Flashcards in Prunuske: GI Function and Physiology Deck (145):
What is a bolus?
rounded mass of food ready to swallow
What is borborygmi?
abdominal rumbling sounds
What is chyme?
semifluid mass of partly digested food passed from the stomach to the duodenum
What is the diverticulum?
Outpouching of GI wall
What is eructation?
What is peristalses?
Propulsion of food through the esophagus and intestines
What is posprandial?
What is sitophobia?
fear of eating
What is steatorrhea?
What structures are in the upper GI tract?
What structures are in the lower GI tract?
What are the gross structures of the small intestine?
What are the gross structure of the large intestine?
What are the accessory organs of the GI system?
Salivary glands (parotid, submandibular, sublingual) Exocrine Pancreas
Liver- Hepatic system Gallbladder- Biliary system
What regulates movement in the GI tract and allows some compartments to act as reservoirs?
Internal and external anal sphincters
What sphincters can you voluntarily control?
External Anal sphincter
What is the purpose of mucosal folding? How do villi differ between the small intestine and the colon?
Sml intesetine- villi + crypts
Colon/stomach- flatter + crypts
How are GI cells unique from other cells?
They undergo constant renewal!
Describe how GI cells are renewed.
STEM cells at the base of the crypts divide, differentiate and migrate to the tips of the villi
How often are GI cells shed and why is this important
Apoptosis occurs every 3-6 days and sheds cells into the lumen
Prevents the accumulation of mutations d/t toxin exposure
What are the 4 major layers of the GI wall?
What is a diverticulum?
Single pouch protruding from an alimentary tract (usually false)
True- includes all 4 layers
False- includes serosa and mucosal layer
Why is it important to have fiber in your diet?
Lack of fiber can lead to diverticulosis which means you have multiple diverticula, which can progress to diverticulitis...which is bad.
What secretes mucus in the GI tract?
What is MUCIN?
It's a protein that makes mucus!!
protein monomers combined into complexes by disulfide links
How many layers of mucin are there in the stomach and colon vs the small intestine?
2 layers in the stomach and colon, 1 layer in small intestine
What protects the protein core of mucin from proteases?
glycosylation (sugar groups prevent sequence specific recognition by proteases)
What is coated w/ transmembrane mucins?
What are the 4 basic processes facilitating caloric uptake?
Digestion, absorption (excess capacity)
Secretion, motility (under nerual/hormonal control)
How do you make a bolus?
TEETH masticate food and SALIVA provides lubrication and AMYLASE to create a bolus
How do you make chyme?
Stomach movements +
pepsin further digests food>
What type of enzymes does the duodenum contain and what gives it additional digestive enzymes?
brush border enzymes
Where does most abosrption occur?
villi (gives small intestine more SA)
What transports nutrients and ions?
Enterocytes--> CAPILLARIES (transport the absorbed nutrients)
What transports most fats to the lymph?
Lacteals (TOO BIG TO ENTER CIRCULATORY SYSTEM)
What is absorption?
Transport of nutrients across epithelium into blood or lymphatic system
Where are AA, monosaccharides and lipids absorbed?
What helps to facilitate hte absorption of lipids?
Bile salts from the LIVER that act as DETERGENTS
Where are cobalamine (B12) and bile salts/acids absorbed?
usually the ileum-- need to be recycled so you have enough to coat a fat molecule
Where are water and electrolytes absorbed?
small and large intestines
What does absorption depend on?
Splanchnic circulation (all the blood vessels in the gut)!
What is normal blood lfow?
1 to 5 liters/min depending on regulators of vasomotor tone
At rest, how much blood flow goes to the gut?
21% (gut is about 5% of body mass)
Where does all blood flow leaving the intestines go? Why?
Blood supply to the liver comes from....
hepatic artery (1/4 of blood)
portal vein (3/4 of blood)
What happens to blood flow to the gut when it's really active?
it increases 4-5 fold
What happens to blood flow to the gut during exercise and emergency situations?
vasoconstriction SHUNTS blood away
Is it a bad idea to swim after eating?
Only after consuming alcohol.
What is enterohepatic circulation?
Recycling of bile salts/acids through the hepatic portal vein
- don't need to synthesize as many new bile ducts
*some drugs/bacteria can modify bile salts and make them less easier to reabsorb
What are the metabolic vasodilators that increase blood flow?
What can alter blood flow in the gut?
autonomic and enteric neural influence
What happens to chylomicrons?
Too large to pass through capillary cells SO they're absorbed by LACTEALS which empty in to the bloodstream via the THORACIC DUCT
What are the causes of mesenteric ischemia?
1. occlusive mechaisms (thrombi, mesenteric infarction)
2. non-occlusive mechanisms- prolonged reflex vasoconstriction (hypovolemia, HF) or abnormal levels of circulating vasoconstrictors (Epi, Ang II)
How does a pt who has had a mesenteric infarction present clinically?
Post prandial pain
Sitophobia (fear of eating)
What has happened on a physiological level to a pt w/ mesenteric infarction?
