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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
CHAPTER 48LECTURE
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The Digestive System
Chapter 48
Types of Digestive Systems
• Heterotrophs are divided into three groups based on their food sources1. Herbivores are animals that eat plants
exclusively
2. Carnivores are animals that eat other animals
3. Omnivores are animals that eat both plants and other animals
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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
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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
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Types of Digestive Systems
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
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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
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Vertebrate Digestive Systems
• Accessory organs
– Liver• Produces bile
– Gallbladder• Stores and concentrates bile
– Pancreas• Produces pancreatic juice• Digestive enzymes and bicarbonate buffer
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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
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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
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• 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
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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
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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
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Mouth and Teeth
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
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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)
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The Stomach
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The Stomach (Cont.)
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
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The Small Intestine
• About 4.5 m long – small diameter
• Consists of duodenum, jejunum, and ileum
• Receives – Chyme from stomach– Digestive enzymes and bicarbonate from
pancreas– Bile from liver and gallbladder
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• 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
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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
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Accessory Organs
• Liver – 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
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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– Reassembled into triglycerides and then chylomicrons– Enter the lymphatic system and later join the
circulatory system
• Almost all fluid reabsorbed in small intestine
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Absorption
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
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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)
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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
material34
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• 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
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Variations in Digestive Systems
• Foregut fermentation– Convergent evolution– Modified lysozyme to
take on new role of digesting bacteria in stomach
– Same 5 amino acids changed
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Cow Horse Human BaboonRat Langur
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 feces38
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
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Regulation of the Digestive Tract
• Gastrointestinal activities are coordinated by the nervous and endocrine systems
• Nervous system stimulates salivary and gastric secretions in response to sight, smell, and consumption of food
• In the stomach, proteins stimulate the release of gastrin– Triggers the secretion of HCl and pepsinogen
from the gastric glands40
Regulation of the Digestive Tract
• Enterogastrones or duodenal hormones– Inhibit stomach contractions and prevent
additional chyme from entering duodenum– Cholecystokinin (CCK), secretin, and gastric
inhibitory peptide (GIP)• All inhibit gastric motility and secretions
– CCK also stimulates gallbladder contraction and pancreatic enzyme secretion
– Secretin also stimulates the secretion of pancreatic bicarbonate
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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
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Accessory Organ Function
• Regulation of blood glucose– After a carbohydrate-rich meal
• Insulin stimulates removal of excess blood glucose by liver and skeletal muscles (glycogen)
– When blood glucose levels decrease• Glycogenolysis – glucagon stimulates liver to
break down glycogen to release glucose into blood• Gluconeogenesis – liver converts other molecules
into glucose if fasting continues
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Food Energy
• Ingestion of food serves two primary functions1. Source of energy
2. Source of raw material
• Basal metabolic rate (BMR)– Minimal amount of energy consumed under
defined resting conditions
• Continued ingestion of excess food energy results primarily in accumulation of fat
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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
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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• Melanocyte-stimulating hormone (-MSH) which
suppresses it
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Essential Nutrients
• Animal cannot manufacture these for itself but are necessary for health and so must be obtained in the diet
• Vitamins– Humans, apes, monkeys, and guinea pigs have lost
the ability to synthesize ascorbic acid (vitamin C)• Amino acids – humans require 9• Long-chain unsaturated fatty acids
– Vertebrates can synthesize cholesterol, a key component of steroid hormones, but some carnivorous insects cannot
• Minerals51