GI Physiology Overview Flashcards
What is a bolus?
a rounded mass of food ready to swallow
What do we call abdominal rumbling sounds
borborygmi
What is chyme?
a semifluid mass of partly digested food passed form the stomach to the duodenum
What is the technical term for eructation?
belching
What’s sitophobia?
fear of eating
What is the term for fatty stools?
steatorrhea
What organs make up the upper GI system?
oral cavity pharynx esophagus stomach stmall intestine
WHat makes up the lower GI system?
large intestine
What are the sections of the small intestine?
duodenum
jejunum
ileum
What are the sections of the large itnestine?
cecum, colon, rectum, anus
What are the accessory organs of the GI system?
salivary glands
exocrine pancreas
liver
gallbladder
What regulates movement in the GI tract and allow some compartments to act as reservoirs?
the sphincters
Can you name all the sphincters in the GI tract?
the upper and lower esophageal sphincters
pylorus leaving the stomach
sphincter of oddi to the liver
ileocecal valve between the small int and the cecum
lastly the internal and external anal sphincters
What sphincters do we ahve voluntarily control over?
upper esophageal and external anal
What organs have vili and crypt? Which have only cryps?
the small intestine has both.
the stomach and large intestine just have the crypts
How do GI epithelial cells differ from most other cell types?
they undergo constant renewal
Describe how GI epithelial cells are renewed?
Stems cells at the base of the crypts divide and differentiate as they migrate up to the tips of the villi where they then slough off as new cells push their way up?
How many days do GI epithelial cells live?
3-6 days
Why is this constant GI epithelial renal so important?
they’re exposed to toxins all the time, which increases their rate of mutation. so sloughing them off is a really good idea
What makes diverticuli of the esophagus “false”?
it means that not every layer bulges out - it’s just the mucosa and serosa that pushes thorugh the others
What is mucus made of?
mucin
Describe the structure of mucin
why is this structure important?
it’s a protein monomer combined into complexes by disulfide links
highly glycosylated to protect protein core from proteases
How may layers of mucus are in the stomach an colon? In the small intestine?
stomach and colon have 2 layers
small intestine has 1
What are the four basic processes that facilitate caloric uptake? Which have excess capacity and which require some control?
digestion, absorption (those two have excess capacity)
secretion and motility (under neural and hormonal control)
What do the teeth and saliva do for digestion?
teeth masticate the food and saliva produces lubrication and amylase to create a bolus
What will further digest the bolus to create chyme?
stomach movements and pepsin
In GENERAL TERMS, what does the small intestine use to digest down the chyme?
enzymes on its brush border and enzymes from the pancreas
Why are the villi so important?
they massively increase surface area for massive absorption of nutrients
What structures are located in the villi to absorb nutrients?
capillaries absorb most nutrients, but lacteals are necessary for fat absorption - the fat is too big to get into the capillaries so they go through the lymph first
In what sections of the small intestine are amino acids, monosaccharides and lipids absorbed?
the duodenum and jejunum
What chemical form the liver helps facilitate absorption of lipids?
bile salts
Where is cobalamine absorbed in the mall intestine?
the ileum
How about the bile salts and acids?
ileum also
About how much of the body’s blood flow goes to the gut at rest?
around 21%, which is amazing because th gut is only 5% of body mass
All the blood flowing thorugh the intestines leaves through what vein? Why?
through the portal vein to the liver for detoxification
THe livers receives approximately 1/4 of it’s blood supply from where? 3/4 from where?
1/4 from hepatic artery and 3/4 from portal vein
How much can blood flow increase during heavy digestion?
4-5 times
In general terms, how are bile salts recycled?
thorugh the enterohepatic circulation
secreted from liver and then brought back in the blood
What are some metabolic vasodilators that increase blood flow in the small intestine?
