Week 4a Gastrointestinal Physiology Flashcards
How much body mass is the GI system?
5%
What are the functions of the GI system?
- motility
- digestion
- absorption
- excretion
- circulation
All regions of the GI system work together to:
regulate the intake, processing & absorption of ingested nutrients & the disposal of waste products
What does the intestines interface with?
the immune system, with a diverse array of antigens present in food and gut microbes
What are the two primary movements of the GI system?
- mixing
2. propulsive (peristalsis)
What are the layers of the GI wall?
Serosa Longitudinal muscle layer Circular muscle layer Submucosa Mucosa Epithelium
Define the serosa
smooth membrane of thin connective tissue
What does the longitudinal muscle layer do?
contracts in order to shorten the length
What does the circular muscle layer do?
contracts to decrease the diameter
What does the submucosa do?
contains the submucosal (Meissner) plexus, which
transmits information from the epithelium to the enteric
and central nervous systems (CNS)
What is the mucosa layer?
functions to move the villi and contains blood vessels,
nerve endings, and immune and inflammatory cells
What layer of the GI system contains the immune and inflammatory cells?
mucosa
What is the GI system epithelium layer?
contents are sensed and where secretion of enzymes, absorption of nutrients, and excretion of waste products
occur
How is the GI system innervated?
- Extrinsic
SNS (primarily inhibitory)
PSNS (primarily excitatory) - Enteric (controls motility, secretion, & blood flow)
What does the SNS do in the GI system?
primarily inhibitory as stimulation can decrease or cease GI motility
primary neurotransmitter is norepinephrine
What does the PSNS do in the GI system?
- primarily excitatory as it activates GI motility and function
• primary neurotransmitter is acetylcholine
What does the enteric nervous system do in the GI system?
independent nervous system of the GI tract, which controls motility, secretions, and blood flow
What do vagus nerve fibers innervate in the GI system? (5)
[celiac plexus]
- esophagus
- stomach
- pancreas
- small intestine
- first half of large intestine
What do pelvic nerve fibers innervate in the GI system? (3)
[inferior hypogastric plexus]
- second half of the large intestine
- sigmoid
- rectal/anal regions
What is myenteric (Auerbach’s) plexus?
Linear chain of many interconnecting neurons that
extends the entire length of the gastrointestinal tract
→ Lies between the longitudinal and circular layers of the
intestinal smooth muscle
What does stimulation of myenteric plexus result in?
- Tonic contraction
- Increased intensity of rhythmic contractions
- Slight increase in rate of rhythmic contractions
- Increased velocity of peristaltic waves
What is the submucosal (Meissner’s) plexus?
• Controls inner wall of each minute segment of the intestine
• Originates in the epithelium
• Helps control local secretions, absorption, and contraction of the
submucosal muscle
What is Meissner’s plexus also called?
submucosal plexus
What is Auerbach’s plexus called?
myenteric plexus
What is another name for the submucosal plexus of the GI system?
Meissner’s plexus
What is another name for the myenteric plexus?
Auerbach’s plexus
What does the submucosal plexus control?
inner wall of each minute segment of the intestine
Where does the submucosal plexus originate?
in the epithelium
What is the excitatory neuron secreted by enteric neurons?
- acetylcholine
What are the two inhibitory neurons secreted by enteric neurons?
- norepinephrine
- epinephrine
Where do peristalsis contractions occur?
esophagus
stomach
small intestine
Where do segmentation contractions occur?
small & large intestine
What type of GI contraction is a mixing movement?
segmentation contractions
Where is the esophagus located?
C6-T11
What is the job of the esophagus?
propels food from the pharynx to the stomach
What is the upper 1/3 of the esophagus composed of?
striated muscle
What is the lower 2/3 of the esophagus composed of?
smooth muscle
What prevents aspiration?
the epiglottis moves upward in a protective mechanism over the larynx and trachea to prevent aspiration during swallowing
What is the level of the upper esophageal sphincter? & what is the resting tone?
at the level of the cricoid cartilage; C6
resting tone ranges 30-200mmHg
What innervates the lower esophageal sphincter?
