GI

Question 1

layers of GI wall

Answer 1

1. serosa: outer layer
2. smooth muscle
   * outer longitudinal muscle
   * inner circular muscle
3. submucosa
4. mucosa = lumen ➞ continous w/ external envir
   
   1. acidic pH in stomach
   2. harsh enzymes that hydrolyze food = destructive to tissues ➞ synthesized in inctive form & converted to active form in lumen
   3. millions of microorganisms in GI ➞ detrimental if inside body

Question 2

digestive processes

Answer 2

1. motility: muscular contractions mix & move food
   
   1. propulsivemovements: moving forward/down
   2. mixing movements:
      
      1. aid digestion by mixing w/ digestive juices
      2. facilitate absorption by exposing food to absorbing surfaces
2. secretion: juices secreted into lumen by exocrine glands (ex: pancreas)
3. digestion: biochemical breakdown process where structurally complex food is converted to smaller absorbable units
4. absorption: transfer of absorbable units + water + vit + electrolytes from GI into blood/lymph
   
   • mostly occurs in SI

Question 3

salivary glands

Answer 3

• cephalic phase response: produces saliva in anticipation of food prepares for food
  
  1. prepare food digestion & nutrient absorption efficiency
  2. w/out: delay in absorption, slow speed, longer digestion
  3. energy & homeostasis
• secrete saliva in response to autonomic stimulation
• contains:
  
  • mucus for moisture
  • lysozyme for bacteria
  • bicarbonate buffers for when stomach acid that comes back up
  • amylase: begins chemical digestion by cleaving polysaccharides to maltose

Question 4

mouth

Answer 4

• lips & tongue contain food in mouth ➞ guide food during chewing & swallowing
• teeth begin mechanical breakdown by chewing
• palate = roof of oral cavity
  
  • separates oral cavity from nasal passage
  • simultaneous chewing & breathing
• uvula: soft tissues hangs from rear of mouth ➞ seals nasal passage during swallowing

Question 5

pharynx + esophagus

Answer 5

• pharynx: common passageway for GI & resp system in rear of mouth
• esophagus: muscular tube that extends between pharynx & stomach
• 2 sphincters:
  
  1. pharyngoesophageal sphincter = upper ➞ normally closed to prevent air from entering esophagus (excessive eructation)
  2. gastroesophageal sphincter = lower ➞ normally closed to prevent acid reflux

Question 6

swallowing

Answer 6

entire process of moving food mouth ➞ esophagus ➞ stomach

• brain stem
• propulsive movements
• all or none reflex: initiated when food bolus is pushed by tongue into pharynx ➞ cannot be stopped after initiated
  
  • pressure stimulates receptors that activate swallowing center in medulla
  • swallowing center reflexively activates appropriate muscles to swallow
• 2 stages
  
  1. oropharyngeal stage: 1 sec ➞ moving bolus from mouth through pharynx into esophagus
  2. esophageal stage: 5-9 sec ➞ peristaltic movement through esophagus into stomach

Question 7

stomach: general anatomy & fxs

Answer 7

• fundus: above gastroesophageal sphincter
• body = middle portion
• antrum = bottom portion
  
  • thick layer of smooth muscle
  • connected to SI by pyloric sphincter
• fxs:
  
  1. stores ingested food & delivers to SI at appropriate rate
  2. secretes HCl, pepsinogen, & intrinsic fators to begin protein digestion
  3. produces chyme: thick liquid mixture made by pulverizing ingested food & mixing w/ gastric secretions

Question 8

mucus

Answer 8

• alkaline
• from surface epithelial cells
• covers surface of gastric mucosa to protect from:
  
  • mechanical injury: lubrication
  • self-digestion: inhibits pepsin
  • acid injury: mucus = alkaline ➞ neutralizes HCl

Question 9

gastric mucosa

Answer 9

lining of stomach divided into 2 areas
1. oxyntic mucosa: lines body & fundus ➞ secretes mucus, pepsinogen, HCl, & intrinsic factor
2. pyloric gland area (PGA): lines the antrum

• acid ≠ secreted
• secretes mucus, pepsinogen, & gastrin

Question 10

gastrin

Answer 10

• hormone secreted from pyloric antrum
• circulates back to body & fundus of stomach
• stimulates parietal & chief cells to produce highly acidic gastric juice & pepsinogen

Question 11

pepsinogen

Answer 11

• secreted by chief cells in stomach
• precursor of pepsin that breaks down peptides
• inactive form
• cleaved by HCl to form pepsin:
  
  • can autocatalytically convert
  • initiates protein digestion by splitting peptide bonds
  • self-digestive: stored & secreted in inactive form & only active in gastric lumen