CO2
H+
K+
Adenosine
How do lacteals eventually empty into the blood stream for the fats to get used?
via the thoracic duct
What are the general causes of mesenteric ischemia?
occlusive mechanisms like thrombi (infarction)
non-occlusive mechanisms like prlonged reflex vasoconstriction due to hypovolemia or abnormal levels of circulating vasoconstrictors like epinephrine or angiotensin 2
WHat are the effect sof mesenteric ischemia?
postprandial pain and sitophobia
necrosis of the tips of villi
loss of varrier function fo the wall fo the gut with uptake of vasodilator endotoxin from bacteria in the gut resulting in septic shock
Why is the fluid environment of the lumen important?
it aids in the uptake of nutrients and minimizes damage to epithelium
Where does the fluid in the lumen come from?
1 ingested fluid - 2 L or so
- 1.5 L in saliva
- .5 L from bile
- 1.5 L from pancreas
- 1.5 from intestinal secretions
Where is most of the water reabsorbed?
the majority is from th esmall intestine - about 8.5 L!
But the large intestine absorbed about .4 L, which is 90% of what it receives, so it’s considered the “dessicator”
How does the water get into the lumen?
- through the cells with the help of aquaportins via osmosis
- between cells according ot osmotic gradient through junctions
so the conductivity of the epithelia depends on the variable presence of tight junctions
What largely mediates fluid uptake in the small intestine?
the nutrients are being absorbed and the fluid follows via osmosis
What are the two general types of diarrhea?
osmotic: something is making there be more nutrients or other particles in the lumen, keeping the water inside
secreotry” something is making the secretion of water into the crypts increased (typically by increased secretion of Cl)
What are the effects of diarrhea (either type)?
dehydration and electrolyte imbalances with metabolic disturbance
What are the two main things that fluid flux depends on ?
surface area available for ion transport and residence time in the lumen
(this is why some antidiarrheals work to slow transit time to increase fluid absorption - loperamide)
What controls length of time for digestion and absorption?
motility
What is the average transit time for the following: esophagus stomach small intestin large intestine
esophagus; 10 sec
stomahc: 4-5 hours
small intestine: 2.5-3 hours
large intestine: 30-40 hours!
How are the smooth muscle cells coupled to enable contractile waves to spread?
gap junctions or nexuses
What are the two kinds of contractions that occur?
rhythmic PHASIC contractions that last for seconds
long TONIC contractions that last for minutes to hours
Where do we particularly want basal resting tension “tone” in the IG system?
sphincters
True or false: the basal tone is maintained with an increase in intracellular Ca++ with energy expenditure in the pshincters.
false - no increase in intracellular Ca and no energy expenditure
True or false: GI smooth muscle has a remarkable ability to shorten
true - can shorten to 50% of its resting length
What mechanical occurence can trigger depolarization leading to contration?
stretch
What will trigger relaxation of a sphincter and what will trigger contraction of a sphincter?
proximal pressure relaxes
distal pressure contracts
this is what makes them one-way valves
WHat cells initiate the slow wave electrical activity in the GI system?
interstitial cells of cajal
What are slow waves generated due to?
increase in Ca followed by repolarization of K channels
Is it the amplitude or frequency of slow waves that can ve altered by signals releasing calcium from internal stores or opening Ca channels on plasma membrane?
amplitude
NOT frequency
What contrations will propel intestinal contents forward?
peristaltic contractions
What contractions will mix contents but not propel them forward?
segmenting contractions
What does the migrating motor complex do?
it’s relaxation of sphincters and contractions in the stomach and small intestine during FASTING in order to clear out things that aren’t digestable
this is why gum won’t stay in you for 7 years
what hormone controls the migrating motor complex?
motilin
What sets of nerves can operate autonomously to mediate short reflexes INDEPENDENT of input from the CNS?
the submucosal nerve plexus and the myenteric nerve plexus
Where are the submucosal and myenteric located? both in terms of where in the layers and in which secctions….
submucosal: in the submucosa of the small and large intestine (meissner’s)
myenteric: between the circular and longitudinal muscle lsyers from the esophagus all the way to internal anus
What are the types of receptors in the GI tract that detect changes?
