stomach
- the bolus of food has now entered the stomach, a J-shaped chamber
- its structure is arbitrarily divided up into 3 areas
- it is separated from the small intestine by a barrier called the pyloric sphincter
3 areas of the stomach
- fundus: this is the part of the stomach that lies above the esophageal opening
- body: main part of the stomach
- antrum: the very muscular lower section of the stomach. the barrier between the stomach and the small intestine is the pyloric sphincter
3 main functions of the stomach
- to store ingested food unitl it is ready to be released to the small intestine
- to secret HCl and enzymes necessary for protein digestion
- to mechanically mix the food with gastric secretions to produce the thick fluid known as chyme
4 aspects of gastric motility - part 1
gastric filling
- when empty, the stomach volume is about 50ml
- it can easily expand to accommodate a meal of about 1000ml and even expand to about 4000ml
- the stomach has deep folds that get smaller and flatten out as the stomach expands
- this allows expansion without an increase in tension or pressure
- this occurs by the vegally mediated process called receptive relaxation
4 aspects of gastric motility - part 2
gastric storage
- pacemaker cells in the fundus generate slow-wave potnetials that travel down the length of the stomach at a rate of about 3 per minute
- these slow-wave potentials do not neccessarily reach threshold, it depends on the level of smooth muscle excitability
- when threshold is reached, a peristaltic wave sweeps over the fundus down toward the pyloric sphincter
- most food is stored in the body of the stomach and gradually moved into the muscular antrum where mixing occurs
4 different aspects of gastric motility - part 3
gastric mixing
- with each peristaltic wave, the chyme is pushed towards the pyloric sphincter
- however, the pyloric sphincter is usually in an almost closed positio so that only fluids can pass through
- when the peristaltic wave pushes the chyme to the pyloric sphincter and it cannot pass through it, the chyme folds back upon itself only to be propelled forward and folded back over and over
- this process is called retropulstion and ensures the chyme is thouroughly mixed until the particles are small enough for emptying
4 aspects of gastric motility - part 4
gastric emptying
- with each peristaltic wave some chyme is pushed through the pyloric sphincter
- the volume of chyme in the antrum is about 30ml, a few ml of which will be pushed through with each peristaltic wave
- when very strong waves occur, a greater volume of chyme will pass through before the sphincter tightens again
- the intensity of the peristaltic waves is under the influence of various signals
- within the stomach, the main factor influencing the strength of contraction is the amount of chyme in the stomach and its fluidity
- the greater the volume of chyme, the more distention and therefore more contractions
- the faster chyme becomes a liquid, the faster it will pass through the pyloric sphincter
gastric empyting
- the duodenum also influences the rate of gastric empyting
- unless the duodenim is ready to receive chyme, it will send signals to reduce gastric empyting
- there are neuronal responses mediated by both the intrinsic nerve plexus and the autonomic nerves, collectively called the enterogastric reflex
- there are also hormone responses as several hormones are released from the duodenal mucosa
- two most important hormones are secretin and cholecystokinin (CCK)
4 duodenum stimuli that affect gastric empyting
- fat
- acid
- hypertonicity
- distention
fat stimuli
- the most potent stimulus for inhibiting empyting
- it takes a long time for fat to be digested and absorbed in the lumen of the small intestine so the presence of fat reduces gastric emptying to allow more time to process it
- high fat meals may remain in the stomach for up to 6 hours whereas high protein or carb meals may empty in about 3 hours
acid stimuli
- because the stomach secretes a lot of HCl and it is mixed in with the chyme, the duodenum must neutralize it to prevent damage to the tissue and digestive enzymes
- it does so by secreting, mainly from the pancreas, NaHCO3
- high levels are acid in the duodenum will reduce gastric emptying
hypertonicity stimuli
- as proteins and carbs are broken down into smaller molecules, this can have a dramatic increase in osmolarity
- since water moves freely across the duodenal wall, it moves into the lumen by osmosis
