled release of digesta to small intestine n Horse has small capacity – requires increased number of smaller sized meals n Mixing food n Mechanical breakdown of feed n Hydrolytic digestion by acid and enzymes n Mainly protein n Kill bacteria n Secrete intrinsic factor: needed for vitamin B12 absorption n Hormone production  Stomach Regions n Esophageal n Non-glandular n Cardiac n Secretes mucus n Fundic n Parietal cells n Chief cells n Pyloric n Mucus Gastric Pits n Formed by numerous folds in the epithelium n Glands empty into the gastric pit n Many types of glands may empty into one gastric pit  Gastric Glands Gland Type of Main constituents secretion Cardia Mucous Mucin Pylorus or Antrum Mucous Mucin Fundus acid Chief cells Enzyme Pepsinogen Pepsin Parietal cells Acid HCl, intrinsic factor  Stomach Secretions n HCl n Pepsinogen n Decreases pH (~2-3) n Activated form is n Denatures protein pepsin n Kills bacteria n Hydrolyzes protein n Activates pepsinogen n Rennin (abomasum) n Mucus n Clots milk § Protects lining from acid and enzymes n Lipase § No “autodigestion” n Some species § Lubricant Gastric Motility and Emptying n Motility aids mixing, mechanical and hydrolytic reduction of feed to chyme n acid pulp n Emptying is stimulated by distension of antral wall and presence of liquid chyme Gastrointestinal Hormones n Gastrin n Origin: Stomach, Abomasum n Stimulus: Food in stomach n Function: Stimulates HCl & pepsinogen secretion, increases stomach motility n Secretin n Origin: Duodenum n Stimulus: Acid n Function: Stimulates pancreatic secretions. Slows stomach motility and acid production Gastrointestinal Hormones n Cholecystokinin (CCK) n Origin: Duodenum n Stimulus: Fat & protein in duodenum n Function: Stimulates bile and pancreatic secretions n Also regulates appetite and feed intake n Gastric Inhibitory Protein (GIP) n Origin: Duodenum n Stimulus: Fats and bile n Function: Inhibit stomach motility and secretion of acid and enzymes Small Intestine n Composed of 3 segments (proximal to distal) n Duodenum n Releases bile and pancreatic secretions n Active site of digestion n Jejunum n Active site of nutrient absorption n Ileum n Active site of nutrient absorption n Most water, vitamins & minerals n Some bacterial presence n Fermentation «The pH of the small intestine increases towards 7.0 as food moves from the duodenum to the ileum Nutrient Absorption in the Small Intestine n Principal site of absorption of amino acids, vitamins, minerals and lipids n Glucose and other sugars in monogastrics n Generally, most absorption occurs in the proximal (upper) part of the small intestine but some absorption occurs in all segments n Duodenum, jejunum and ileum n Digestion and absorption within SI is rapid n Within 30 minutes of entering SI Nutrient Absorption n Variety of mechanisms n Diffusion n Facilitated diffusion n Active transport n Pinocytosis or endocytosis n Dependent upon n Solubility of the nutrient (fat vs. water) n Concentration or electrical gradient n Size of the molecule to be absorbed  Diffusion n Water and small lipid molecules pass freely through membrane n Move down concentration gradient to equalize concentrations Facilitated Diffusion 1) Carrier loads particle on outside of cell 2) Carrier releases particle on inside of cell 3) Reverse Allows equalization of concentrations across membrane Active Transport 1) Carrier loads particle on outside of cell 2) Carrier releases particle on inside of cell 3) Carrier returns to outside to pick up another particle Active Transport n Unidirectional movement n Transports nutrients against concentration gradient  Pinocytosis or Endocytosis n Substance contacts cell membrane n Membrane wraps around or engulfs substance into sac n Sac formed separates from the membrane and moves into cell Secretions Entering SI n Intestinal mucus Secreted from n Brush border enzymes within SI n Pancreatic juices Produced & stored in pancreas n Enters n Bile from n Produced in liver ducts into n Stored in gallbladder SI n Horse has no gallbladder n Direct bile secretion into duodenum n Cannot store bile—continuous intake of food Intestinal Mucus n Secreted by glands in wall of duodenum n Brunner’s glands n Acts as lubricant and buffer to protect duodenal wall Primary Enzymes for Carbohydrates Nutrient Enzyme Origin Product Starch, glycogen, Amylase Saliva & Maltose & dextrin pancreas Glucose Maltose Maltase SI Glucose Lactose Lactase SI Glucose & galactose Sucrose Sucrase SI Glucose & fructose  Primary Enzymes for Proteins Nutrient Enzyme Origin Product Milk protein Rennin Gastric mucosa Curd Proteins Pepsin Gastric mucosa Polypeptide Polypeptides Trypsin Pancreas Peptides Chymotrypsin Pancreas Peptides Peptides Carboxypeptidase Pancreas Peptides & Aminopeptidase Small intestine amino acids  Primary Enzymes for Lipids Nutrient Enzyme Origin Product Lipids Lipase & colipase Pancreas Monoglycerides & free fatty acids Bile n Green, viscous liquid n Alkaline ph (neutralize acidic chyme) n Secreted by liver via bile duct to duodenum n Stored in gall bladder (except in horses) n Functions to emulsify fats n Composition n Bile salts (glycocholic and taurocholic acids) n Bile pigments (bilirubin and biliverdin) n Cholesterol n 95% reabsorbed and returned to liver n NOT AN ENZYME Nutrient Digestion - Lipids Large Lipid Droplet Action of bile salts Lipid emulsion Small Bile salts & pancreatic lipase and colipase Water soluble micelles  Pancreatic Juice n Clear, watery juice n Contains n Enters duodenum n HCO3- via pancreatic duct n Trypsinogen Chymotrypsinogen Pro- Aids in fat, starch, n n enzymes Procarboxypeptidase and protein n Amylase digestion n n Lipase n Nuclease Importance of Pancreas for Digestion n Produces enzymes responsible for n 50% of carbohydrate digestion n 50% of protein digestion n 