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As I understand it, the stomach digests the bolus into chyme and then passes the latter on to the small intestine. This process takes between 40 minutes and a few hours.

However, what happens when the processing of the bolus is not completely finished and new food arrives in the stomach?

Will the stomach pass on all the food, including the only partially digested "new food" to the small intestine when the time for digesting the first load is up? Or will it keep all of the food until all of it is completely processed? Or does it have some way of allowing only the processed parts to pass on and keeping the unfinished parts behind, like a gate control?

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    $\begingroup$ Stomach has gates- an entry gate (GI sphincter) & exit gate (pyloric sphincter). Stomach keeps on passing some amount of chyme continuously as it gets digested. The new bolus mixes in with food in stomach, until stomach reaches its max (thats when you vomit) ;) $\endgroup$ – another 'Homo sapien' Jul 3 '16 at 12:29
  • $\begingroup$ Thank you, @user19099, does that mean that the small size of the sphincter-opening makes is possible for (finer) chime to exit while it keeps the (larger) bolus in? Or how does the stomach "know" what to pass on and what to retain? $\endgroup$ – user6268 Jul 10 '16 at 14:34
  • $\begingroup$ Well I'm not expert in this field, but it is related to the small size of the pyloric sphincter due to which bigger particles retain inside while the semi-liquid chyme passes on. $\endgroup$ – another 'Homo sapien' Jul 11 '16 at 10:21
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The regulation of emptying of food from the stomach to the duodenum takes place with the help of feedback mechanisms involving the duodenum and stomach.

Nature of the chyme which inhibits stomach emptying by the duodenum:

  • Excessive acidic chyme.

  • Chyme contains too much undigested or unprocessed proteins and fats due to which its tonicity and irritability changes.

Regulation by the duodenum:

  1. Nervous regulation: The enteric nervous system,the vagus and the prevertebral sympathetic ganglia play the key roles in nervous regulation of this feedback mechanism because when food enters the duodenum they provide stimuli that activates them which inturn inhibits pyloric pump.

  2. Hormonal regulation: Cholecystokinin, secretin and gastric inhibitory peptide inhibits pyloric pump,gastric motility,gastrin effect on stomach emptying respectively.

Thus,when much undigested chyme passes to the duodenum,duodenum regulates if the digestion process should be prolonged or not in the stomach.

Factors promoting emptying of stomach:

Gastric food volume and gastrin hormone promotes emptying by stimulating the enteric nervous system for emptying and enhancing pyloric pump respectively.

Thus, the distinguish between a digested or undigested chyme to be passed to the duodenum or the time for which the food must stay in the stomach for digestion is entirely under the regulation of these factors.

References:

https://www.ncbi.nlm.nih.gov/pubmed/1126597

https://www.ncbi.nlm.nih.gov/pubmed/7943335

https://en.m.wikipedia.org/wiki/Gastrin

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