GI Physiology Flashcards
3 fundamental processes of digestive system
1) secretion (enzymes, mucus, ions, hormones)
2) absorption (water, ions, nutrients)
3) motility (crush, mix, propel)
where does the most chemical digestion occur?
small intestine
total amount of water secreted
9.3 L
total amount of water reabsorbed
9.2 L
only lose .1 L
GI sphincters
1) upper esophageal
2) lower esophageal
3) pyloric
4) oddi
5) ileocecal
6) internal anal
7) external anal (under voluntary control)
function of GI sphincters
keep digestive material moving in one direction
regulation of UES during swallowing
by enteric nervous system
Sensory, modulatory, and motor neurons innervate circular and longitudinal layers
basic structure of GI tract
epithelial cells lamina propria muscularis mucosa submucosal plexus circular muscle myenteric plexus longitudinal muscle serosa
major salivary glands
all PAIRED
1) parotid
2) submandibular
3) sublingual
minor salivary glands
600 in mouth
Sampled via mucosa when testing for abnormalities
major functions of saliva
1) moistens mucosa
2) moistens dry food and cools hot food
3) taste
4) acts as a buffer (high conc of bicarbonate ions)
5) digestion (a-amylase, lingual lipase)
6) controls bacterial flora
7) mineralization of new teeth and repair of enamel lesions (high calcium/phosphate)
8) antibacterial
alpha amylase in saliva
Breaks 1-4 glycoside bonds
only works for a short time bc only functional at neutral pH
lingual lipase
breaks down fats
only works for a short time bc only functional at neutral pH
How to ductal cells modify saliva as it is secreted?
Saliva becomes more isotonic as it passes along duct.
Fewer aquaporins –> force water to stay in saliva.
Low sodium content of saliva aids in detection of salt in diet
causes of increased saliva formation (positive regulators)
Conditioning (pavlov).
Food.
Nausea.
Smell.
causes of decreased saliva formation (negative regulators)
Dehydration.
Fear.
Sleep.
Anticholinergic drugs.
regulation of saliva formation occurs through ____ branch of the autonomic nervous system
Parasympathetic branch.
via ACh
gastrin (source, target, action)
Source: antrum of stomach
Target: parietal/chief cells in stomach
Action: increase H+ / intrinsic factor secretion, increase pepsinogen secretion
34aa, 17 aa
CCK (source, target, action)
Source: duodenum and jejunum
Target: pancreas and gallbladder
Action: increase enzyme secretion, increase contraction
33 aa
secretin (source, target, action)
Source: duodenum
Target: pancreas ducts and bile ducts
Action: increase HCO3/fluid secretion by pancreatic/bile ducts
27 aa
gastrin releasing peptide (GRP)
source, target, action
Source: vagal nerve endings
Target: antrum of stomach
Action: increase gastrin release
somatostatin (source, target, action)
Source: stomach and duodenum
Target: stomach, pancreas, liver
Action: decrease gastrin release, decrease endocrine/exocrine secretions, decrease bile flow
gastrin inhibitory peptide (GIP)
source, target, action
Source: duodenum and jejunum
Target: pancreas
Action: decrease fluid absorption
chemical digestion in stomach
Protein digestion begins (main stomach fxn).
Acid denatures protein, activates protein digesting enzyme.
Mucus prevents stomach cells from being digested.
functions of stomach
1) reservoir - compliance allows increase in volume with little increase in pressure
2) storage - allows digestion by salivary/lingual enzymes
3) mixing - fluid/food/gastric enzymes
4) kneading of food - to less than 1 mm
5) metered emptying - in response to duodenal feedback
6) vomit - defense against harmful substances ingested
how do we get rid of things that we can’t digest larger than 1 mm from stomach?
Corn, fiber, etc.
Emptied between meals
stomach secretions
1) hydrogen ion
2) pepsinogens
3) mucus
4) intrinsic factor
5) water (2.5L/day)
function of hydrogen ion in stomach
Converts pepsinogen into pepsin.
Kills microbes.
Denatures protein
intrinsic factor
Produced in stomach.
Aids in absorption of vit B12
D cells
Secrete somatostatin (inhibits gastrin release)
Triggered by low stomach pH
G cells
Secrete gastrin (stimulate acid secretion, pepsinogen secretion)
gastrin release stimulators
Peptides/amino acids. Stomach stretch (via GRP release).
gastrin release inhibitors
H+ ions (low pH)
H+ stimulates D cells, which then inhibit G cells
gastrin effect chain on other cells
Stimulates enterochromaffin-like cell –secretes histamine–> chief cell –secretes pepsinogen
Stimulates chief cells directly to produce pepsinogen.
Stimulates parietal cells directly to secrete H+
gastric pit components
Parietal cells (acid, intrinsic factor)
ECL cell (histamine secretion)
Chief cell (pepsinogen)
resting parietal cell
pH 1-2
H+/K+ ATPases are sequestered in tubulovesicles in parietal cells.
Minimal ATPases in cell membrane
activated parietal cell
Stimulation causes vesicles to fuse with cell membrane, leading to more H+/K+ ATPases inserted into membrane.
Allows increased movement of hydrogen ions into the stomach.
Stimulated by ACh, histamine, gastrin
__% of the pancreas is dedicated to producing digestive enzymes
80%
which pancreatic cells synthesize and secrete the digestive enzymes?
acinar cells
pancreatic duct cells
secrete bicarbonate
pancreatic ducts
transport enzymes to beginning of small intestine
active pancreatic enzymes
Lipase.
a-Amylase.
Trypsin inhibitor.