Necrosis of the tips of the villi>
Loss of barrier function of the wall of the gut and uptake of vasodilator toxins from gut> SEPTIC SHOCK
Why is a fluid environment vital to the GI system?
supports uptake of nutrients and minimizes DAMAGE to epithelium
What supplies fluid to the GI tract?
Organs that drain into the GI tract
GI epithelial cells
What does fluid in the GI tract contain?
What happens to the majority of fluids?
reabsorbed in the small intestine w/ nutrients
What regulates secretion?
What is transmural water movement?
Water moves across the epithelium driven by PRESSURE created by OSMOTIC gradients
a) THROUGH CELLS (transcellular route) perhaps with the help of specific water channels (aquaporins)
b) BETWEEN CELLS (paracellular route) according to osmotic gradients driven by active electrolyte transport
What affects conductivity (leakiness) in the GI tract?
Presence or absence of tight jxns
What mediates fluid uptake in the small intestine?
absorption of NUTRIENTS in the small intestine (SGLT-1)
What is diarrhea?
an increase in water in the lumen
What is osmotic diarrhea?
small bowel overgrowth (bacteria/defect in digestion of carbs)>
increased production of ORGANIC ACIDS in the LUMEN>
pulls water from blood stream by OSMOSIS
What causes secretory diarrhea?
infection leads to EXCESS secretion of Cl drawing water INTO the lumen
What are the effects of diarrhea?
(When 8.5 L aren't absorbed)
What affects fluid flux?
SA available for ion transport
Residence time in the lumen
-decrease transit time and INCREASE absorption
How does loperamide work?
SLOWS transit to increase fluid absorption
What controls the length of time for digestion and absorption?
What is the average transit time in the esophagus, stomach, SI and LI?
4-5 hrs stomach
2.5-3 sml intestine
30-40 lg intestine
What is most of the GI tract composed of?
Smooth muscle (long slender cells separated into branching bundles and covered by CT)
Where is striated muscle in the GI tract located?
external anal sphincter
What are gap junctions or nexuses in smooth muscle?
low ressistance electrical coupling between cels to enable the conteracation wave to spread to adjacent cells
What modulates activity/frequency of the smooth muscle/gap junctions?
NT released form intermittent swellings along the nerve axon
What is the difference between a phasic and a tonic contraction?
TONIC contractions (min to hrs)--> most important at sphincters
How is basal resting tension or TONE in GI smooth muscle maintained?
WIHTOUT elevation of intracellular Ca and WITHOUT energy expenditures (sphincters)
How much can GI smooth muscle shorten by?
What stimulates GI smooth muscle to contact?
What are sphincters?
one way valves
What causes relaxatin of a sphincter?
What causes contraction of a sphincter?
What initiates slow wave electrical activity in GI smooth muscle?
Interstitial cells of cajal initiate then
propagated over a few cm
What causes slow wave generation in GI smooth muscle?
Increase in Ca followed by repolarization by K channels
How do you alter the AMPLITUDE (not the frequency) of the slow waves?
signals releasing calcium from internal stores or opening Ca++ channels on plasma membrane
What initiates excitation contraction coupling in smooth muscle cells?
Increase in intracellular Ca ion concentration
Describe the pathway for contraction of GI smooth muscle.
ACh binds MR>
increased influx of Ca into cell>
activation of calmodulin-dependent myosin light chain kinase>
phosphorylation of myosin>
increased myosin ATPase activity and binding of myosin to actin>
What are peristatltic contractions?
Propel intestintal contents FORWARD
What are segmenting contractions?
contractions of circular muscles in small and
large intestine that locally mix contents (churning), nonpropulsive
What is the migrating motor complex?
relaxation of sphincters and contractions in stomach and small intestine occurring during fasting controlled by hormone MOTILIN
Does it take 7 years to digest gum?
NO migrating motor complex helps clear things out that we're not able to digest
What is the mini brain of the gut?
enteric nervous system (100 million neurons)--> can act alone to mediate short reflexes INDEPENDENT of input from the CNS
(Submucosal and Myenteric nerve plexuses)
Where is the submucosal nerve plexus located?
SMALL and LARGE intestine
fxn: sensory and regulates blood flow,
What is the myenteric nerve plexus?
between circular and longitudinal muscle layer from esophagus to internal anus
What detects changes in the GI tract?
intrinsic sensory receptors:
stretch receptors, osmoreceptors, and chemoreceptors
Activation of receptors in the GI tract leads to...
release of signaling molecules or upregulation of transporters
Do chemosensitive taste receptors always evoke taste?
NO, but they do localize to multiple spots in the body!
What does enteric nervous system consist of?
afferent sensory neurons
What excites the afferent sensory neurons of the gut?
1. fast distension of the gut
2. chemical signals from the lumen
What stimulates the sensory neurons?
serotonin (5HT) released form ECL (monitor what's going on)
-->respond w/ a few AP followed by hyperpolarization/adaption after a while
What do the afferent sensory neurons do?
transfer information about gut environment to INTERNEURONS in myenteric plexus which relay signals up and down the gut
Where are efferent motorneurons primarily found?