90% of lipid digestion n Produces sodium bicarbonate for neutralization of chyme in duodenum  Activation of Pancreatic Enzymes n Enterokinase n Secreted from crypts in duodenum n Trypsinogen trypsin n Trypsin then converts: n Trypsinogen trypsin n Chymotrypsinogen chymotrypsin n Procarboxypeptidase carboxypeptidase Overview of Digestive Enzymes n Stomach n Pepsinogen n Brush Border (SI) n Chymosin (rennin) n Sucrase n Pancreas n Maltase n Trypsinogen n Lactase n Chymotrypsinogen n Aminopeptidase n Procarboxypeptidase n Dipeptidase n Amylase n Enterokinase n Lipase n Nuclease Large Intestine n Composed of three segments n Cecum n Colon n Rectum n Function n Fermentative digestion n No enzyme secretion n Relies on microbes or secretions washed out of the SI n Absorption of remaining water, volatile fatty acids (VFAs) from microbial fermentation and minerals n Digesta storage Degree of development is species dependent Monogastric Cecum n Located at junction of small and large intestine n Function similar to rumen in ruminants n Microbial activity and digestion of feeds n Contains a microbial population similar to the rumen n Cellulolytic & hemicelluloytic bacteria n Since cecum is located AFTER major site of nutrient absorption (small intestine), then microbial cell proteins are not available to the animal n Fecal loss Monogastric Large Intestine n Function: n Absorption of liquid n Mass movements move fecal matter to anus n Usually only a few times a day n Associated with defecation Bacteria n Cellulolytic – digest cellulose (forages) n Amylolytic – digest starches and sugars (concentrates or grains) n Other types: n Proteolytic n Clostridium n Organic acid utilizers n Methanogens n Produce CO2, H2, formate, CH4 Rectum n Muscular area of large intestine used for storage of feces and ultimately for defecation n Feces includes sloughed cells, undigested food and microbial matter Avians (Poultry) Mouth n No teeth, rigid tongue n Poorly developed salivary glands n Saliva contains amylase n Beak is adapted for prehension and mastication Avians (Poultry) Esophagus n Enlarged area called crop n Ingesta holding and moistening n Location for breakdown of carbohydrate by amylase n Fermentation Proventriculus (stomach) n Release of HCl and pepsin (gastric juices) n Ingesta passes through very quickly (14 seconds) Avians (Poultry) Gizzard (ventriculus) n Muscular area with a hardened lining reduces particle size n Muscular contractions every 20-30 seconds n Includes action of grit n HCl and pepsin secreted in proventriculus Small intestine n Similar to other monogastrics n No Lacteals Avians (Poultry) Ceca and large intestine n Contain two ceca instead of one as in other monogastrics n Large intestine is very short (2-4 in) and empties into cloaca where fecal material will be voided via the vent n Water resorption n Fiber fermentation by bacteria n H2O soluble vitamin synthesis by bacteria Ruminant Digestive System Modified to handle the breakdown of large amounts of fiber Ruminant Digestive System n Mouth n no upper incisors, hard palate n molars for grinding coarse vegetation n saliva does not contain enzymes n Esophagus n muscular tube connecting the mouth to the stomach Ruminant Digestive Systems The Four Compartmented Stomach n Rumen: storage area and fermentation vat n Reticulum: nails and wire may be found here-hardware stomach n Omasum: eliminates excess water from feed n Abomasum: true stomach, gastric juices and enzymes are secreted Ruminant digestion Ruminant An animal with a polygastric digestive system. Rumination:Part of the process of digestion in a polygastric system where a bolus is regurgitated, re-masticated, re ensilavated and re-swallowed. Ruminant Digestive Systems Stomach (cont.) n Regurgitation: first step in rumination n large quantities of roughage are consumed and are chewed just enough to swallow n after swallowing, regurgitation (“cud chewing”) takes place, food is re-chewed Ruminant Digestive System n Small Intestine n connects stomach to large intestine n food nutrients absorbed into blood n contains bile and pancreatic juices n pushes food through by muscle contractions Ruminant Digestive System n Large Intestine n Contains Cecum, Colon and Rectum n Cecum: sac at junction of small intestine and large intestine n Colon and rectum: at end of system n not as long as small intestine, but larger in diameter n water and some nutrient absorption occurs here n where residue solidifies before excretion Review Monogastric A digestive system with one stomach compartment Polygastric A digestive system capable of breaking down cellulose into digestible protein and carbohydrates. The stomach is divided into compartments each with a specialized function. 56 Ruminant Stomach Abomasum Site of chemical digestion in a polygastric digestive system. Reticulum “hardware stomach” holding area for foreign materials in a polygastric system. Rumen Largest compartment of the polygastric system, serves as a storage area. Omasum Muscular portion of the polygastric system that squeezes excess water from digested food products. 57 Equine Digestive Systems Characterized by non-ruminant animals that consume and digest feeds high in fiber Equine Digestive Systems n Mouth n intact top and bottom incisors n molars adapted to chewing fibrous feeds n no digestive enzymes in saliva n Esophagus n not well adapted for regurgitation n connects mouth and stomach Equine Digestive System n Stomach n similar to monogastric system n Small intestine n similar to monogastric and ruminant systems n no gall bladder to store bile n enlarged cecum to aid in fiber breakdown Equine Digestive System n Large Intestine n similar to monogastric systems n cecum (at junction of small and lare intestines) and colon take up most of the volume of the equine digestive system