GI physiology Flashcards
What are the layers of the GI tract
- serosa
- muscularis propria
- submucosa
- mucosa
Describe the serosa
Outermost layer of GI tract
- several layers of connective tissue
- covered by simple squamous epithelium called mesothelium
Describe the muscularis externa
- outer longituidinal muscle layer
- inner circular muscle layer
- site of Auerbach’s (myenteric) plexus
Describe the submucosa
loose connective tissue with collagen and elastin fibers
- site of Meissner’s (submucosal) plexus
Describe the mucosa
Innermost layer comprised of
- muscularis mucosa
- lamina propria
- epithelium
Where is the myenteric plexus
(Auerbach’s plexus)
- between inner and outer muscle layers of muscularis externa
What is the function of the myenteric plexus?
Because the motor neurons of the myenteric plexus supply the longitudinal and circular smooth muscle
layers of the muscularis, this plexus mostly controls GI tract motility, particularly the frequency and
strength of contraction of the muscularis.
Where is the submucosal plexus?
(Meissner’s plexus)
- located in the submucosa
What is the function of the submucosal plexus?
- Senses the environment within the lumen
- Regulates GI blood flow
- Controls epithelial cell function
The motor neurons of the submucosal plexus supply the secretory cells of the mucosal epithelium, controlling the secretions of the organs of the GI tract.
Describe the action of the circular smooth muscle
contraction causes constriction of the gut
Describe the action of the longitudinal smooth muscle
contraction causes shortening of gut
How is peristalsis achieved?
Smooth muscle functions to move the food bolus along the GI tract. The two types complement each
other to achieve peristalsis
Name the three salivary glands
- parotid gland
- submandibular gland
- sublingual gland
Which nerves innervate the salivary glands
CN VII and IX
How much saliva is produced per day?
0.5L
What do salivary secretions contain?
- mucus (lubricate)
- amylase
- bicarbonate
- thiocyanate & lysosyme
What is the function of amylase?`
digest starch
what is the function of bicarbonate
neutralise acid
what is the function of thiocyanate and lysosyme?
bacteriocidal agents
What is the purpose of mastication?
- reduces particle size
- mixes with saliva (lube and enzymes)
- increases surface area
How is mastication controlled?
voluntary and involuntary (mostly latter)
Once started it becomes involuntarily
Where is the swallowing centre found?
within the reticular formation of the brainstem
What are the voluntary movements of swallowing?
Tongue pushes food bolus in oropharynx superiorly and posteriorly
Bolus is pushed against the soft palate, so that it does not enter the nasopharynx
After this point (past soft palate), swallowing is involuntary
What is the function of the oesophagus?
Acts as a conduit to propel bolus from pharynx to stomach
What features are found on the oesophagus?
an upper and lower sphincter (thickened band of circular and some longitudinal muscle)
What is the function of the striated muscle in the oesophagus?
mostly voluntary control (initial swallowing)
What is the function of smooth muscle in the oesophagus?
automated peristalsis, towards the distal end of the oesophagus
What is the innervation of the oesophagus?
Afferent and efferent vagal pathways
o Mechanoreceptors in the oesophagus stimulate afferent vagal pathways
o Contractions are mediated by efferent vagal pathways
What is achalasia?
disease of the muscle of the lower esophageal body and the lower esophageal sphincter that prevents relaxation of the sphincter and an absence of contractions, or peristalsis, of the esophagus
What imagine is used to confirm achalasia?
barium swallow
What is the appearance of achalasia on a barium swallow x ray?
Dilated oesophagus on either side that fails to contract
Very tight LES
What are the symptoms of achalasia?
Inability to swallow causes regurgitation and vomiting
What is the treatment of achalasia?
balloon dilation on endoscopy to widen LES
What is the side effect of treatment for achalasia?
Patients tend to suffer from heartburn afterwards (due to reflux)
Describe the innervation of the oesophagus
Innervated by both sympathetic and parasympathetic nervous systems
Parasympathetic regulates peristalsis via the vagus nerve (CN X)
Myenteric plexus – major nerve supply to circular and longitudinal muscle
What is the arterial supply of the oesophagus?
