1. CHAPTER 48 LECTURE SLIDES
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2. The Digestive System
Chapter 48

3. 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

4. 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

5. 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

6. Types of Digestive Systems

7. 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

8. 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

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

11. 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

13. 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

14. 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

15. 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

16. 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

17. Mouth and Teeth

18. 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

20. 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)

21. The Stomach

22. The Stomach (Cont.)

23. 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

24. 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

25. 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

26. 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

27. 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

29. 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

30. Absorption

31. 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

33. 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)

34. 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

36. 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

37. 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
Rat
Human
Baboon
Langur

38. 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

39. 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

40. 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 glands

41. 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

43. 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

44. 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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47. Food Energy Ingestion of food serves two primary functions
Source of energy
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

48. 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

49. 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 (a-MSH) which suppresses it

51. 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
Minerals