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The Digestive System Chapter 48

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

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

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

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

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

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

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

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

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

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

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

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

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)

The Stomach

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

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

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

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

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

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

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

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)

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

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

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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cow Horse Rat Human Baboon 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 feces

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

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

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

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

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 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

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

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

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