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54 Cards in this Set
- Front
- Back
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Digestion |
From food, humans must get basic organic molecules to make ATP, build tissues, and serve as cofactors and coenzymes.
-Digestion breaks polymers (carbohydrates, fats, and proteins) into monomer building blocks. (Via hydrolysis reactions)
Absorption takes these monomers into the bloodstream to be allocated. |
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Digestive Tract |
Open at both ends and continuous with the environment -Considered “outside”the body -Materials that cannot be digested (cellulose) never actually “enter”the body |
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Events in Digestive Tract: Motility |
Motility Ingestion: taking food into the mouth Mastication: chewing Deglutition: swallowing Peristalsis: one-way movement through tract Segmentation: churning/mixing |
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Events in Digestive Tract: 2-4 |
2. Secretion -Exocrine: digestive enzymes, acid, mucus -Endocrine: hormones to regulate digestion
3. Digestion Breaking food down into smaller units
4. Absorption Passing broken-down food into blood or lymph |
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Events in Digestive Tract: 5-6 |
5. Storage and elimination -Temporary storage and elimination of undigested food
6.Immune barrier -Simple columnar epithelium with tight junctions prevents swallowed pathogens from entering body. |
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Digestive System Divisions |
Gastrointestinal tract: 30 feet long, from mouth to anus Mouth --> Pharynx --> Esophagus --> Stomach --> Small intestines --> Large intestines --> Anus
Accessory organs: teeth, tongue, salivary glands, liver, gallbladder, pancreas |
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Regulation of the GI Tract (parasympathetic) |
Parasympathetic division:
Stimulates esophagus, stomach, small intestine, pancreas, gallbladder, and first part of large intestine via vagusnerve
Spinal nerves in sacral region stimulate lower Large intestine. |
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Regulation of the GI Tract (sympathetic) |
Sympathetic division:
Inhibits peristalsis and secretion Stimulates contraction of sphincters
Hormones: From brain or other digestive organs |
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Intrinsic regulation of the GI Tract |
Intrinsic sensory neurons in gut wall help in intrinsic regulation via separate enteric nervous system -2ndbrain: is an integrating center that can function independent of the CNS -Has cells similar to astrogliaand a diffusion barrier (like blood-brain barrier) -Responds to amount of food and nutrient quality of food -Extends from esophagus to anus -Gets lots of info from PS and S systems
(works a lot by paracrine signals) |
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Paracrine signals |
Cells in tissue send out signals to other cells in same tissue |
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Mouth |
Point of entry Mastication: Chewing breaks food down into smaller pieces for deglutition and mixes it with saliva. Saliva: contains mucus, an antimicrobial agent, and salivary amylase to start digestion of starch. -Enhances taste and begins digestion
Deglutition: process of swallowing food and passing it on to stomach through esophagus -Voluntary and reflex muscle actions -Epiglottis ensures that food enters esophagus and not laryanx
See video on Beachboard
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Stomach: Functions (6) |
1. Stores food 2. Churns food to mix with gastric secretions 3. Begins protein digestion -Does not hydrolyze (break apart peptide bonds) proteins, but acid conditions cause loss of tertiary, secondary structures (denaturation)
4. Kills bacteria in the food (acid) 5. Secretes intrinsic factor (read about it!) No typical food absorption in stomach! Only alcohol and NSAIDs
6. Moves food into small intestine in the form of chyme -Semifluid mass of partially digested food -Result of mechanical and chemical breakdown of bolus -Contains partially digested food, water, HCl, digestive enzymes
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Stomach Structure |
Food is delivered to cardiac region. Upper region = fundus Lower region = body Distal region = pyloris -Ends at pyloric sphincter
Lining has folds called rugae. |
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Gastric pits Goblet cells |
-Part of stomach structure Gastric pits at base of folds lead to gastric glands with secretory cells: Goblet cells secrete mucus to help protect stomach lining from acid (pH < 2). |
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Stomach Structure |
Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor (helps small intestine to absorb vitamin B12).
Chief cells secrete pepsinogen
Enterochromaffin-like (ECL) cells secrete histamine and serotonin (paracrine signals). •G cells secrete gastrin (hormone). •D cells secrete somatostatin(hormone). |
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Stimulation of HCl Secretion |
Gastrin: made in G cells; carried to parietal cells in blood -Stimulates the secretion of HClfrom parietal cells -Also stimulates ECL cells to make histamine
Histamine: also stimulates parietal cells via H2 histamine receptors -Examples: Tagamet and Zantac: block H2 receptors.
Parasympathetic neurons: stimulate parietal and ECL cells |
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Three Functions of HCl |
Reduces pH to 2: -Proteins are denatured (allows enzymes access). -Pepsinogen (Chief cells) is converted to active pepsin (digests proteins). -Serves as the optimal pH for pepsin activity
-ogen = inactive form
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Stomach Defenses |
Acid and pepsin could eat the stomach lining.