Question 12

intrinsic factor

Answer 12

required for absorption of vit B12

• only secreted from parietal cells
• glycoprotein
• binds B12 to form complex that binds to intestinal receptor cubulin triggering receptor-mediated endocytosis
• vit B12 essential for normal RBC fx
  
  • no B12 = anemia

Question 13

mucosal gland cells

Answer 13

mucous neck cells secrete thin watery mucus

chief cells secrete pepsinogen: precursor of pepsin that breaks down peptides

• pepsinogen stored in zymogen granules: vesicles inside chief cells
• HCl cleaves of pepsinogen forming pepsin

parietal (oxyntic) cells secrete HCl & intrinsic factors

• H+ & Cl- are actively secreted by separate pumps to protect against HCl acid-degradation
• intrinsic factor: required for absorption of vit B12
• degeneration of parietal cells leads to:
  
  • loss of intrinsic factor ➞ most important: only secreted from parietal cells
  • loss of pepsinogen
  • loss of HCl
  • loss of pepsinogen & HCl can be compensated for by enzymes produced in SI & pancreas

surface cell epithelium secrete thick alkaline mucus

Question 14

HCl

Answer 14

• secreted by parietal (oxyntic) cells
• H+ & Cl- are actively secreted by separate pumps ➞ protection against HCl acid-degradation
• H+ actively transported against large [gradient]
  
  • derived from metabolic process: H2O mol form inside parietal cell
• new H+ generated from carbonic acid when H+ secreted
• Cl- transported against much smaller [gradient]
• fxs:
  
  1. activates pepsinogen w/ acid envir
  2. acid breakdown of connective tissue
  3. kills harmful microorganisms from food

Question 15

gastric filling

Answer 15

stomach can stretch up to 20x b/c of smooth muscle

• stomach smooth muscle maintains constant tension of range of lengths by slow-wave potentials

Question 16

gastric mixing & emptying

Answer 16

strong antral peristaltic contractions
* propulsive movements
* gastric mixing in antrum
* mix food w/ gastric secretions to produce chyme
* propel chyme to pyloric sphincter ➞ small amount goes in duodenum
* sphincter only allows some food to enter duodenum ➔ then closes & pushes remaining chyme back into stomach to prevent too much or too big of particles
* factors controlling gastric mixing & emptying
1. in the stomach:
* volume of chyme: distension stimulates motility & emptying via:
1. direct effects of stretch on smooth muscle
2. intestinal plexuses & vagus nerve
3. gastrin (stomach hormone)
* fluidity of chyme: contents must be in fluid form to exit ➞ more liquidy = faster emptying
2. in the duodenum: presence of fat, acid, hypertonicity, & distension inhibit gastric motility & emptying until duodenum has broken down & passed on/digested
* fat cannot empty from stomach until the fat in SI is processed (is only digested & absorbed w/in SI)
* acid: highly acidic chyme from stomach must be neutralized with NaHCO3 (sodium bicarbonate) from pancreas
* hypertonicity: high concentration of AA & glucose in duodenum ↑ osmolarity (more solute in water)
* more solute inside than outside (hypertonic) causes water to diffuse into duodenum & swell ➔ triggers reflex to reduce gastric emptying
* distention = distended (extended & expanded)

Question 17

gastrin

Answer 17

• hormone released from pyloric gland area in stomach into blood
• stimulate parietal cells & chief cells to secrete HCl & pepsinogen
• enhances gut motility
• stimulates ileal motility
• relaxes ileocecal sphincter

Question 18

pancreas

Answer 18

• located behind & below stomach
• all pancreatic enzymes work best in alkaline envir
• made of endocrine & exocrine tissue (exocrine mainly for GI, endocrine for hormones)
• acinar cells: (at bottom) release 3 types of enzymes
  
  • all 3 pancreatic enzymes are packaged into common vesicles ∴ fat or protein triggers release of all 3 ➔ carbs do not trigger release
    1. proteolytic enzymes for protein digestion
    
    1. trypsinogen (inactive) ➞ trypsin (active) by enterokinase = enzyme in luminal border of duodenum
       
       • trypsin can autocatalytically trigger further conversion
         
         1. chymotrypsinogen ➔ chymotrypsin
         2. procarboxypeptidase ➔ carboxypeptidase
       • trypsin inhibitor: substance in pancreas that blocks trypsin in case trypsinogen inadvertently converts trypsin
         
         3. pancreatic amylase for carbohydrate digestion
            * salivary gland also releases amylase
            * not harmful ∴ secreted as active form
         4. pancreatic lipase = only enzyme in digestive tract that can digest fat
• duct cells: release sodium bicarbonate (NaHCO3)

