Session 4- The Stomach Flashcards
functions of the stomach
storage
start digestion
a little bit of carb and fat digeston
disinfect
what is chyme
resuly of stomach beginning digestion of proteins continuing digestion of fats and carbs
what is the stomach made up of
fundus
body
antrum
what type of epithelia is present
simple columnar
where are the cardia
where the oesophagus joins the stomach close to heart
receptive relaxtation of the stomach
peristalsis causes reflex relaxation of proximal stomach which causes the fundus to distend and then the stomach can fill without significant rise in pressure
what are the muscles from innermost to outer
oblique
circular
longitudinal
they mechanically break down food by forceful contractions
as you go down the stomach how does the tickeness of the wall change
gets thicker and more muscular
what do parietal cells produce
hcl
what do chief cells produce
pepsinogen which is converted to pepsin by the action of HCL which is produced by the parietal cells
what is a G cell
enteroendocrine cell which is more prevalent in antral section in stomach
produce gastrin
What do stomach epithelia secrete as protection
HCO3- into the mucus so eventually you get a layer that is neutral compared to the stomach lumen
how do parietal cells produce acid
proton pump exchanges K+ with H + into the stomach lumen
what stimulates acid production
smell taste sight stretch presence of amino acids as small peptides chyme in duodenum presence of partially disgested proteins
what stimulates a parietal cell - gastrin
peptides in the stomach lumen stimulate g cells to produce gastrin which binds to CCK receptors
parietal cell stimulation- vagal
vagal stimulation of the cell through Mcr
also ACh binds ro G cells causing them to release gastrin which binds to CCK receptors
how is an entero-chromaffin cell stimulated
vagal stimulation of entero-chromaffin like cell-ECL
histamine released which binds to H2 cell
there are also CCK receptors on the ECL so can be stimulated by gastrin
how is acid secretion inhibited
D cells are stimulated by a drop in pH so produce somatostatin
how is HCL produced
hydrgen ions from the H+/K+/ATPase combine with Cl- which diffuse into the stomach lumen
the hydrogen ions come from
H20 + C02 = H2C03 -carbonic anhydrage
the Cl- ions come from the HCO3-/Cl- exchanger on the basal surface
Briefly explain to a friend why the parietal cell is unable to secrete acid during its resting
phase
During its resting phase the apical membrane of the parietal cell does not have any proton pumps (H+/K+ ATPases) associated with it as these are found (in the resting phase) within tubulovesicles in the cytosol of the cell.
Like wise the tubulovesicles only contain H+ component of the proton pump and need to be associated with the apical membrane to become functioning.
Therefore in its resting phase, the acid secreting mechanism (the proton pump) is
dissociated from itself and therefore no acid can be secreted.
When the parietal cell is in its resting phase, what cytosolic structures contain the proton
pumps?
tubovesicles
What receptor does the hormone Gastrin bind to on the parietal cell
CCK receptor
How does the stomach separate out larger food molecules, that need to remain in the
stomach for further processing, from the smaller food molecules that can pass into the
duodenum?
The shape of the stomach is a funnel which means that when the stomach muscles contract the smaller molecules accelerate more quickly than the larger molecules, which separates them out, with the smaller molecules moving distally and the larger molecules staying in the body of the stomach for further processing.
In simple terms explain why, when food enters the stomach, acid secretion is triggered
Food stretches the stomach activates vagal nerve stimulation of acid production
Food effectively raises the pH of the stomach by buffering the acid secretions. This removes the negative feedback on acid secretion that a low pH causes…therefore acid can be secreted.
Small peptides and amino acids are produced because of the action of pepsin in the
stomach and these stimulate acid secretion
What are the various mechanisms that the stomach uses to resist injury from its own secretions
The stomach produces acid and proteases both of which could damage its structural
integrity (the stomach is made of protein).
The stomach is lined with surface mucous cells that secrete a mucus layer. Into this layer
bicarbonate ions are secreted so that a pH neutral barrier is formed.
The stomach mucosa also has a good blood supply that helps it H+ molecules that get
through the mucus barrier.
The stomach epithelia are replaced regularly so that damaged cells do not pose a threat to the integrity of the mucosa
Prostaglandins are released which boost the protective mechanisms in many ways,
especially by ensuring adequate mucosal blood flow.