SNS and PSNS
What is the resting tone of the lower esophageal sphincter?
10-45mmHg
What controls the lower esophageal sphincter?
it responds to esophageal distention via myogenic & neurohormonal mechanisms
How much does the stomach store?
1.5-2L
What does the stomach do?
Breaks down food into chyme through physical/chemical mechanisms
- Solids must be broken into 1-2mm particles to enter the duodenum
- mixes food with gastric secretions
What are the 3 types of cells in the stomach?
- G cells - secrete gastrin (stimulate parietal cells)
- Parietal cells – secrete hydrochloric acid (lowers pH)
- Chief cells – secrete pepsin (enzyme to digest PRO, needs acidic pH 1.5-2 to secrete)
What is the innervation of the stomach?
intrinsic & extrinsic
Parasympathetic stimulation to the vagus nerve increases the number and force of contractions
• Sympathetic stimulation inhibits these contractions via the splanchnic nerve
PSNS signal is sent to the stomach via ___
the vagus nerve
SNS signal is sent to the stomach via ___-
the splanchnic nerve
What 2 hormones increase the strength and frequency of stomach contractions?
- gastrin
2. motilin
What does gastric inhibitory peptide do?
inhibits contractions
What hormone inhibits stomach contractions?
gastric inhibitory peptide
What prompts emptying of the stomach? (3)
gastric distention, gastrin, nitric oxide
What inhibits emptying of the stomach?
duodenal distention, increased fat content via release of cholecystokinin
What does cholecystokinin do?
inhibits stomach emptying
What volume of gastric secretions are secreted per day?
~2L
What are 2 types of tubular glands in the stomach?
- oxyntic glands; aka gastric glands
2. pyloric glands
What do the oxyntic glands secrete?
[acid forming glands]
hydrochloric acid,
pepsinogen, intrinsic factor, mucus
Where does hydrochloric acid come from?
oxyntic glands
What do the pyloric glands secrete?
mainly mucous for protection from stomach acid,
gastrin
What does hydrochloric acid do in the stomach?
kills bacteria
aids in protein digestion
releases pepsin from pepsinogen
When parietal cells are stimulated, they secrete ____. At the same time, ___ is secreted
parietal cells secrete an acid solution of hydrochloric acid,
at the same time, HCO3 diffuses into the blood so that gastric venous blood has a higher pH than arterial blood when the stomach is secreting acid
How does H+ occur?
hydrogen-potassium pump
What is the pH of the acid secreted by parietal cells?
0.8
H+ concentration is about 3 million x that of arterial blood
What is the function of the small intestine?
to circulate the contents and expose them to the mucosal wall in order to maximize absorption of water, nutrients, and vitamins before entering the large intestine
How many liters of secretions are released per day?
7L
How much fluid enters the small intestine each day?
9L
How many liters of fluid make it to the colon from the small intestine?
1-2L
What are the 3 portions of the small intestine?
- duodenum
- jejunum
- ileum
Where is the duodenum and how long is it?
inferior to stomach, 25-30cm long
Where does the small intestine start?
begins at the pyloric sphincter, ligament of Treitz
What happens in the duodenum?
chemically digests chyme for absorption
What part of the small intestine is the jejunum?
the proximal 2/5
What happens in the jejunum?
absorption of nutrients
By the time chyme enters the ileum (from the jejunum), how much of the nutrients have been absorbed?
almost 90%
What is the ileum?
the final section (distal 2/5) of the small intestine
What vitamin is absorbed in the ileum?