Question 19

acinar cells

Answer 19

• cells of the pancreas
  1. proteolytic enzymes for protein digestion
  
  1. trypsinogen (inactive) ➞ trypsin (active) by enterokinase = enzyme in luminal border of duodenum
     
     • trypsin can autocatalytically trigger further conversion
       
       1. chymotrypsinogen ➔ chymotrypsin
       2. procarboxypeptidase ➔ carboxypeptidase
     • trypsin inhibitor: substance in pancreas that blocks trypsin in case trypsinogen inadvertently converts trypsin
       
       2. chymotrypsinogen ➔ chymotrypsin
       3. procarboxypeptidase ➔ carboxypeptidase
          2. pancreatic amylase for carbohydrate digestion
          * salivary gland also releases amylase
          * not harmful ∴ secreted as active form
          3. pancreatic lipase = only enzyme in digestive tract that can digest fat

Question 20

duct cells

Answer 20

cells of the pancreas that release sodium bicarbonate (NaHCO3)

Question 21

regulation of pancreatic secretions

Answer 21

• hormonally regulated
• duodenum releases 2 major enterogastrones in response to chyme:
  
  1. secretin release triggered by acidic chyme ➞ acts on duct cells to secrete NaHCO3 & inhibits gastric motility and secretion
  2. cholecystokinin (CCK) release triggered by fat & protein in chyme
     
     • inhibits gastric motility and secretion
     • stimulates pancreatic enzyme secretion
     • stimulates gallbladder contraction
     • acts as a satiety signal

Question 22

secretin

Answer 22

• secreted from duodenum triggered by acidic chyme
• acts on duct cells to secrete NaHCO3 & inhibits gastric motility and secretion

Question 23

cholecystokinin (CCK)

Answer 23

secreted from duodenum triggered by fat & protein in chyme
* inhibits gastric motility and secretion
* stimulates pancreatic enzyme secretion
* stimulates gallbladder contraction
* acts as a satiety signal

Question 24

biliary system: general

Answer 24

• liver, gallbladder, & associated ducts
• liver produces bile salts
  
  • much higher concentration emptied into duodenum each meal than in body
  • recycled through enterohepatic circulation
  • lost bile salts replaced by newly synthesized ones
• bile secreted by liver ➔ enters duodenum during meals
• between meals sphincter of oddi closes & bile is diverted to gallbladder for storage until next meal when it reopens
• bile stored in gallbladder
• gallbladder ≠ essential

Question 25

biliary system function

Answer 25

1. aid in fat digestion: **act as detergent to emulsify fat** * **detergent action**: bile salts convert large fat globules to lipid emulsion of many small fat droplets suspended in aqueous chyme ➔ ↑ SA for action by pancreatic lipase * mechanism: * to digest fat lipase must come into direct contact with triglyceride * bile salts dissolve & absorb in surface of fat droplet leaving charged water-soluble part projecting from surface * negatively charged portion cause droplets to repel ➞ cannot recombine into big fat droplet 2. **facilitate fat absorption by forming fat micelles** * micelle: bile salts + lecithin aggregate in small clusters forming hydrophobic core & hydrophilic shell * formed from bile salts + cholesterol + lecithin (also constituent of bile) * if cholesterol is out of proportion with bile salts/lecithin ➔ cholesterol precipitates to form gallstones

Question 26

SI: general

Answer 26

* most digestion & absorption in SI ➞ no digestion occurs after SI * very large SA for absorption: inner surface has circular folds + surface of circular folds has villi + surface of villi have microvilli (brush border) * total ↑ SA 600x

Question 27

SI: movement

Answer 27

* **segmentation:** * **primary method** of motility * oscillating ring-like contractions (not peristalsis) * absent between meals but strong during & after a meal * both **mixes & propels chyme** * chyme moves forward b/c frequency of contractions ↓ gradually along length of SI

Question 28

SI: digestion

Answer 28

* inside lumen with pancreatic enzymes * fat digestion enhanced by bile secretions & completed extracellularly ➞ in lumen * protein & carb digestion completed intracellularly ➞ in brush border * does not synthesize digestive enzyme secreted into lumen * synthesizes digestive enzymes that act intracellularly w/in brush-border membrane of epithelial cells lining lumen * 3 brush-border enzymes: 1. **enterokinase**: activates pancreatic enzyme trypsinogen 2. the **disaccharidases**: maltase, sucrase, & lactase ➞ complete digestion of carbs 3. the **aminopeptidase** complete digestion of proteins