Defenses that help prevent this: -Adherent layer of mucus with bicarbonate -Tight junctions between epithelial cells -Rapid epithelial mitosis that replaces epithelium every three days
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Peptic Ulcers –what causes them? |
Peptic ulcers: erosions of the mucosa of the stomach or duodenum 1. Helicobacter pylori: bacterium that reduces mucosal barriers to acid 2. The most wide-spread chronic infection in the world 3. 2005 Noble Prize for Medicine
Treatment for ulcers combines K+/H+pump inhibitors (Prilosec) and antibiotics. |
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Small Intestine |
~ 3 meters long, not so “small”
3 basic parts: 1. Duodenum 2. Jejunum 3. Lleum |
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Small Intestine Functions |
Complete digestion of carbohydrates, proteins, and fats
Absorption of nutrients: -Sugars, lipids, amino acids, calcium, and iron absorbed in duodenum and jejunum -Bile salts, vitamin B12, water, and electrolytes in ileum -Very rapid due to villi and microvilli
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Small Intestine Structure |
Lots of structure Mucosa and submucosa folded into: plicaecirculares ---> villi ---> microvilli---> |
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Villi and Microvilli |
Columnar epithelium with goblet cells (mucus)
Capillaries absorb sugars and amino acids
Lacteals: lymphatic vessels -absorb fatty acids. |
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Intestinal Contractions/Motility |
Smooth muscle contractions occur automatically. Contractions can be influenced by the autonomic nervous systems
Peristalsis is weak, relative to esophagus and stomach.
Movement of food is faster at pyloric end and slower at distal.
Segmentation: muscular contractions of lumen in different segments of intestine -Effective mixing of chymeand causing slow movement toward distal end. |
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Digestion in small intestine |
Even though most digestion and absorption occurs in SI, most of the digestion is through the use of pancreatic enzymes
Some of the pancreatic enzymes are activated by the brush border enzymes of the SI
Brush border enzymes: enterokinase, disaccharidaases, peptidases, phosphatases
Most digestion and absorption occurs in duodenum and jejunum |
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Intestinal Enzymes |
SI has no glands Has brush border enzymes; -These enzymes are not released into lumen, but stay attached to plasma membrane with active site exposed to chyme
**Enterckinase
-table 18.1 |
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Activation of pancreatic juices |
Enterokinase (brush border enzyme) -Activates trypsin
Trypsin (pancreatic juice) -Activate the other enzymes of the pancreatic juices |
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Digestion and Absorption: Carbohydrates |
Starch digestion begins in mouth with salivary amylase and continues in intestines with pancreatic amylase.
Brush border enzymes finish breaking down resulting products and other disaccharides (maltose, sucrose, lactose).
Monosaccharides are absorbed across the epithelium into capillaries |
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Digestion and absorption: Proteins |
Begins in stomach with pepsin to produce short-chain polypeptides
Finishes in duodenum and jejunum with pancreatic trypsin,chymotrypsin, elastase, and carboxypeptidase, and the brush border enzyme aminopeptidase.
Free amino acids are also absorbed into capillaries |
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Transport of nutrients: |
importance of Sodium-Potassium ATPase.
Sodium gradient allows secondary active transport of other nutrients (sugars, amino acids, ions) |
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Digestion and Absorption: Fats |
Fat digestion begins in duodenum when bile emulsifies the fat and the pancreatic enzyme lipase breaks it down into fatty acids.
Phospholipase A (from pancreas) digests phospholipids into fatty acids.
Fatty acids and monoglycerides move into bile micelles and are transported to brush border.
Inside the epithelial cells, they are regenerated into triglycerides, cholesterol, and phospholipids and combined with proteins to form chylomicrons. These enter the lacteals: -Chylomicrons are too big to be dealt with by capillaries
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Large Intestine Structure |
Chymefrom ileum passes through ileocecal valve into: Cecum--> Ascending colon --> Transverse colon --> Descending colon --> Sigmoid colon --> Rectum --> Anal canal --> Anus |
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Large Intestine Function |
Absorption of water, electrolytes, vitamin K, and some B vitamins
Production of vitamin K and B vitamins via microbial organisms
Storage and compaction of waste products: feces
Little to no digestive function -Already taken care of “upstream” |
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Absorption of Fluids |
Most absorption occurs in small intestine, but some is left for large intestine. Water is absorbed passively following an osmotic gradient set up by active Na+/K+pumps. -Similar to PCT of kidney -Aldosterone stimulates greater salt and water absorption here.
