GI physiology
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91 terms
Terms | Definitions |
|---|---|
 GI smooth muscle is | spontaneously acitve, contracts without external input |
| What is the basic electrical rhythm? | spontaneous variations in membrane potential that produce action potentials- determines frequency of reactions |
| What is the strength of contraction of GI muscle regulated by? | hormones, and nervous |
| What determines the type of motility pattern that occurs in the GI tract? | ENS |
| What are the 2 types of motility patterns? | Fasting motility pattern and feeding motility pattern |
| What is the fasting motility pattern? | migrating motor complex (MMC)- involves stomach, SI, LI, 4-5 hrs after a meal, repeated every 2 hrs. Function: keep digestive system clear |
| What are the 3 periods of the MMC? | inactivity (1 hr), intermittent activity (50min), Intense activity |
| What is the feeding motility pattern? | Storage (in stomach, relaxation of smooth muscle allow volume of luminal contents to increase without increasing pressure), Peristalsis, Segmentation |
| Peristalsis | Eosphagus, stomach, SI, LI- propulsive |
| Segmentation | SI, LI, mixing and exposure to absorptive surfaces, periods of contraction and relaxation of SI tract |
| What are the specific motility patterns? | chewing and swallowing, gastric motility, Intestinal motility |
| Chewing | reduce size of food, increase SA, mix with saliva, tasting. Control= voluntary (skeletal muscle), reflex control of strength, frequency, side of chewing |
| Swallowing and deglutination | rapidly transfer food from mouth to stomach. Control initiated at will, proceeds reflexively. (contraction of pharynegeal skeletal muscle and peristaltic contractions of eosphageal skeletal and smooth muscle) |
| Gastric motility | Storage, mechanical digestion, mixing, controlled delivery to duodenum. |
| Funcus and body of stomach functions | storage, smooth muscle layers thin, wall distendible |
| pylorus of stomach functions | thick muscle layer- peristalsis and mechanical digestion |
| Stomach in fasting | shrinks to small volume (50mL), migrating motor complex |
| Stomach in feeding | motility patterns associated with storage, mixing, mechanical digestion, controlled delivery to duodenum (1L) |
| What is receptive relaxation? | increase in volume of luminal contents without an increase in pressure, relaxation of smooth muscles. Nervous reflex, involves vagus nerve |
| Peristaltic waves | intiated on the greater curvature, aprox 3min-1, spreads to antrum, first 60min= more gentle activity, 60-300 min= more intense activity. Function in stomach: propulsions of food toward pyloric sphincter,combined with pyloric sphincter: retropulsion |
| Rate of emptying food in stomach | solids slower than liquids, fats slower than gastric emptying |
| How does the duodenum control the stomach? | Hormonal GIP or neural (enterogastric reflex)- increase volume of pH stimulates a parasympathetic pathway, inhibits smooth muscle action in stomach |
| Main motility pattern in SI | feeding: mainly segmenation, peristalysis, fasting: MMC |
| Motility pattern of LI | inactivity, segmentation (one per 30 min), mass movements 3-4 times a day, force faeces into rectum |
| Function of exocrine secretions? | digest food, dilute food, optimal pH, protection and lubrication |
| Major exocrine secretions | enzymes, mucus, electolytes |
| What is the volume of salivary secretion produced? | 1.5L/day, rate: basal=0.3mL/min stimulated=1.5mL/min |
| What is the composition of saliva? | mucus, dilute solution of NaCl/ NaHCO3, enzymes a-amylase and lipase |
| How is salivary secretion regulated? | Autonomic nervous system: parasympathetic= Ach- copious volume, sympathetic= adrenaline- small volume of viscous fluid |
| Functions of saliva | lubrication, hygiene, digestion |
| What is the volume of Gastric secretion produced per day? | 2-3L, between meals released at slow rate (15-30mL/hr)- mainly mucus, Eating: 150mL/hr of 150mM HCL, pH=1, Mucus, pepsinogen, intrinsic factor |
| Function of gastric sectetion | mucus: protection, Intrinsic factor: absorption of vitamin B12 in SI, Pepsin: digestion of proteins, Gastric acid: denatures proteins |
| What are the 3 phases that coordinate the secretion of components when eating? | cephalic phase, Gastic phase, Intestinal phase |
| Cephalic phase | 30-40% of secretion, stimuli: smell, sight, chewing, brain controls secretion: PNS via ENS stimulates parietal cells (neural), stimulates gastrin secretion (hormonal) |
| Gastric phase | 60% of secretion, Stimuli: stretch, products of digestion, Respone: local reflex (ENS), external reflex (PNS), stimulate parieal cell, gastrin production |
| Intestinal phase | 5-10% of secretion, intestine controls amount of acid delivered to SI, stimuli: stretch of duodenum, arrival of acid in duodenum, reponse: GIP, CCK, and secretin inhibit secretion, enerogastric reflex (vagus- inhibits secretion) |
| Volume of pancreatic secretion | 1-1.5L/day, enzymes (acinar cells) and alkaline fluid (produced in ducts) |
| Enzymes -acinar cells | include lipolytic (fats), amylytic (carbohydrates), proteolytic (proteins). Function: digest food |
| How are proteolytic enzymes activated? | In SI, trypsinogen is converted to trypson by enterokinase, trypsin activates other enzymes |
| HCO3 solution (ducts) function | neutralise acidic chyme, creates optimum pH for pancreatic and intestinal enzymes |
| CCK | produced by duodenal endocrine cells in response to digestive products, stimulates secretion of enzymes by acinar cells |
