describe the oral phase (saliva) what are the functions
- salivary secretions
- under autonomic control (mostly parasympathetic)
- lubrication and moistening of food
- solubilization of material for taste
- initial digestion of starches (salivary amylasee)
- lingual lipase contributes minorly to lipid hydrolysis
- antibacterial actions
-> lysozyme = antibacterial enzyme
-> immunoglobins = antibodies
describe the oral phase (the swallowing reflex)
- tongue pushes bolus against soft palate and back of mouth, triggering swallowing reflex
-> soft palate elevates closing off the nasopharynx
-> tonically contracted upper esophageal sphincter - breathing is inhibited as the bolus passes the closed airways
-> epiglottis folds down to help keep swallowed material out of airways
- upper esophageal sphincter relaxes - food moves downward into the esophagus, propelled by peristaltic waves and aided by gravity
what are the functions of the gastric phase (integrated response to a meal)
- digestion (mechanical and chemical)
-> gastric contractions continue to break food up and help mix it with gastric acid and secretions for chemical digestion
-> produces chyme (soupy mixture of partially digested food) - production
-> acidic environment destroys pathogens
->mucus and bicarbonate to protect itself - storage
-> release of chyme into small intestine is regulated
what are the 3 sections of the stomach
- fundus
- body
- antrum
- rugae = surface folding that increases area
explain the mucus-bicarbonate buffer
- protects the gastric epithelium
- gastric mucous cells (epithelial cells that secrete mucus)
- mucus layer = physical barrier (secretes bicarbonate) -> chemical barrier that neutralizes acid
- stomach lumen = gastric juice (pH = 2)
explain how pepsin starts protein digestion
- pepsinogen = inactive protease secreted by chief cells of the gastric glands
- pepsinogen -> pepsin (activated by H+)
- pepsin is optimally active between pH 1.8-3.5
- pepsin begins protein digestion by cleaving specific peptide bonds of polypeptides
-> protein product entering small intestine is a mixture of intact protein, polypeptides and some free amino acids
overview of gastric chemical digestion (protein, fats, carbohydrate)
protein
- low pH denatures proteins
- pepsin begins breaking polypeptides into smaller fragments
fats (minor 10% digested in stomach)
- gastric motility -> forms an emulsion of lipids and gastric lipase
- gastric lipase -> hydrolyzes triglycerides into monoglycerides and FFAs
carbohydrate
- none
- salivary amylase is inactivated at low pH
what are the cell types of the gastric mucosa (gland) , the substances they secrete, the stimulus for release, the function of secretion
- mucous surface -> secrete mucus -> tonic secretion with irritation of mucosa -> physical barrier between lumen and epithelium
- neck cells -> secrete bicarbonate -> secreted with mucus -> buffers gastric acid to prevent damage to epithelium
- parietal cells -> secrete gastric acid (HCl) -> acetylcholine (PNS)-> activates pepsin, kills bacteria
- chief cells -> secrete pepsin and gastric lipase -> acetylcholine and acid secretion -> digests proteins, digests fats
what are the main functions of the small intestine phase
- regulation of gastric emptying
-> chyme in small intestine triggers feedback to regulate further release from the stomach - secretions
-> epithelial cells = mucus, digestive (brush border) enzymes
-> pancreas = HCO3-, digestive enzymes
-> liver and gallbladder = bile - digestion
-> fats, carbohydrates, proteins/peptides - absorption
-> nutrients, vitamins, ions, minerals, water
what are the components of the wall of the small intestine
- enterocytes = transporting epithelium
-> microvilli increase SA and create brush border
-> digestive enzymes on the the brush border
-> nutrient absorption occurs here along villi - crypt epithelial cells
-> secretion of ions, water and hormones
-goblet cells
-> secrete mucus for protection and lubrication
- capillaries
-> transport absorbed nutrients
-lacteals
-> transport absorbed fats via lymphatic system
what is the exocrine pancreas
- most secretions responsible for enzymatic digestion are from the pancreas
= acinar + duct cells
1. acinar cells secrete digestive enzymes
-> some are inactive precursors (zymogens)
-> active enzymes (proteases, amylases, lipase)
- duct cells secrete aqueous solutions of NaHCO3 (helps neutralize chyme from stomach)
- exocrine secretions travel from pancreatic acini to main pancreatic duct -> into common bile duct -> release into duodenum is controlled by sphincter of oddi
explain biliary secretions
- bile is a non-enzymatic solution of bile salts, bile pigments, and cholesterol
- produced in the liver
- between meals bile is diverted into the gallbladder where it is concentrated and stored
- bile salts help with digestion and absorption of fats
explain the oral, gastric, and intestinal phase of carbohydrate digestion