stretch receptors
osmoreceptors
chemoreceptors
True or false: there are taste receptors throughout the GI system.
true, but they don’t really give the sensation of taste per se
What will excite the afferent sensory neurons of the enteric nervous system?
fast distention of the gut wall or chemical signals from the lumen of the gut transmitted to sensory neurons
What neurotransmitter will many of the sensory neurons in the gut be activated by?
serotonin released form the mucosal enterochromaffin cells
Which plexus contains the majority of efferent motorneurons in the enteric nervous system?
the myenteric plexus
What information will be carried by the unipolar efferent motorneurons bia sustained trains of action potentials?
efferent info to GI smooth muscle, vascular smooth muscle, GI exocrine secreotry cells and GI encodcrine secretory cells
What will the excitatory fibers of the efferent motorneurons release?
acetylcholine
neurokinin K
substance P
WHat will the inhibitory fibers of the efferent motorneurons release?
vasoactive intestinal peptide and nitric oxide onto smooth muscle cells
Can quadriplegics still digest food despite no long reflexes from the CNS?
Yes, but control of secreiton and motility may be less than optimal
What is the extrinsic autonomic nervous system signal to the GI system made up of?
parasympathetic from the vagus nerve stimulates activity
sympathetic inhibits activity
Again, neural activity can increase or decrease contractile activity by influencing the ____ of the slow electrical waves.
the amplitude (not frequency)
Describe the vago-vagal reflex.
vagal afferent info is transmitted to ANS centers in the medulla
vagal efferents coordinate excitatory and inhibitory activity within the ENS to mediate peristalsis
What is released on the smooth muscle proximal to a bolus? How about distal to a bolus?
ACh is released proximally to contract
NO is release distally to relax
What NT from the vagal nerve stimulates G cell secretion of gastrin?
gastrin releasing peptide
What excitatory transmitter is generally go-released with ACh?
substance P
What does gastrin do?
the G cells in the antrum of the stomach detect amino acids and rlease gastrin, which increases pepsiongen and H release
What cells release chleycystokinin in response to fat and amino acids?
I cells in the D/J junction
What does cholecystokinin do?
triggers the secretion of pancreatic enzymes and bile salts to aid in fat uptake
What cells will secrete secretin? In response to what?
S cells in the D/J junctin detect acid and secrete secretin
What does secretin do?
stimulates the pancrease to secrete bicarb to neutralize the acid
also inhibits gastric motility
What do K cells in the D/J secrete in response to carbs and fat?
gastric inhibitory peptide (GIP)
What does GIP do?
it inhibits gastric acid secretion and stimulates insulin release from the pancreas
What’s secreted by endocrine cells cyclically during fasting?
motilin - to initiate the migrating motor complex
What do EDL cells release to further modulate the GI response?
Histamine (in reponse to gastrin release) to increase H release
What will release somatostatin to further modulate the response?
D cells - inhibits acid release
Describe digestive enzyme release in general terms?
synthesis by specialized cells
packaged into zymogen granules and stores
secretion after stimulus-induced increase in either cAMP or Ca2+ leading to xymogen granule docking and dumping
What are the ways gas is produced in the GI system?
- air swallows
- neutralizaiton of gastric acid by bicarb generates CO2
- gas byproducts of fermentation of fiber in colon
What are some sympmtoms or issues that could arise from bacterial overgrowth syndrome due to delayed small intestine transit and diverticulum?
gas and bloating from fermentaitn
compete for B12 leading to anemia
deconjugate bile acids leading to steatorrhea
produce toxins altering epithelium
How can you diagnose bacterial overgrowth syndrome
hydrogen breath test
What are some properties of the gut that limit colonization by microorganisms?
- IgA, lactoferrin and lysozyme in saliva and mucus
- acidic
- lots of lymphocytes
- paneth cells secrete antimicrobial peptides
- digestive enzymes
- diarrea/vomiting
- normal flora
- peristalsis, ileoceccal valve