Superior & inferior thyroid arteries
Branches of bronchial, intercostal, descending aorta arteries
Branches of left gastric, left inferior phrenic and splenic arteries
Dense anastomosis within submucosa, meaning infarction of oesophagus is very rare because of dense
collateral circulation
Describe the venous drainage of the oesophagus
More clinically significant than arterial supply
Portal vein – main route of portal system
o Left gastric vein, splenic gastric vein, short gastric veins
What is the result of cirrhosis on the venous drainage of the oesophagus?
Cirrhosis causes portal hypertension, which causes an extension of HTN to the left gastric and splenic
veins
o Causes splenomegaly and oesophageal varices
What is a sengstaken tube and its use?
Goes down the oesophagus into the stomach
Tube is inflated and pulled to create traction against
the cardia
Tampenades the vein to stop bleeding
50% of patients re-bleed when the balloon is
deflated because it does not really address the
underlying HTN
What is the function of the stomach?
Functions primarily as a reservoir to store large quantities of ingested food
Initiates the digestive process
Acid secretion (via proton pump)
Releases contents in a controlled fashion into the duodenum, which has a smaller capacity
How is motility of the stomach controlled?
vagus nerve
Describe the stomach at rest
contracted at rest and there is very little or no
peristalsis
Vago-electrical rhythm has a sub-threshold amplitude at this
point
What happens to the stomach on eating
Stomach distends when food is swallowed
Distension is sensed by mechanoreceptors = activate vagus
afferents
Vago-vagal reflex as vagal efferents are activated
This increases the amplitude of the vaso-electrical rhythm motility increases resulting in peristalsis
As food moves to duodenum, enterogastrin and secretin cause negative feedback
What are the gastric secretions?
- Hydrogen ions (produced by parietal cells)
- Pepsin (precursor ‘pepsinogen’ produced by peptic or chief cells)
- Intrinsic factor forms a complex with B12 to aid absorption
- Mucus
- Water
Why is gastric acid necessary?
o Converts inactive pepsinogen to the active enzyme pepsin
o Kills a large number of bacteria that enter the stomach
What is the normal pH of blood?
7.4
What is the gastric pH at high rate?
~1
Why is lots of energy expended in the stomach?
Creating this massive difference in pH expends a lot of energy
o Parietal cells therefore expend a lot of energy
What are Oxyntic glands?
Simple tubes, two or more of which open into a single duct.
o Oxynticmeans acid-secreting
o Secretehydrochloric acid(HCl) andintrinsic factor
Where are oxyntic glands found?
in the fundus and body of the stomach
How is bicarbonate produced?
H 2 O =>OH- + H+
OH - + CO 2 => HCO3- (catalysed by carbonic anhydrase (CA))
How are hydrogen ions moved?
actively “pumped” into the lumen by the H + -K + -ATPase
What is the pH of gastric venous blood?
Bicarbonate enters the blood = gastric venous circulation is therefore more alkaline
Where is gastric acid produced?
parietal cell
What controls the parietal cells ability to secrete acid?
active transport
stimulation by several sources
How does bicarbonate enter blood?
Bicarbonate is transported out of the basolateral membrane in exchange for chloride ions → chloride
shift
This outflow of bicarbonate into blood results in a slight elevation of blood pH known as the
alkaline tide.
As a result, blood draining away from the stomach is slightly more alkaline than normal blood
This process serves to maintain intracellular pH in the parietal cell, preventing alkalosis.
How is carbonic acid formed?
Carbon dioxide diffuses into the cell and combines with water to form carbonic acid
This is catalyzed by carbonic anhydrase
What happens to carbonic acid?
Carbonic acid dissociates into hydrogen ions and bicarbonate ions
How are hydrogen ions pumped out of the cell
Hydrogen ions are then pumped out of the cell, into the lumen, in exchange for potassium by H + -K + -
ATPase
What is the purpose of hydrogen ions entering the lumen?
Accumulation of osmotically-active hydrogen ions in the lumen generates an osmotic gradient across the
membrane
This results in outward diffusion of water→ gastric juice
How is potassium recycled in gastric acid production?
Chloride anions enter the lumen of the gastric gland together with K + by facilitated diffusion
K + is thus recycled
What is HCL secretion stimulated by?
Histamine – Produced by ECL cells (enterochromaffin like cells) near parietal cells
Gastrin – Secreted by G cells, acts on the cholecystokinin-2 receptor
Acetylcholine – released by parasympathetic neurons (vagus nerve), acts at muscarinic receptor
What targets can be used to stop excess acid secretyion?