What type of structure do efferent motor neurons have? What excites them? How do they respond?
sustained trains of APs
What organs do efferent motorneurons affect?
carry “efferent” information to GI smooth muscle, vascular smooth muscle, GI exocrine secretory cells and GI endocrine secretory cells
What is the the difference between excitatory and inhibitory fibers that affect efferent motorneurons?
Excitatory fibers release acetylcholine, neurokinin A and substance P
Inhibitory fibers release vasoactive intestinal peptide (VIP) and nitric oxide (NO) on smooth muscle cells.
Short neural reflexes affecting GI function involve...
nerves of the enteric plexus
Long neural reflexes affecting GI funciton involve...
both CNS and ANS
_________ activate both autonomic nervous system and enteric nervous system promoting secretion and motility.
How does being a quadraplegic (w/out higher center) affect your GI control?
GI can operate fine, but control of secretion and motility may be less than optimal
Afferent information leaving the GI tract is carried by....
the Extrinsic autonomic nervous system
How does parasympathetic activity affect the GI tract? Through which nerve?
Stimulates activity of the enteric plexuses, increases GI MOTILITY and SECRETORY activity.
REST AND DIGEST
Cholinergic of VAGUS nerve
How does sympathetic activity affect the GI tract? through which nerve?
INHIBITS activity of the enteric plexuses, decreases GI motility, contracts GI sphincters, constricts GI microvasculature
What is the difference between parasympahetic activity and sympathetic activity on contractile waves?
parasympathetic increases activity, cholinergic, vagus
“rest and digest”
- sympathetic stimulation decreases activity. EPINEPHRINE
What is the vago-vagal reflex?
Vagal afferent info is transmitted to autonomic centers in the medulla
Vagal efferents coordinate excitatory and inhibitory activity within the Enteric Nervous System to mediate peristalsis
the primary EXCITATORY transmitter from sensory cells and from motoneurons to muscle, epithelium, secretory cells and at interneuronal junctions.
INCREAES INTRACELLULAR Ca++
* can also cause release of NO/VIP which is inhibitory
Gastrin release peptide (GRP)
released from vagal nerve endings to stimulate G cell secretion of GASTRIN
GASTRIC ACID SECRETION
Substance P (tachykinin)
an EXCITATORY transmitter generally co-released with acetylcholine
Vasoactive intestinal peptice (VIP)
Promotes motility (FORWARD MOVEMENT)
RELAXES smooth muscle in esophagus and stomach
Stimulates fluid secretion and promotes dilation of the GI vasculature. INCREASES cAMP
**IMPT in relaxing tonically active sphincter
an inhibitory transmitter co-release with VIP from inhibitory motoneurons, hydrophobic- intracellular targets
G cells in antrum of stomach detect AA leading to PEPSINOGEN and H+ release from parietal cells
As cells move from stomach to sml intestine:
I cells in D/J detect FAT and AA>
secretion of PANCREATIC ENZYMES and BILE SALTS
involved in fat uptake
S cells primarily in D/J detect ACID stimulates secretion of BICARBONATE (pancreas) and inhibits gastric motility
Gastric Inhibitory Peptide or Glucose Dependent Insulinotropic Peptide (GIP
K cells in D/J detect CARB and FAT to inhibit gastric acid secretion and stimulate INSULIN release from pancreas.
secreted by endocrine cells; released cyclically during FASTING STATE to initiate Migrating Motor Complex
Portal blood concentration of which substance will increase when the lumen of the small intestine is perfused with a buffered solution of pH3?
What are paracrine regulators?
Histamine (ECL cells)
Somatostatin (D cells)
Where does synthesis of secretions occur?
In specialized cells and products are packaged into ZYMOGEN granules and stored
What stimulates secretion?
stimulus-induced increase in either cAMP or [Ca+2]i (or both) … which then leads to zymogen granule docking and dumping
What is the forgotten organ of the gut?
Human body 10^13 cells
Gut 10^14 microorganisms (500 different species, mostly ANAEROBES- difficult to culture) and quadrillion viruses
The lumen of the gut is equivalent to being outside the body
Regional and developmental variability
Create physiological inflammation within GI track
Reside in outer mucus layer of the large intestine
Constitute 60% of fecal mass
Changes in microbiota are associated with obesity and inflammatory diseases
What causes burping?
Air swallowed during eating is released by eructation
What generates CO2 in the gut?
neutralization of gastric acid by bicarb
Femermentation of fiber in the colon can lead to gas byproducts. What can use these byproducts in the gut?
Excreted by the lungs
What is borborygmi?
Movement of gas in the BOWELS not stomach
Why are some people's flatulence flammable?
Only some people have the methanobrevibcter smithii in their microbiome
What can cause bacterial overgrowth syndrome?
Delayed small intestine transit and diverticulum
-gas and bloating from fermentation
How can bacterial overgrowth syndrome lead to anemia?
bacteria compete for B12 uptake
How can bacterial overgrowth syndrome lead to steatorrhea?
Deconjugate bile acids
How do you diagnose bacterial overgrowth syndrome?
Hydrogen breath test