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The Digestive System Chapter 48 Mechanical Breakdown of Food Heterotrophs are divided into three groups based on their food sources -Herbivores are animals that eat plants exclusively -Carnivores are animals that eat other animals -Omnivores are animals that eat both plants and other animals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Herbivore Carnivore Omnivore Horse Lion Human Incisors Canines Premolars Molars 2 “Simple” Digestive Strategies Food Hydra (a Cnidarian) Mouth Wastes Tentacle Body stalk Gastrovascular cavity -Single-celled organisms as well as sponges digest their food intracellularly -Other multicellular animals digest their food extracellularly, within a digestive cavity -Cnidarians and flatworms, have a gastrovascular cavity...has only one opening, and no specialized regions 3 Specialization in Digestive Systems -Specialization occurs when the digestive tract has a separate mouth and anus -Nematodes have the most primitive digestive tract ...tubular gut lined by an epithelial membrane -More complex animals have a digestive tract specialized in different regions 4 Specialization in Digestive Systems *Mouth and pharynx = Entry *Esophagus = Delivers food to stomach *Stomach = Preliminary digestion *Small intestine = Absorption *Large intestine = Concentration of wastes *Cloaca or rectum = Waste storage *Liver=Produces bile *Gallbladder=Stores and concentrates bile *Pancreas=produces pancreatic juice and bicarbonate buffer 5 Copyright © The McGraw-Hill Companies, Inc. 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Four GENERAL Layers of Gastrointestinal Tract Gland outside gastrointestinal tract Blood vessel Lumen Mucosa Epithelium that lines the lumen Submucosa Connective tissue Gland in submucosa Submucosal plexus Muscularis Circular layer Smooth muscles Longitudinal layer Serosa Epithelial tissue layer Covers the external surface of the tract Nerve Myenteric plexus 6 Initial Digestion & Swallowing -In the mouth, saliva contains salivary amylase, which initiates the breakdown of starch -Tongue moves food to the back of the mouth -Soft palate seals off nasal cavity -Elevation of the larynx (voice box) pushes the glottis against the epiglottis 7 -Keeps food out of respiratory tract Swallowing *The esophagus is a muscular tube to the stomach *It actively moves a processed lump of food, called a bolus, through muscular action *Swallowing center in brain stimulates successive waves of contraction *Peristalsis moves the food down to the stomach Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peristalic movement Esophagus Relaxation Contraction Food Bolus Relaxation 8 The Stomach *The stomach has convoluted surface for expansion *Contains an extra layer of smooth muscles for mixing food with gastric juice...gastric juice + food = chyme *Has two kinds of secretory cells -Parietal cells – Secrete HCl lowers pH for protein digestion. Also secretes intrinsic factor (for vita. B12 absorption) -Chief cells – Secrete pepsinogen, the inactive form of pepsin *In Stomach, No significant digestion of carbohydrates or fats occurs Chyme exits through the pyloric sphincter 9 The Small Intestine *The small intestine is primary site for carbs and fat digestion, and absorption into the blood stream -Consists of duodenum, jejunum and ileum *Epithelial wall is covered with villi, which in turn are covered by microvilli...greatly increase surface area *Microvilli also participate in digestion -Many adults lack the enzyme lactase...are lactose intolerant 10 The Pancreas & the Duodenum -Langerhans islet cells secrete pancreatic fluid into the duodenu through the pancreatic duct -Host of enzymes: trypsin, chymotrypsin, pancreatic amylase, and lipase Digest proteins into smaller polypeptides, polysaccharides Pancreatic islet into shorter sugars, and fats (of Langerhans) into free fatty acids & cell monoglycerides From liver cell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Common bile duct Pancreas Gallbladder Pancreatic duct Duodenum 11 The Liver & The Duodenum -The liver is the body’s largest internal organ -It secretes bile into the duodenum during digestion of a meal...bile consists of bile pigments (waste products) and bile salts (for emulsification of fats) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreatic islet (of Langerhans) From liver cell cell Common bile duct Pancreas Gallbladder Pancreatic duct Duodenum -Gallbladder Stores and concentrates bile 12 Absorption *Blood carries amino acids and monosaccharides to the liver via the hepatic portal vein *Fatty acids and monoglycerides, diffuse into epithelial cells, they are assembled into triglycerides & then chylomicrons, and then, enter the lymphatic system and then, they join the circulatory system 13 Remember the Hepatic portal vein leads into the Liver...and then The Liver: -Chemically modifies the substances absorbed from the digestive tract before they reach the rest of the body -Removes toxins, pesticides, & carcinogens, converting them to less toxic forms -Regulates levels of steroid hormones -Produces most proteins found in plasma 14 Water Absorption in Small Intestine About 9 liters of fluid pass through the small intestine per day -Only about 50 g of solid and 100 mL of liquid leave the body as feces -The normal fluid absorption efficiency of the human digestive tract is 99% (via osmosis) 15 The Large Intestine Main function is waste material concentration -Compacted feces are stored in the rectum, until it can be eliminated through the anus Most mammals have a rectum -Most vertebrates have a common cavity, the cloaca, where the urinary, reproductive, and gastrointestinal tracts join 16 Large intestine, or colon, is much shorter than small intestine, but has larger diameter Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Small intestine empties directly into the large intestine at a junction were two vestigial structures, cecum & appendix, remain Ascending portion of large intestine No digestion occurs Only 4% absorption Ileocecal valve ...water, remaining Last portion of electrolytes & vitamin K... small intestine vitamin K is made by intestinal Cecum bacteria in mammals and is Appendix involved in blood clotting 17 Variations in Digestive Systems The digestive tracts of some animals contain bacteria and protists that convert cellulose into substances the host can absorb Ruminants have a 4-chambered stomach -Rumen, reticulum, omasum, abomasum -Rumen has cellulose-degrading microbes -Contents can be regurgitated from the Rumen & rechewed...called rumination 18 Animals, such as horses, deer and rabbits, digest cellulose in the cecum... such animals practice coprophagy... eat their feces to absorb nutrients on the second passage of food... Food is past the stomach so not regurgitated Protein diets are more easily digested so have shorter tract and few specialized pouches. 19 Digestive Tract & Hormones Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach Liver pH Proteins Gastrin (+) (+) ( – ) Chief cells Parietal cells GIP Pepsin (+) HCl Bile Pancreas Enzymes Bicarbonate Gallbladder (+) Acinar cells (+) CCK Secretin Duodenum 20 Regulation of blood glucose Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolism Eating carbohydraterich meal Fasting or exercise Increasing blood glucose Decreasing blood glucose Pancreatic Islets Pancreatic Islets Insulin secretion Insulin secretion Glucagon secretion Glucagon secretion Formation of glycogen (in liver) and fat (in adipose tissue) Breakdown of glycogen (in liver) and fat (in adipose 21 tissue) glycogenolysis Hormones involved in the control of feeding and energy 22 Essential Nutrients Essential nutrients...exist because animals have lost the ability to manufacture some nutrients but they are still necessary for health and so must be obtained in the diet -These include some of the: -Vitamins -Amino acids -Long-chain unsaturated fatty acids -Minerals 23