- vagus nerve innervation
- histamine
- proton pump
How is the vagus nerve targeted to stop excess acid secretion
Vagotomy
o Oldest intervention
o Strip away the vagus nerve from around the
stomach
o The vagus nerve has many other actions and therefore this surgery had many side effects
What are the side effects of targeting the vagus nerve?
Vagus nerve is also responsible for peristalsis and emptying
o Therefore a side-effect is delay in emptying
o For this reason was generally performed with pyloroplasty or gastroenterostomy
What drug is used against H2 receptors to stop excess acid secretion?
anti-histamines:
- cimetidine
What drug is used against proton pumps to stop excess acid secretion?
Proton pump inhibitor (PPI)
o Omeprazole, lansoprazole, esomeprazole
What are the side effects of proton pump inhibitors?
Because gastric acid kills a large number of bacteria, PPIs are associated with increased risk of nosocomial
infections including Clostridium difficile
How is gastric acid secretion inhibited?
Gastrin is inhibited at lower pH (<3) (after food leaves)
The low pH also stimulates somatostatin, a broadly inhibitory hormone. Inhibits HCl secretion by:
o Inhibiting gastrin secretion by G cells
o Inhibiting histamine release by enterochromaffin cells
o Inhibiting HCl release by parietal cells
Duodenal enterogastrones are also released:
o GIP
o Secretin
o CCK
What are the phases of gastric secretion
- Cephalic phase (sight/smell/taste food)
- Gastric phase (food in stomach-duodenum)
- Intestinal
Describe the cephalic phase
Sight/smell/taste of food
VAGUS activates parietal and gastrin cells
Moderate stimulation of HCl / Pepsinogen
Describe the gastric phase
Distension of stomach + Proteins in antrum
VAGO-VAGAL REFLEX + GASTRIN (+ histamine)
Strong stimulation of HCl / Pepsinogen
Some foods directly stimulate gastrin release:
o Protein/peptides
o Coffee
o Calcium rich food
o * Alcohol oddly doesn’t appear to stimulate gastrin release (although it is associated with excess
acid/gastritis)
Describe the intestinal phase
Proteins in duodenum gastrin (excitatory)
HCl in duodenum secretin (inhibitory effect on peristalsis and secretion)
Lipids in duodenum Peptide YY (inhibitory)
Summarise the events in the stomach during digestion
Acid is produced to convert pepsinogen to pepsin
Pepsin is produced for protein digestion
Mucus is produced for lubrication, and to prevent acid and pepsin from contacting the mucosa
Intrinsic factor is produced to form a complex with B12 for absorption in the terminal ileum
Water produced
What is the function of the small intestine?
Mixing of foodstuffs and digestive secretions/enzymes
Circulation of all intestinal contents to facilitate contact with mucosa
Propulsion of intestinal content towards terminal ileum
Describe the anatomy of the small intestine
(increase surface area):
o Kerckring’s folds – (plicae circularis) circular folds,
large valvular flaps projecting into the lumen of the
small intestine
o Villi
o Microvilli on surface of enterocytes
What are Brunners glands?
glands in duodenum submucosa which secrete bicarbonate
What are peyer’s patches?
lymph follicles found in the ileum
What motility processes occur in the small intestine?
- Segmentation contractions (non-synchronised contractions) of circular smooth muscle to mix chyme
- Pendular contractions (back and forth) of longitudinal muscle to mix chyme
- Villus movements brought about by muscularis mucosae
- Peristaltic waves arising from duodenal bulb propelling food towards terminal ileum
What are secreted from the crypts?
o NaCl / NaHCO 3 - = neutralises gastric HCl
o Amylase = aids in starch digestion
o Enteropeptidase = activates trypsinogen
What are secreted from the villus tips?
brush border enzymes
Describe the pancreas
can be divided into two parts:
- endocrine pancreas
- exocrine pancreas
Describe the endocrine pancreas
Throughout parenchymal tissue, islands of
endocrine tissue – islets of Langerhans
o β cells producing insulin
o α cells producing glucagon
Describe the exocrine pancreas
Aqueous / bicarbonate component
Enzymatic component
What is secreted from the acinar cells?
- pro-enzymes
- NaCL
What is secreted from the pancreatic duct?
Bicarbonate