- carbs can only be absorbed as monosaccharides (glucose, galactose, fructose)
oral phase - mechanical digestion by chewing
- chemical digestion begins with salivary amylase
gastric phase - mechanical digestion due to peristaltic contractions (mixing, churning)
- chemical digestion paused in stomach (too acidic)
intestinal phase - most digestion occurs in SI
- continued mechanical digestion (segmental contractions)
- pancreatic amylase secreted into the lumen of the duodenum
- brush border enzymes on the apical surface of epithelial cells in the duodenum and jejunum
-> sucrase, maltase, lactase
explain carbohydrate digestion to monosaccharides
- in the lumen = amylase
-> glucose polymers -> disaccharides - at the brush border = maltase, sucrase, lactase
-> disaccharides -> monosaccharides - the end products of carbohydrate digestion = glucose, fructose, galactose
- absorbed by specific transporters (uniport or symport)
explain the oral, gastric, and intestinal phase of protein digestion
oral phase
- mechanical digestion only
gastric phase
- chemical protein digestion initiated in the stomach
-> HCl denatures protein; pepsin cleaves peptide bonds
- mechanical digestion by peristaltic mixing/churning
- results in a mixture of intact protein, large polypeptides and some free amino acids
intestinal phase
- mechanical digestion by segmental contractions
- chemical cleavage by peptidases
-> pancreatic enzymes (endo and exopeptidases)
-> trypsin, chymotrypsin, carboxypeptidase
-> brush border enzymes: exopeptidase (aminopeptidase)
-> endopeptidase cleave internal peptide bonds
-> exopeptidase cleave terminal peptide bonds
-products = di and tri peptides and free amino acids (some small peptides)
explain protein digestion to amino acids and pepdtides
- endopeptidases digest internal peptide bonds
-> pepsin, trypsin, chymotrypsin
-> into 2 smaller proteins - exopeptidases digests terminal peptide bonds to release amino acids
-> aminopeptidase (brush border of SI)
-> carboxypeptidases (pancreatic juice)
-> into amino acids and peptides
what are fats (functions) and the oral, gastric, and intestinal phase
- dietary lipids
-> triglycerides, cholesterol, phospholipids, fatty acids - provide both an energy source and reserve
- functions = form hormones, part of cell membranes, insulation & shock absorption, transport fat soluble vitamins and other compounds
oral phase
- mechanical digestion only
gastric phase
- begin chemical digestion by gastric lipase
intestinal
- major chemical digestion by pancreatic lipase
what is the problem with lipids being hydrophobic and what is the solution
- creates a challenge to digestive and absorptive processes
solution - coarse emulsification in the stomach by mixing
(large fat droplets suspended in chyme) - association with bile salts in small intestine
-> fine emulsification increase surface area for digestion and eventually forms a micellar solution (small lipid droplets)
explain how lipids are absorbed
- bile salts first create a stable emulsion of lipid droplets in solution
-> pancreatic lipase digests triglycerides with the help of colipase (coenzyme that helps pancreatic lipase break down fats) - small disks called micelles are formed and are brought close to enterocytes for absorption
what are the steps of fat absorption
- bile salts from liver coat fat droplets
- pancreatic lipase and colipase break down fats into monoglycerides and fatty acids stored in micelles
3.a) monoglycerides and fatty acids move out of micelles and enter cells by diffusion
3.b) cholesterol is transported into cells - absorbed fats combine with cholesterol and proteins in the intestinal cells to form chylomicrons
- chylomicrons are removed by the lymphatic system -> lymph to vena cava
- monoglycerides and fatty acids reform triglycerides inside the cell
what are chylomicrons
- large fat droplet, must be packed into secretory vesicle by golgi for exocytosis
what are the main functions of the large intestine
- approx. 1.4L/day of unabsorbed liquid enters into the large intestine
-> most is absorbed - further absorption of water and electrolytes
- supports an environment that facilitates colonization by healthy bacteria
- digest some fiber (bacteria)
- storage of waste in rectum until it can be eliminated
what are the major anatomical features of the large intestine
- cecum
- ileocecal sphincter (terminal portion of small intestine)
- ascending colon
- transverse colon
- teniae coli = longitudinal layer
- descending layer
- haustra
- sigmoid colon
- rectum
- internal and external anal sphincter
- one under autonomic control one under voluntary
what is colon microflora
- bacterial ecosystem that contributes to normal GI physiology in several ways
- digest dietary fiber
- produce vitamin K (50% of total need)
- limit growth and invasion of pathogenic microorganisms
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