Question 29

carb digestion & absorption in SI

Answer 29

**carbohydrate digestion completed intracellularly** polysaccharides broken down into monosaccharides & enter blood stream via passive diffusion * glucose & galactose via passive carriers ↓[gradient] * fructose via facilitated diffusion

Question 30

protein digestion & absorption in SI

Answer 30

1. hydrolyzed into small peptides & AA by pepsin & pancreatic proteolytic enzymes 2. AA absorbed into cell via Na+ & active transport 3. small peptides broken down into AA by intracellular peptidases & aminopeptidases 4. AA enter blood ↓[gradiets]

Question 31

fat digestion & absorption in SI

Answer 31

1. emulsified by detergent action of bile salts 2. lipases hydrolyze triglycerides into monoglycerides & free fatty acids 3. hydrophobic mol move interior of micelles to cell surface 4. monoglycerides & free fatty acids diffuse into cell then re-synthesize into triglycerides 6. triglycerides coated w/ lipoprotein & form chylomicrons that are exocytosed from cell 7. chylomicrons enter lymphatic vessels

Question 32

LI: fxs

Answer 32

drying & storage

Question 33

LI anatomy

Answer 33

* drying & storage organs * **cecum**: blind-ended pouch below ileocecal valve * **appendix**: finger-like projection of lymphoid tissue) * **colon**: ascending, transverse, descending, & sigmoid * **rectum**: straight, connected to anal canal

Question 34

LI: movement

Answer 34

* one way flow of contents from ileum to cecum to keep bacteria out of SI * ileocecal velve opens in 1 direction only * ileocecal sphincter separates ileum & cecum ➞ opens/closes for regulation * **haustral contractions** slowly shuttle contents of large intestine to aid absorption of H2O & salt * gradually change location forming haustra: pouch * **mass movements**: large contractions in ascending & transverse colon that rapidly drive contents forward * occur 3-4x/d through gastrocolic reflex: triggered by gastrin & extrinsic autonomic nerves * **defecation reflex** causes internal anal sphincter (smooth muscle) to relax & rectum & sigmoid colon to contract * defecation if external anal sphincter is also relaxed * external anal sphincter = skeletal muscle ➞ voluntary control * **constipation**: defecation is delayed too long & too much water is absorbed from feces * **appendicitis**: if hardened feces gets lodged in appendix & obstructs normal circulation & mucus secretion

Question 35

trypsinogen

Answer 35

* inactive form * converted to **trypsin** by **enterokinase**: enzyme in luminal border of duodenum * secreted by acinar cells in duodenum * proteolytic enzyme ➞ acts on proteins * optimal pH = alkaline

Question 36

amylase

Answer 36

* active form * secreted by salivary glands & acinar cells in exocrine pancreas * acts on carbs * optimal pH = 7

Question 37

chymotrypsinogen

Answer 37

* proteolytic enzyme ➞ acts on proteins * secreted by acinar cells in exocrine pancreas * inactive form * converted to **chymotrypsin** by autocatylitic conversion of trypsin

Question 38

enterokinase

Answer 38

* enzyme in luminal border of duodenum * converts trypsinogen to trypsin

Question 39

pancreatic lipase

Answer 39

* fat digestion * only enzyme in GI that can digest fat * released from acinar cells in exocrine pancreas * secreted in active form ➞ no risk of self-digestion

Question 40

bile salts

Answer 40

* derivitives of cholesterol * produced by liver * recycled through enterohepatic circulation 1. aid in fat digestion by acting as detergent (emulsifying fat) 2. facilitate fat absorption by forming fat miscelles * convert large fat globules to lipid emulsion made of many small fat droplets suspended in aqueous chyme * ↑ SA for pancreatic lipase to digest fat * cause fat droplets to repel each other ensuring droplets don't reform to 1 large one

Question 41

gallstones

Answer 41

precipitate that forms in gallbladder when amount of cholesterol is disproportionate to bile salts & lecithin

Question 42

sphincter of Oddi

Answer 42

muscle at the opening of the bile duct into the duodenum * prevents bile from entering the duodenum except during digestion of meal * when sphincter is closed bile is diverted back up into the gallbladder

Question 43

gallbladder

Answer 43

* small, saclike structure tucked beneath but not directly connected to the liver * stores and concentrates bile between meals

Question 44

lecithin

Answer 44

component of bile that helps form micelles because of hydrophilic & hydrophobic properties

Question 45

enterogastrones

Answer 45

hormones secreted by the duodenal mucosa that regulate gastric emptying & pancreatic secretions * inhibit gastric motility and secretion * two hormones 1. **secretin** acts on duct cells to secrete NaHCO3 & stimulated by acidic chyme in duodenum 2. **cholecystokinin (CCK)** acts on acinar cells