Vitamin B12
Where does the ileum end?
the ileoceal valve
What is the ileocecal valve?
a circular muscle that serves to prevent reflux of colonic contents into the small intestine
What is the function of the large intentine?
reservoir for waste and indigestible material before elimination and extracts any remaining electrolytes and water
Where is the highest concentration of organisms in any ecosystem located?
in the large intestine
What are the 7 portions of the large intestine?
cecum appendix ascending colon transverse colon descending colon sigmoid colon rectum
Where is water absorbed in the large intestine?
throughout the ascending, transverse, and descending portions
What promotes relaxation of the internal anal sphincter to allow for defecation?
rectal distention & nitric oxide release
What does poor motility through the large intestine cause?
greater absorption of water and hard feces in the transverse colon → constipation
The pancreas is a part of what organ systems? (2)
digestive
endocrine
How much of the pancreas is endocrine?
1%
What is the endocrine function of the pancreas?
regulates blood sugar by secreting insulin, glucagon, & somatostatin
What percent of pancreas function is exocrine?
99%
What is the exocrine function of the pancreas?
secretes digestive enzymes
Where is the pancreas located? and how does it function?
behind the stomach & functions as a gland
How does the pancreas perform the exocrine function?
glands lined with epithelial (pancreatic acini) cells, secrete digestive
enzymes and sodium bicarbonate into ducts that drain via the
pancreatic duct and the common bile duct into the duodenum
What are the epithelial cells of the pancreas that line the gland?
pancreatic acini
What are pancreatic acini?
epithelial cells that line the gland
What does the pancreas secrete?
digestive enzymes and bicarbonate
Where does the pancreas secrete enzymes to?
into ducts that drain via the pancreatic duct and common bile duct into the duodenum
What portion of the small intestine does the pancreas secrete into?
the duodenum
What ducts does the pancreas use to secrete enzymes?
the pancreatic duct & the common bile duct
In what form are the enzymes produced by the pancreas?
inactive precursors (zymogens) that are converted into active forms once in the duodenum
What are zymogens?
inactive precursor enzymes secreted by the pancreas
Where are pancreatic enzymes converted into their active form?
in the duodenum
What stimulates the pancreas to secrete?
Pancreatic juice is secreted in response to the type of chyme in the upper portions of the small intestine
What is the pathway of secretion of pancreatic juice?
Flow is through a long pancreatic duct that normally joins the common
bile duct immediately before it empties into the duodenum through the papilla of Vater, surrounded by the sphincter of Oddi
Where does pancreatic juice empty into the duodenum (name)?
papilla of Vater
What surrounds the papilla of vater?
the sphincter of Oddi
Where does the pancreas secrete insulin?
into the bloodstream via the islets of Langerhans
Where are the islets of Langerhans located?
in islet patches throughout the pancreas
What hormones/neurotransmitters stimulate exocrine secretion by the pancreas?
Acetylcholine
Cholecystokinin (CCK)
Secretin
How is acetylcholine involved in the exocrine stimulation of the pancreas?
released from the parasympathetic vagus nerve endings and from other cholinergic nerves in the enteric nervous system
How is cholecystokinin (CCK) involved in exocrine stimulation of the pancreas?
- secreted by the duodenal and upper jejunal mucosa when food
enters the small intestine
How is secretin involved in the exocrine stimulation of the pancreas?
- secreted by the duodenal and jejunal mucosa when highly acidic food enters the
small intestine
What enzyme is secreted/works in the esophagus?
salivary amylase
starch → maltose
What does salivary amylase do?
starch → maltose
What are the 2 categories of enzymes secreted by the stomach?
- proteases; protein → polypeptides
2. stomach acid; chemical digestion
What enzymes are released from the liver/gallbladder?
bile salts; emulsification of lipids
What is the function of bile salts?
emulsification of lipids
What do proteases do?
protein → polypeptides
The pancreas secretes these 5:
- amylase; starch → maltose
- lipase; triglycerides → fatty acids
- endopeptidase; peptides → amino acids
- nuclease; DNA/RNA → nucleosides
- bicarbonate ions; neutralize stomach acid
What enzymes are found in the small intestine?
membrane-bound enzymes (eg. maltase)
What are 4 digestive secretions of the stomach?
