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CHAPTER 48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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1 CHAPTER 48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Types of Digestive Systems
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

3 Types of Digestive Systems
Single-celled organisms and sponges digest their food intracellularly Other multicellular animals digest their food extracellularly Within a digestive cavity Cnidarians and flatworms have a gastrovascular cavity Only one opening, and no specialized regions

4 Types of 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

5 Types of Digestive Systems

6 Types of Digestive Systems
Ingested food may be stored or first subjected to physical fragmentation Chemical digestion occurs next Hydrolysis reactions liberate the subunit molecules Products pass through gut’s epithelial lining into the blood (absorption) Wastes are excreted from the anus

7 Vertebrate Digestive Systems
Consists of a tubular gastrointestinal tract and accessory organs Mouth and pharynx – entry Esophagus – delivers food to stomach Stomach – preliminary digestion Small intestine – digestion and absorption Large intestine – absorption of water and minerals Cloaca or rectum – expel waste


9 Vertebrate Digestive Systems
Accessory organs Liver Produces bile Gallbladder Stores and concentrates bile Pancreas Produces pancreatic juice Digestive enzymes and bicarbonate buffer

10 Vertebrate Digestive Systems
Gastrointestinal tract is layered Mucosa – innermost Epithelium that lines the interior, or lumen, of the tract Submucosa Connective tissue Muscularis Circular and longitudinal smooth muscle layers Serosa – outermost Epithelium covering external surface of tract


12 Mouth and Teeth Many vertebrates have teeth used for chewing or mastication Birds Lack teeth Break up food in a two-chambered stomach Gizzard – muscular chamber that uses ingested pebbles to pulverize food

13 Carnivores – pointed teeth that lack flat grinding surfaces
Herbivores – 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

14 Mouth and Teeth Inside the mouth, the tongue mixes food with saliva
Moistens and lubricates the food Contains salivary amylase, which initiates the breakdown of starch Salivation is controlled by the nervous system Tasting, smelling, and even thinking or talking about food stimulate increased salivation

15 Mouth and Teeth Swallowing Starts as voluntary action
Continued under involuntary control When food is ready to be swallowed, the 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

16 Mouth and Teeth

17 The Esophagus Muscular tube connecting the esophagus to the stomach
Actively moves a bolus through peristalsis Swallowing center in brain stimulates successive one-directional waves of contraction Sphincter opens to allow food to enter stomach Humans lack a true sphincter here


19 The Stomach Saclike portion of tract
Convoluted surface allows expansion Contains 3rd layer of smooth muscles for mixing food with gastric juice 3 kinds of secretory cells Mucus-secreting cells Parietal cells Secrete HCl and intrinsic factor (for vitamin B12 absorption) Chief cells Secrete pepsinogen (inactive form of pepsin)

20 The Stomach

21 The Stomach (Cont.)

22 The Stomach Low pH in the stomach helps denature food proteins
Activates pepsin and keeps it functioning No significant digestion of carbohydrates or fats occurs Absorption of some water (aspirin and alcohol) Mixture of partially digested food and gastric juice is called chyme Peptic ulcer – commonly caused by bacteria Leaves the stomach through the pyloric sphincter to enter the small intestine

23 The Small Intestine Consists of duodenum, jejunum, and ileum Receives
Chyme from stomach Digestive enzymes and bicarbonate from pancreas Bile from liver and gallbladder

24 Epithelial wall is covered with villi
Villi are covered by microvilli Greatly increase surface area Microvilli participate in digestion and absorption Brush border enzymes Many adults lack the enzyme lactase Have lactose intolerance

25 Accessory Organs Pancreas
Pancreatic fluid is secreted into the duodenum through the pancreatic duct Enzymes Trypsin and chymotrypsin – proteins into smaller polypeptides Pancreatic amylase – polysaccharides into shorter sugars Lipase – fats into free fatty acids and monoglycerides Bicarbonate neutralizes acidic chyme Exocrine and endocrine gland

26 Accessory Organs Liver Gallbladder Body’s largest internal organ
Secretes bile Bile pigments (waste products) and bile salts (for emulsification of fats) Gallbladder Stores and concentrates bile Arrival of fatty food in the duodenum triggers a neural and endocrine reflex that stimulates the gallbladder to contract, causing bile to be transported through the common bile duct and injected into the duodenum


28 The Large Intestine (colon)
Much shorter than small intestine, but has larger diameter Small intestine empties directly into the large intestine at a junction where two vestigial structures, cecum and appendix, remain No digestion occurs Function to reabsorb water, remaining electrolytes, and vitamin K Prepare waste for expulsion


30 The Large Intestine Many bacteria live and reproduce within the large intestine Feces compacted and passed to rectum Feces exit anus Smooth muscle sphincter (involuntary) Striated muscle sphincter (voluntary)

31 Variations in Digestive Systems
Digestive tracts of some animals contain bacteria and protists that convert cellulose into substances the host can absorb Minor in humans Essential to some animals Herbivores have longer digestive tracts Greater time for digestion of cellulose Modifications to enhance digestion of plant material


33 Ruminants have a four-chambered stomach
Rumen, reticulum, omasum True stomach – abomasum Rumen has cellulose-degrading microbes Contents can be regurgitated and rechewed Rumination Evolved only once

34 Variations in Digestive Systems
Rodents, horses, deer, and rabbits digest cellulose in the cecum Regurgitation of contents is not possible However, some such animals practice coprophagy Eat their feces to absorb nutrients on the second passage of food Cannot remain healthy if prevented from eating feces

35 Variations in Digestive Systems
All mammals rely on intestinal bacteria to synthesize vitamin K, which is required for blood clotting Birds, which lack these bacteria, must consume the required quantities of vitamin K in their diet

36 Accessory Organ Function
Liver Chemically modifies the substances absorbed from the digestive tract before they reach the rest of the body Ingested alcohol and other drugs are taken into liver cells and metabolized Removes toxins, pesticides, and carcinogens, converting them to less toxic forms Regulates levels of steroid hormones Produces most proteins found in plasma

37 Regulation of Food Intake
Control mechanism links food intake to energy balance Leptin – peptide hormone Key to appetite control Produced by adipose tissue Leptin receptor located in hypothalamus Reduced leptin signals brain to intake food Research on leptin in humans ongoing

38 Regulation of Food Intake
Other hormones involved in the control of feeding and energy include Insulin, GIP, and CCK, which signal satiety Ghrelin which stimulates food intake Efferent control of feeding Neuropeptide Y (NPY) induces feeding activity

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