Types of Digestive Systems The Digestive System Single-celled organisms as well as sponges Chapter 48 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 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Types of Digestive Systems Food Wastes Specialization occurs when the digestive tract Mouth has a separate mouth and anus Tentacle -Nematodes have the most primitive Body stalk digestive tract Gastrovascular -A tubular gut lined by an epithelial cavity membrane -More complex animals have a digestive tract specialized in different regions 3 4 Types of Digestive Systems Types of Digestive Systems Ingested food may be stored or first subjected to Physical digestion -Chemical digestion occurs next, involving hydrolysis reactions that liberate the subunit molecules from food -Products pass through gut’s epithelial lining into the blood (absorption) -Wastes are excreted from the anus 5 6 1 Vertebrate Digestive Systems The digestive system consists of a tubular gastrointestinal tract and accessory organs -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 7 8 Vertebrate Digestive Systems Vertebrate Digestive Systems Accessory organs The gastrointestinal tract has four layers -Liver -Mucosa = Epithelium that lines the interior, -Produces bile or lumen, of the tract -Gallbladder -Submucosa = Connective tissue -Stores and concentrates bile -Muscularis = Double layer of smooth -Pancreas muscles -Produces pancreatic juice and -Serosa = Epithelium that covers the external surface of the tract bicarbonate buffer 9 10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gland outside gastrointestinal tract Blood vessel Mouth and Teeth Lumen Many vertebrates have teeth used for Mucosa chewing or mastication (PD) Submucosa Birds, which lack teeth, break up food in their Gland in submucosa two-chambered stomach Submucosal plexus -Gizzard = Muscular chamber that uses Muscularis Circular layer ingested pebbles to pulverize food (PD) Longitudinal layer Serosa Nerve Epithelial tissue layer Myenteric plexus 11 12 2 Mouth and Teeth For food storage Vertebrate teeth are adapted to their nutritional source -Carnivorous mammals have pointed teeth that lack flat grinding surfaces -Herbivores have large flat teeth suited for grinding cellulose cell walls of plant tissues -Humans have carnivore-like teeth in the front and herbivore-like teeth in the back 13 14 Mouth and Teeth Mouth and Teeth Inside the mouth, the tongue mixes food with Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Herbivore Carnivore Omnivore saliva -Moistens and lubricates the food -Also contains salivary amylase, which initiates the breakdown of starch (CD) Horse Lion Human Salivation is controlled by the nervous system Incisors Premolars -Tasting, smelling, and even thinking or Canines Molars talking about food stimulate salivation 15 16 Mouth and Teeth Mouth and Teeth Swallowing is initiated by voluntary action, The image cannot be displayed. Your computer may not have enough memory to open then is continued under involuntary control the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then -When food is ready to be swallowed, the insert it again. tongue moves it to the back of the mouth -Soft palate seals off nasal cavity -Elevation of the larynx (voice box) pushes the glottis against the epiglottis -Keeps food out of respiratory tract 17 18 3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peristalic The Esophagus movement The esophagus is a muscular tube Esophagus connecting the esophagus to the stomach Relaxation -It actively moves a processed lump of food, called a bolus, through muscular action Contraction -Swallowing center in brain stimulates Food Bolus successive waves of contraction -Peristalsis moves the food down to Relaxation the stomach 19 20 The Stomach The Stomach The stomach is the saclike portion of tract The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then -Has convoluted surface, allowing expansion insert it again. -Contains an extra layer of smooth muscles for mixing food with gastric juice -Has two kinds of secretory cells -Parietal cells – Secrete HCl and intrinsic factor (for vita. B12 absorption) -Chief cells – Secrete pepsinogen, the inactive form of pepsin 21 22 The Stomach The Small Intestine The low pH in the stomach helps denature The small intestine is about 4.5 m long food proteins -Consists of duodenum, jejunum and ileum -No significant digestion of carbohydrates or Epithelial wall is covered with villi, which in fats occurs turn are covered by microvilli The mixture of partially digested food and -Greatly increase surface area gastric juice is called chyme Microvilli also participate in digestion -Leaves the stomach through the pyloric -Many adults lack the enzyme lactase sphincter to enter the small intestine -Have lactose intolerance 23 24 4 The Small Intestine Accessory Organs Pancreas -Pancreatic fluid is secreted into the duodenum through the pancreatic duct -Host of enzymes: trypsin, chymotrypsin, pancreatic amylase, and lipase -Digest proteins into smaller polypeptides, polysaccharides into shorter sugars, and fats into free fatty acids & monoglycerides 25 26 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pancreatic islet Accessory Organs (of Langerhans) β cell Liver From liver α cell -The liver is the body’s largest internal organ -It secretes bile into the duodenum during Common Pancreas digestion of a meal bile duct -Consists of bile pigments (waste products) and bile salts (for emulsification of fats) Gallbladder Gallbladder Pancreatic duct -Stores and concentrates bile Duodenum 27 28 Absorption Absorption Amino acids and monosaccharides are transported through epithelial cells to blood -Blood carries these products to the liver via the hepatic portal vein Fatty acids and monoglycerides diffuse into epithelial cells -They are reassembled into chylomicrons -Enter the lymphatic system and later join the circulatory system 29 30 5 Absorption The Large Intestine About 9 liters of fluid pass through the small The large intestine, or colon, is much shorter intestine per day than small intestine, but has larger diameter -Only about 50 g of solid and 100 mL of Small intestine empties directly into the large liquid leave the body as feces intestine at a junction where two vestigial -The normal fluid absorption efficiency structures, cecum and appendix, remain of the human digestive tract is 99%! No digestion occurs Only 4% absorption -Water, remaining electrolytes & vitamin K 31 32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Large Intestine Main function is waste material concentration -Compacted feces are stored in the rectum, Ascending portion of large intestine until it can be eliminated through the anus Ileocecal valve Most mammals have a rectum Last portion of -Most vertebrates have a common cavity, small intestine the cloaca, where the urinary, reproductive, Cecum Appendix and gastrointestinal tracts join 33 34 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 four-chambered stomach -Rumen, reticulum, omasum, abomasum -Rumen has cellulose-degrading microbes -Contents can be regurgitated and rechewed -Rumination 35 36 6 Variations in Digestive Systems Animals, such as horses, deer and rabbits, digest cellulose in the cecum -Regurgitation of contents is not possible -However, such animals practice coprophagy -Eat their feces to absorb nutrients on the second passage of food 37 38 Variations in Digestive Systems Accessory Organ Function Some animals digest waxes with the help of Liver microorganisms -Chemically modifies the substances absorbed from the digestive tract before All mammals rely on intestinal bacteria to they reach the rest of the body synthesize vitamin K, which is required for -Removes toxins, pesticides, & carcinogens, blood clotting converting them to less toxic forms -Birds, which lack these bacteria, must -Regulates levels of steroid hormones consume the required quantities of vitamin -Produces most proteins found in plasma K in their diet 39 40 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Accessory Organ Function Metabolism Eating carbohydrate- Fasting or Regulation of blood glucose rich meal exercise -After a meal, increased secretion of insulin Increasing blood glucose Decreasing blood glucose promotes deposition of glycogen and fat -During fasting or exercising, an increased Pancreatic Islets Pancreatic Islets secretion of glucagon promotes the Insulin secretion Insulin secretion breakdown of glycogen (glycogenolysis) Glucagon secretion Glucagon