Nucleases.
inactive pancreatic enzymes (proenzymes)
[all are proteolytic] Trypsinogen. Chymotrypsinogen. Proelastase. Procarboxypeptidase A/B
If they were secreted in an active form, they would damage the ducts.
major route of pancreas stimulation
Partially digested fats/proteins cause CCK release, increasing amount of digestive enzymes. (acts on acinar cells)
Acidity in intestine causes increased sodium bicarbonate release from ductal cells. (acts on ductal cells)
minor route of pancreas stimulation
seeing/smelling food causes parasympathetic impulses along vagus (CN X) nerves.
Which enzyme activates all pancreatic proteases?
Trypsin
enterokinase
cleaves trypsinogen to active form (trypsin)
apical/intrinsic (trapped in brush border)
first pass effect
Hepatic metabolism of pharmalogical agent when it is absorbed from the gut and delivered to liver via portal circulation.
Greater first pass effect = lower distribution of drug (if administered orally)
basolateral side of hepatocytes vs apical side
basolateral: blood flows thru cords (portal vein, cords, central vein)
apical: secrete bile (bile flows thru canaliculi to duct)
the liver stores:
Glucose in form of glycogen. Fat soluble vitamins (ADEK). Folate. Vitamin B12. Minerals (copper/iron)
bilirubin
Only component of bile that is not made/secreted by liver.
From macrophages clearing RBCs.
function of water, ions, HCO3 in bile salts
Neutralize pH.
Dilute chyme.
synthesis of bile acids
All start from cholesterol.
Primary: made exclusively in liver
Cholic acid.
chenodeoxycholic acid.
secondary bile acids
Made by gut bacteria.
Conjugated with taurine or glycine.
Functionally equivalent to primary bile salts.
bile acid structure
Ampipathic.
Hydrophobic side is attracted to fat.
Hydrophilic side binds to water.
Allows emulsification of fat, micelle formation.
gall bladder when there’s no food
Gall bladder expands.
Sphincter of Oddi is closed.
Gallbladder pumps out ions, so water follows, concentrating the bile.
gallbladder after ingestion of food
Gallbladder contracts.
Sphincter of Oddi relaxes.
via CCK and neural stimulation.
emulsification of fat
1) bile acids emulsify fat droplets.
2) hydrolysis of triglycerides to FAs and monoglycerides
3) dissolve FAs/MAGs into micelles (mixed micelles)
why form micelles?
Micelle solubilization of polar lipids greatly increases their rate of diffusion to epithelial surface of small intestine.
Essential for absorption of lipids.
micelle structure
Hydrophobic core:
Cholesterol,
TAGs,
Phospholipid tails.
Hydrophilic external:
Bile salts,
Phospholipid heads,
MAG heads.
major regulation of bile secretion
FAs and AAs in chyme entering duodenum stimulate secretion of CCK into blood. CCK: opens sphincter of Oddi, contraction of gallbladder.
Acidic chyme stimulates secretion of Secretin into blood. Secretin: secretion of bicarbonate from liver bile ducts.
minor regulation of bile secretion
Parasympathetic impulses from vagus nerve (CN X) stimulate bile production by liver.
mechanisms to increase surface area of small intestine
Cylinder.
Folds.
Villi.
Microvilli.
Amplified 20x
small intestine epithelial cells
1) absorptive cell: AAs, glucose, monoglycerides, membrane enzymes
2) goblet cell - mucin
3) enteric endocrine cell: gastrin, CCK, secretin
4) stem/progenitor cell: cell renewal
5) paneth cell: lysozyme to kill bacteria
absorptive cell of small intestine
Absorbs AAs, glucose, monoglycerides
contains membrane enzymes
enteric endocrine cells
Small intestine.
Secrete gastrin, CCK, secretin.
secreted digestive enzymes
Amylase. Lipase. Pepsin. Trypsin. Other proteases.
All enter small intestine, mix with chyme.
cell membrane bound digestive enzymes
Enterokinase.
Disachharidases.
Peptidases.
Final products of digestion.
Close to cell transporters for easy/quick uptake into cell.
NOT secreted. Digestion occurs at cell surface.
main digestive enzymes: carbohydrates
Amylase (salivary glands, pancreas). [secreted] - break sugars down to disaccharides
Disaccharidases (small intestine) [cell membrane bound] - break disaccharides to monosaccharides
transport of glucose into enterocytes
Uses glucose/Na+ co-transporter
main digestive enzymes: protein
Pepsin (stomach) [secreted]
Acid (stomach) [secreted} - denatures protein.
Proteases: trypsin, chymotrypsin, carboxypeptidase, elastase (pancreas, secreted) - each cleave at a different AA to form different smaller pieces
Peptidases (small intestine, membrane bound) - cleave to individual AAs
transportation of amino acids
through bloodstream
transportation of fats
through lymph.
eventually to bloodstream
main digestive enzymes: fat
Lingual lipase (mouth, secreted).
Pancreatic lipase, phospholipase, cholesterol esterase (pancreas, secreted)
process of fat digestion and absorption
1) bile salts coat fat droplets.
2) pancreatic lipase breaks fats down into MAGs and FAs stored in micelles.
3) MAGs/FAs move out of micelles and enter cells by diffusion.
[cholesterol is transported into cells]
4) absorbed fats combine with cholesterol/proteins in enterocytes to form chylomicrons.
5) chylomicrons removed by lymphatic system
transport of protein components into enterocytes
amino acids transported by Na+ co-transporter
small peptides (2-3 aa) have separate transporter - must be further digested by peptidases in cytoplasm before transport in capillaries