| Secretin | produced by duodenal endocrine cells in response to increase in H+, stimulates HCO3 secretion by duct cells- neutralises acidic chyme |
| Volume of biliary secretions | 0.5mL day |
| Composition of Biliary secretions? | excretory products: bile pigments (waste products), cholesterol (excreted by liver), Bile salts, and lecithin, HCO3 fluid |
| What is the function of Biliary secretions? | Bile salts and lecithin: fat digestion and absorption, HCO3 neutralises acidic chyme, Bile pigments- secretion |
| Control of Biliary secretions? | bile to SI: hormonal control, CCK, controls contraction of gall bladder and relaxation of sphincter of oddi |
| Volume of SI secretions? | 1.5L per day |
| Composition of SI secretions? | mucus, NaHCO3- neutralises acid, dilutes food |
| Composition of LI secretions? | Mucus |
| Control of Intestine secretions? | hormonal- vasoactive intestinal peptide- released by endocrine cells, stimulates ion secretion by epithelial cell |
| What are the main nutrients that undergo chemical digestion? | carbohydrates, fats, proteins |
| Structure of carbohydrates? | long chains of monosaccharides (glucose) joined by a 1-4 glycosidic bonds (starch and glycogen major carbohydrates in diet) |
| Cellulose | B 1-4 glycosidic bonds |
| List the 3 main disaccharides we ingest? | Sucrose, Lactose, Maltose |
| Amount of Carbohydrates ingested? | 250-800g |
| Amount of proteins ingested? | 70-100g |
| Structure and function of proteins? | long chains of aa joined by peptide bonds, required for aa |
| Amount of lipids ingested? | 100-150g |
| Structure of lipids | consist mainly of TAGs |
| Structure of TAGs | glycerol backbone with 3 fatty acids attached by ester bonds |
| What are the 2 steps of chemical digestion? | luminal digestion and contact digestion in SI |
| Luminal digestion | involves enzymes secreted into lumen, salivary glands (amylase), stomach (pepsin), SI (trypsin, chymotrypsin, lipase, amylase) |
| contact digestion | digestion completed by enzymes produced by enterocyte, attached to brushborder of enterocyte |
| Chemical digestion of carbohydrates | polysaccharides converted to disaccharides by salivary and pancreatic amylase, disaccharides converted to monosaccharides by sucrase, lactase and maltase bound to brush border |
| Chemical digestion of proteins | pepsin (stomach) and trypsin, chymotrypsin convert proteins into polypeptides and proteases, Peptidases attached to brushborder convert polypeptides to amino acids |
| Chemical digestion of lipids | no contact digestion, divided up into several stages: emulsification, stabilization, digestion, formation of micelles |
| Emulsification of lipids | breaks large fat droplets down into smaller droplets, increases SA for digestion, occurs in stomach (retropulsion) and SI (segmentation) |
| Stabilisation of lipids | Lecithin and bile salts stabilise emulsion droplets, allow formation in smaller droplets, occurs in SI |
| Digestion of lipids | Involves lipase and colipase (cofactor) (secreted by pancreas), co-lipase anchors to surface of droplet, lipase converts TAGs to Monoglycerides and free fatty acides, occurs in lumen of Si at surface of emulsion droplets |
| Formation of micelles | products of fat digestion insoluble in water, kept in solution by formation of micelles |
| What are micelles? | small droplets (4-6nm dia) consist of bile salts/ lecithin, fatty acids, monoglycerides |
| Where does most absorption occur? | SI- 90%, 9% in LI, mouth, eosophagus, stomach |
| What is absorption? | passage of substances into blood |
| What factors affect absorption? | motility, SA, passage of molecules across availiable SA |
| What are the 2 pathways of absorption? | paracellular (between cells) and cellular (through cells) |
| Paracellular pathway | only barrier is tight junction, relatively non-selective, is passive- required gradient |
| cellular pathway | solute must cross 2 membranes, substances not lipid soluble require transport proteins- selective, active transport |
| How do you maximise abosorption across available SA? | reduction in size, transport proteins |
| How much water has to be absorbed each day? | 1.5L |
| How much of salivary, gastric, pancreatic, liver and SI do we secrete into GI tract? | Salivary: 1.5L, Gastric: 3L, Pancreatic: 1.5L, Liver: 0.5L, SI: 1.5L |
| What is the total water delivered to the SI? | 9-10L/ day |
| How is Na+ absorbed? | passive movement via paracellular pathway of active transport via cells |
| What are the 3 transport mechanisms for active absorption of Na+? | Na+ alone, Na+ coupled to glucose, Na+ coupled to aa |
| How are products of carbohydrate digestion absorbed? | passive: monosaccharides (glucose, galactose, fructose) diffuse down gradient via paracellular pathway. Active: cotransport with Na+ (glucose, galactose) |
| How are the products of protein digestion absorbed? | Passive: aa diffuse down gradient via paracellular pathway. Active: cotransport of Na+, aa coupled to Na+ |
| How are the products of fat absorption absorbed? | lipid soluble, can diffuse across lipid membrane |
| What happens to product of fat digestion in the cell? | synthesised into TAGs, packaged into chylomicrons, exit via exocytosis and enter lacteals |
| How are bile salts absorbed? | diffusion in jejunum, active transport in ileum |
| How are fat soluble vitamins absorbed? | A, D, E, K absorbed with fats |
| How are water soluble vitamins absorbed? | Na+ dependent absorption (C) |
| How is vitamin B12 absorbed? | intrinsic factor binds to vitamin B12, receptors in ileum bind intrinsic factor, vitamin B12 actively absorbed |
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