gastric physiology 2 Flashcards
which cells secrete protease
chief cells produce pepsinogen
why is pepsin secreted as inactive zymogen
it is stored this way to prevent it digesting the chief cells and the rest of the body
how is pepsin secreted
synthesised i’m inactive form (zymogen)
pepsinogen mediated by input from enteric nervous system (ACh)
secretion parallels HCL secretion
luminal activation
how is protease activated
conversion of pepsinogen to pepsin is pH dependent
When pepsinogen is released into the stomach lumen, the low pH (generated by HCL) of the stomach activates a rapid autocatalytic process in which pepsin is produced from pepsinogen
- Once some pepsin is prodcued, it can be used to produce more since it can aid in the cleavage of pepsinogen - positive feedback
most efficient when pH <2
positive feedback loop (pepsin also catalyses the reaction)
pepsin only active at low pH
irreversible inactivation in small intestine by HCO3-
what does HCO3- do
HCO3- released in the duodenum irreversibly inactivates pepsin
role of pepsin in protein digestion
not essential (protein digestion can occur if the stomach is removed)
accelerates protein digestion
breaks down collagen in meat - helps shred meat into smaller pieces with greater surface area for digestion
how much of total protein digestion does pepsin account for
about 20%
what happens if pepsin is removed
no vitamin B-12 absorption can occur in the small intestine since the
stomach parietal cells produce intrinsic factor
- essential for vitamin B-12
absorption in the small intestine
volume of empty stomach
about 50mL
volume of stomach when eating
when eating, the stomach can accommodate about 1.5L with little increase in luminal pressure
it does this by smooth muscles in body and fundus undergoes receptive relaxation
what is receptive relaxation
mediated by parasympathetic nervous system acting on enteric nerve plexuses
with coordination provided by afferent input from the stomach via the vagus nerve and by the swallowing centre in the brain
nitric oxide and serotonin released by enteric nerves mediate relaxation
Acetyl choline activates parietal & chief cells & initiates receptive relaxation
what is peristalsis
a series of wave-like muscle contractions that move food through the digestive tract
where does peristalsis begin
in the stomach
it produces peristaltic waves in response to the arriving food
describe peristalsis process
- each wave begins in the body of the
stomach producing a ripple as it proceeds towards the antrum - the initial contraction in the body is too weak to produce much mixing of luminal contents with acid and pepsin
- there are more powerful contractions in the antrum which enables better mixing of the luminal contents
- The pyloric sphincter closes as peristaltic waves reach it
- this contraction, every time a wave reaches it means little chyme (smooth, somewhat orange coloured liquid = the end result of stomach digestion and peristalsis) enters the duodenum
- It also means that the antral contents are forced back towards the body meaning there is more mixing and thus digestion
what determines the frequency of peristaltic waves
pacemaker cells
(INTERSTITIAL CELLS OF CAJAL) in the muscular propria (longitudinal smooth
muscle layer) and is constant (3 every minute)
what is the pyloric sphincter
a ring of smooth muscle and connective tissue between the atrium and duodenum
what is basic electrical rhythm
frequency of peristaltic waves determined by pacemaker cells in muscularis propia and is constant (3/minute)
pacemaker cells undergo slow depolarisation - repolarisation cycles
depolarisation waves are transmitted through gap junctions to adjacent smooth muscle cells
these cells do not cause significant contraction in empty stomach
why does strength of peristaltic contractions vary
excitatory neurotransmitters and hormones further depolarise membranes
action potentials generated when threshold reached
The interstitial cells of cajal are active all the time, but the action potential threshold for muscle contraction can be altered by the enteric nervous system
what increases strength of peristaltic contractions
gastrin
gastrin distension (mediated by mechanoreceptors)
what decreases strength of peristaltic contractions
duodenal distension
- increase in duodenal luminal fat
- increase in duodenal osmolarity
- decrease in duodenal pH
- increase in sympathetic NS action
- decrease in parasympathetic NS action
what is gastric emptying
capacity of stomach > capacity of duodenum
overfilling of duodenum by a hypertonic solution causes dumping syndrome : vomiting, bloating,c cramps, diarrhoea, dizziness, fatigue, weakness, swelling
causes of delayed gastric emptying
idiopathic (unknown cause)
autonomic neuropathies (eg in diabetes mellitus)
drugs
abdominal surgery
parkinson’s disease
multiple sclerosis
scleroderma
amyloidosis
female gender
don’t need to know all of these
what kind of drugs cause delayed gastric emptying
gastrointestinal agents
anticholinergic medications
miscellaneous
symptoms of delayed gastric emptying
nausea
early satiety
vomiting undigested food
GORD
abdo pain / bloating
what happens to pH as gastric contents enter the duodenum
as gastric contents enter duodenum, duodenal pH falls
what regulates gastric emptying
the same things that regulate parietal & chief cells
what is gastroparesis
delayed gastric emptying
can cause matter in the stomach to rot and smell and become a similar
appearance to faeces
summary of peristalsis
Produced in response to arriving food
- Ripple movement begins in body
- More powerful contraction wave in antrum
- Pyloric sphincter closes - Not much chyme can enter duodenum
- Antral contents forced back to body – mixing