- if digestion is faster than absorption, the increase in osmolarity will bring enough water in to distend the duodenum and potentially decrease plasma volume
- becasue of this, increased osmolairty in the duodenum will relfexively inhibit gastric emptying
distention stimuli
the more the duodenum is distended, the slower the rate of gastric empyting
steps that occur within vomiting
- begins with a deep inspiration and closure of the glottis ( to prevent gastric contents going into lungs) and uvula (to prevent gastric contents going into basal cavity)
- the diaphragm then contracts downward and the abdominal muscles contract inwards
- the flaccid stomach is compressed, its contents are forced upwards, through the relaxed esophageal sphincters, and out through the mouth
- this will repeat until the stomach is empty. because the autonomic system is involved, vomiting is usually preceded by profuse sweating, salivation, increased heart rate, and sensation of nausea
causes of vomiting
- touch stimulation to back of throat
- irritation or distention of stomach or duodenum
- elevated intracranial pressure (after head injury)
- rotation or acceleration of head (motion sickness)
- chemical agents or dugs that interact with chemoreceptor trigger zone next to vomiting centre
- psychogenic vomiting
gastric secretions
- within the gastric mucosa of the stomach there are two distinct areas with respect to secretion
- the oxyntic mucosa, which line the fundus and the body, and the pyloric gland area, which lines the antrum
- the luminal surface of the stomach has pockets formed by the in-foldings of the gastric mucosa
- these are called gastric pits, at the bottom of which lie gastric glands
- wihtin oxyntic mucosa, there are 3 types of secretory cells whose exocrine secretions collectively are known as gastric digestive juices
secretions collectively are known as gastric digestive juices
- mucous cells: line the pits and the entrance to the glands, secrete a watery mucus
- chief cells: the more numerous cells in the gastric glands, secrete pepsinogen
-parietal cells: also in gastric glands, secrete HCl and intrinsic factor
hydrochloric acid
parietal cells actively secrete HCl into the gastric pits, which empty into the lumen of the stomach and can decrease the pH to as low as 2
4 functions of HCl within stomach
- activates pepsinogen to the active form pepsin
- helps break down connective tissues and muscle fibres
- denatures proteins
- kills most microorganisms ingested with food
mechanism of HCl secretion - step 1
- within the parietal cells, H2O is broken down inot H+ and OH-
- the H+ is then secreted into the lumen by the H+-K+ ATPase by active transport
- the HCO3- that is simultaneously brought inot the parietal cell passively leaks back into the lumen
mechanism of HCl secretion - step 2
- the generated OH- combines with a H+ from H2CO3 to remake H2O
- since parietal cells have lots of carbonic anhydrase, the H2O combines with CO2 (either from metabolic acitvity or plasma) to form H2CO3, which partially dissociates into H+ and HCO3-, thereby regenerating the H+ that was secreted
mechanism of HCl secretion - step 3
- the HCO3- is moved into the plasma by a Cl- HCO3- exchanger
- this exchanger creates a buildup of Cl- within the parietal cells, which will then move through channels, down its electrochemical gradient, into the gastric lumen
how pepsinogen is converted to its active form in the lumen
- once released, HCl cleaves off a small part of the protein to release the active form of the enxyme pepsin
- pepsin itself can then cleave more pepsinogen to form even more pepsin
- the active pepsin then starts protein digestion by splitting certain amino acid linkages to release smaller amino acid chains. this acitvity of pepsin is dependet upon the acidic environment of stomach
why is pepsin stored in the chief cells as pepsinogen
- since pepsin is able to digest protein, it must be stored in an inactive form to prevent it from digesting the proteins within the chief cells in which it is formed
- it is stored in an inactive form, such that it is harmless to the cell, until it reaches the gastric lumen and is activated by HCl
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Module 2 Section 122
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Module 3 Section 117
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Module 4 Section 132
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Module 5 Section 120
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Module 6 Section 128
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