digestive stystem chapter 48 Flashcards
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
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
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
Ingested food may be stored or first subjected to physical fragmentation—mechanical digestion
Chemical digestion occurs next
Hydrolysis (splitting with water) 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
Mouthoral cavity – entry and mechanical digestion and
beginning of chemical digestion
Pharynx—”common” area of resp. and digest. sys.
Esophagus – delivers food to stomach
Stomach – preliminary digestion
Small intestine – completion of digestion and absorption
Large intestine – absorption of water and minerals
Cloaca or rectum – expel waste
Salivary glands—digestive enzymes, mucus and watery solution Liver Produces bile Gallbladder Stores and concentrates bile (acts as emulsifier) Pancreas Produces pancreatic juices Digestive enzymes and bicarbonate buffer
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
Inside the mouth, the tongue mixes food with saliva
Moistens and lubricates the foodbolus
Contains salivary amylase, which initiates the breakdown of starchdisaccharide maltose
Salivation is controlled by the nervous system
Tasting, smelling, and even thinking or talking about food stimulate increased salivation
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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The Esophagus
Muscular tube connecting the pharynx to the stomach
Actively moves a bolus through peristalsis—wave-like muscular contractions
Sphincter opens to allow food to enter stomach
Humans lack a true sphincter here
The Stomach
Saclike portion of tract
Convoluted surface (folds are called rugae) allows expansion
Contains 3rd layer of smooth muscles (oblique) for mixing food with gastric juice
3 kinds of secretory cells in gastric glands in mucosa
Mucus-secreting cells—protects stomach lining
Parietal cells
Secrete HCl and intrinsic factor (for vitamin B12 absorption)
Chief cells
Secrete pepsinogen (inactive form of pepsin)
HCl + pepsinogenpepsin for protein digestion
Low pH in the stomach helps denature food proteins
Activates pepsin and keeps it functioning
Proteinshorter chains of polypeptides
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, Helicobacter pylori. Attacks mucous cells.
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—makes chyme less acidic Bile from liver and gallbladder
Epithelial wall is covered with villi
Villi are covered by cells with 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 disaccharides lactose and sucrose
Lipase – fats into free fatty acids and monoglycerides
Bicarbonate neutralizes acidic chyme—acini cells
Exocrine and endocrine gland
Liver
Body’s largest internal organ
Secretes bile
Bile pigments (waste products from old red blood cells) 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 in small intestine
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 a lacteal (lymphatic capillary) and later join the circulatory system via subclavian vein
Almost all fluid reabsorbed in small intestine
Mouth—mechanical and chemical digestion
salivary amylasecarbohydratesdisaccharide maltose
Stomach—chemical digestion of proteins
pepsinogen + HClpepsinproteinsshorter chains of polypeptides
Small intestine—chemical digestion
emulsification of fats by bile
pancreatic lipasefatsfree fatty acids + monoglycerides
trypsin and chymotrypsinproteinssmaller polypeptides
pancreatic amylasepolysaccharidesdisaccharides
epithelial cells release last digestive enzymes
fatty acids + monoglyceridesgo into lacteal—lymphatic system
polypeptidesamino acidsgo into bloodliver
disaccharidesmonosaccharidesgo into bloodliver
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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
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
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
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
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
Essential Nutrients
Animal cannot manufacture these for itself but are necessary for health and so must be obtained in the diet
Vitamins—organic substances required in trace amounts
Humans, apes, monkeys, and guinea pigs have lost the ability to synthesize ascorbic acid (vitamin C)
Essential amino acids – cannot manufacture—
humans require 9
Long-chain unsaturated fatty acids
Vertebrates can synthesize cholesterol, a key component of steroid hormones, but some carnivorous insects cannot
Minerals—inorganic substances; trace elements—required in very small amounts