Not all water is absorbed; about 200 ml is left per day to be excreted with feces |
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Microbial Biota |
Several hundred different species of bacteria live in the large intestine. Some of them are useful -Up to 10^14 bacteria -Microbes make vitamin K and some B vitamins -Make fatty acids from cellulose (Can’t absorb them, but serve as direct energy supply for epithelial cells) (Can account for 10% of caloric intake)
Disruption of normal microfloracan lead to irritable bowel disease
Bacterial relationship: mutualism -Immunological control is complicated…
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Pancreas |
Located behind stomach Endocrine and exocrine functions: Endocrine: Islets of Langerhans cells (alpha and beta) make insulin and glucagon. Exocrine: Acini cells make pancreatic juice, which is delivered to the duodenum via the pancreatic duct. ~ 29 digestive enzymes: amylase, lipase, trypsin Enzymes are inactive in pancreatic juice
The ducts secrete bicarbonate -Neutralize HCl from stomach |
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Pancreatic Juice |
Contains water, bicarbonate, & digestive enzymes Digestive enzymes include amylase for starch, trypsin for proteins, and lipase for fats -Brush border enzymes are also required for complete digestion |
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Pancreatic Enzymes and How They Are Activated in Small Intestine |
Most are inactive (Zymogens) until they reach the small intestine. -Enterokinase activates trypsinogen-->trypsin (to digest protein). -Trypsin activates other enzymes.
Due to the high potential activity of pancreatic proteases, pancreatic trypsin inhibitor is also produced in the pancreas, just in case… |
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Liver |
Largest abdominal organ
Has amazing regenerative abilities due to mitosis of hepatocytes Composed of hepatocytes that form hepatic plates separated by capillaries called sinusoids -Very permeable, allowing passage of blood proteins, fat, and cholesterol |
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Liver Functions |
table 18.3 |
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Detoxification of Blood |
The liver can remove hormones, drugs, and other substances in three ways: 1. Secreted into bile 2. Phagocytized by Kupffer cells lining sinusoids 3. Chemically altered by hepatocytes -Ammonia is converted into urea. -Urea is returned to the blood to be filtered by the kidneys. -Steroids are altered and then secreted into bile. |
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Secretion of Glucose |
The liver helps balance blood glucose levels by removing glucose and storing it as glycogen (glycogenesis)/triglycerides (lipogenesis) or by breaking down glycogen (glycogenolysis) and releasing it into the blood.
The liver can also make glucose from amino acids (gluconeogenesis) and convert fatty acids into ketones (ketogenesis). |
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Blood Circulation Through Digestive System and Liver |
Products of digestion absorbed in intestines are delivered to the liver via the hepatic portal vein.
After circulating through liver capillaries, the blood leaves via the hepatic vein.
Liver also receives blood from hepatic artery and it mixes-up with the blood coming through hepatic portal vein |
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Liver Lobules |
Are functional units formed by hepatic plates (hexagons)
In middle of each is central vein
At edge of each lobule are branches of hepatic portal vein & artery which open into sinusoids |
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Liver Lobules |
Blood from hepatic portal vein and hepatic arteries is mixed and processed by hepatocytes (Lots of surface area interaction: efficient liver function)
The processed blood drains into central vein Bile is secreted by hepatocytes in bile canaliculi -Empty into bile ducts which flow into hepatic ducts that carry bile away from liver |
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Bile Production |
The liver makes 250–1,500 ml of bile per day. Bile is composed of: Bile pigments (bilirubin) Bile salts Phospholipids (lecithin) Cholesterol Inorganic ions
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Bile Salts (Bile Acids) |
Bile acids: derived from cholesterolCholicacid and deoxycholicacid Most is recycled in enterohepaticcirculation. ½ gram of cholesterol is broken down and lost in the feces through this pathway.
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Bile Salts |
Form micelles with polar groups toward water -Fats enter the micelle and are emulsified -Allows for lipase enzymes of SI to gain access to lipids and be broken down |
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Gallbladder |
Sac-like organ located below liver Stores and concentrates bile from the liver: Liver --> Bile ducts --> Hepatic duct --> Cystic duct --> Gallbladder --> Cystic duct --> Common bile duct --> (Sphincter of Oddi) --> duodenum
Loss of gallbladder (gallstones) results in a very drastic change in diet, can’t digest fats very well |
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Regulation of Gastric Function |
Gastric motility & secretion occur automatically -ANS & hormonal effects are superimposed on automatic activity
Extrinsic control of gastric function is divided into cephalic, gastric, & intestinal phases
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Cephalic Phase |
Stimulated by sight, smell, & taste of food
Activation of vagus: Stimulates chief cells to secrete pepsinogen Directly stimulates G cells to secrete gastrin Directly stimulates ECL cells to secrete histamine Indirectly stimulates parietal cells to secrete HCl
Continues into 1st 30 min of a meal |
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Gastric Phase |
Arrival of food in stomach stimulates gastric phase
Gastric secretion stimulated by distension of stomach & chemical nature of chyme
Short polypeptides & amino acids stimulate G cells to secrete gastrin & chief cells to secrete pepsinogen -Gastrin stimulates ECL cells to secrete histamine which stimulates parietal cell secretion of HCl |
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Intestinal Phase |
Begins with inhibition of gastric activity when chymeenters SI
Arrival of chymein SI is detected by sensory neurons of vagus -This causes a neural reflex that inhibits gastric motility & secretion -Fat in chymestimulates SI to secrete enterogasterones--hormones that inhibit gastric motility & secretion --Enterogasterones include somatostatin, cholecystokinin (CCK), & glucagon-like peptide-1 (GLP-1) |
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Summary of Gastrointestinal Hormones |
table 18.5 |
