Control of Gastric Emptying
The rate of gastric emptying is strongly influenced by both volume and composition of gastric contents, which makes considerable sense. Consider three examples of something you might ingest and try to anticipate which rate of gastric emptying would be most appropriate:
|A large glass of water: The stomach becomes distended, but there are no solids to grind and liquefy, and after the water reaches the small intestine, no further processing is required before absorption - the rate of gastric emptying should be very fast.|
|A double cheeseburger with fries (or a mouse if you're a cat): The stomach is distended and its contents must be liquefied; you would also want the meal to be retained in the stomach long enough for pepsin and acid to get a good shot at digesting the protein. Additionally, the resulting chyme should be allowed to empty in the small intestine slowly so as to not overload that organ, particularly with regard to digestion of fat - the rate of gastric emptying should be slow.|
|A single chicken nugget (or a grasshopper if you're a cat): The stomach will not be distended after this kind of a "meal" and in the absense of distension, there is relatively little stimulus for gastric motility - the rate of gastric emptying should be slow.|
After consuming a typical solid meal, there is a lag time of 20 to 30 minutes in which there is minimal gastric emptying. This is followed by a phase in which the rate of emptying is roughly linear. In contrast, liquids are generally transported out of the stomach at an exponential rate.
For liquids, the principal determinant of rate of gastric emptying is volume and, secondarily, composition. If the liquid is low in nutrients (e.g. water), there is an exponential relationship between volume and rate of emptying - large volumes empty at an exponentially faster rate than small volumes.
However, if the fluid is hypertonic or acidic or rich in nutrients such as fat or certain amino acids, the rate of gastric emptying will be considerably slower and non-exponential. Indeed, the rate of gastric emptying of any meal can be predicted rather accurately by knowing its nutrient density. Nutrient density is sensed predominantly in the small intestine by osmoreceptors and chemoreceptors, and relayed to the stomach as inhibitory neural and hormonal messages that delay emptying by altering the patterns of gastric motility. The presence of fat in the small intestine is the most potent inhibitor of gastric emptying, resulting in relaxation of the proximal stomach and diminished contractions of the distal, "gastric grinder" - when the fat has been absorbed, the inhibitory stimulus is removed and productive gastric motility resumes.
Understanding the basic principles of gastric emptying facilitates management of gastric motility disorders, which are relatively common in both man and animals.
Advanced and Supplemental Topics
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