- hydrochloric acid
- pepsin
- mucous
- intrinsic factor
What is secreted by gastric glands?
- HCl
- Pepsin
- intrinsic factor
What is secreted by the mucus glands of the stomach?
mucus
What is the acinus?
the functional unit of the exocrine pancreas. it is composed of acinar cells, centroacinar cells, and duct cells.
- a connective tissue matrix (including basal lamina) surrounds the acinus but does not insert itself between individual acinar cells
Where is the spleen located?
Located in the left upper abdominal quadrant, posterior/lateral to the
stomach, and along the tail of the pancreas
What is the blood supply to the spleen?
- splenic artery (branching off of the celiac trunk)
2. splenic vein (uniting with the superior mesenteric vein to form the hepatic portal vein)
What are the 2 major units of the spleen?
White pulp; lymphatic tissue/WBCs
Red pulp: venous sinuses, reservoir for monocytes to aid in wound healing, macrophages for digestion of abnormal or aging RBCs
When does splenic vasculature begin to develop?
in the 5th week of gestation
Where is the primary site of fetal erythropoiesis (until 28 weeks gestation)?
the spleen
What are 4 functions of the spleen?
- filtration
- iron metabolism
- prevention of infection
- red blood cell and platelet storage
How is the spleen connected to the GI tract?
vascular connections between the spleen, pancreas, stomach
the spleen is not ___
directly involved in digestion
What is splanchnic circulation?
blood flow through the gut, spleen, pancreas
that immediately flows to the liver via the portal vein
What cells are responsible for removing bacteria and other particulate matter from the blood? where are these cells?
the reticuloendothelial cells that line the liver sinusoids
How does surgery affect bowel function?
stress = ↑ SNS = ↓ GI moility
*directly proportional to the amount
What pre-op medication has benefits for the GI tract? Why?
midazolam,
will help ↓ the SNS response of the body.
less SNS = less GI inhibition
How do volatile anesthetics affect the GI tract?
Depression of spontaneous activity and changes in intestinal tissue
oxygenation
Small intestine is the first part to recover, followed by the stomach in approximately 24
hours and then the colon 30 to 40 hours postoperatively
What is the first part of the GI tract to recover from volatile anesthetic administration?
the small intestine
How does the administration of propofol affect the GI tract?
no difference in overall recovery and bowel function when
comparing isoflurane/nitrous oxide, propofol/air, and
propofol/nitrous oxide
How does Nitrous Oxide administration affect the GI tract?
30 x’s more soluble than nitrogen in the blood and as such will diffuse into gas-containing cavities from the blood faster than the nitrogen already present in those cavities can diffuse out
AKA…. don’t use!
How do NDMR affect the GI tract?
only affect skeletal
muscle, so GI motility remains intact
How does succinylcholine affect the GI tract?
mimics acetylcholine
- Produces an initial muscle contraction
- Increases intra-gastric pressure, which may be so strong as to overcome the tone of the LES and allow reflux
How does neostigmine administration affect the GI tract?
increases
parasympathetic activity and bowel peristalsis
- partially offset by simultaneous administration of the anticholinergic medications, glycopyrrolate
- Not seen with sugammadex – prudent choice for reversal in tenuous bowel anastomoses
How do opioids affect the GI tract?
reduced GI motility & constipation
central and peripheral receptors, namely mu, delta, and kappa
• Central = analgesia
• Peripheral = adverse effects (mu-opioid receptors in the myenteric and submucosal plexuses)
Depresses peristaltic contraction
Increases GI muscular activity and resting tone
Reduced motility the stool will remain in the gut for a longer period of time à more water is absorbed, and the stool becomes hard and dry leading to constipation
• Use of laxatives, stool softeners, and prokinetic agents, such as metoclopramide and neostigmine, have shown some success in alleviating opioid-induced constipation
