Here's a fantastic (and free) book about just this structure:
Fun fact: I had pyloric stenosis as an infant. IIRC, this condition tends to hit first born males of which I am one.
Dearest mother kept bringing me into the hospital. "He keeps throwing up no matter what." She kept being told the "oh you're just a new mother" song until finally one doctor switched on a lightbulb to check something out.
Next thing is that I'm this infant on an adult stretcher being prep'd for surgery.
My oldest scar.
I swear if I was born 100 years earlier I'd be dead by starvation at no fault of anything but medical innovation.
In any case, congratulations on your continued survival!
Anus inspired honey dispenser https://imgur.com/a/jmQUSYr
You're right though, the engineering that goes into our bodily machines is mind boggling.
https://www.implantica.com/im-refluxstop/patients/gastrointe...
Who knows?
All I know is that it's different for everyone and for lots and lots of people, eating totally normal things is what causes or contributes to lots of bad symptoms misattributed to some disease or disorder. More often than not it's not you, it's the utter garbage food that our society has standardized upon. Just because it's common to eat a sandwich, or pasta, or a bowl of cereal or granola, or a bagel doesn't mean that human evolution has caught up with it yet.
I live on espresso/tea, heavy cream, protein/nutrition shakes, zero-cal flavor syrups, and grilled beef. It's sort of crazy how much healthier I am eating this than if I ate, say, even healthy sandwiches. My body just seems to dig being primarily a carnivore. If I personally liked vegetables and leafy greens more I would probably do vegan keto, as I don't like how unsustainable western meat production is, but I simply can't get into the idea of leafs as food.
Do you have an ELI5 for how this works?
But the vertical fists model helps to visualize the pylorus as a "cylinder" of annular muscle segments that contract and relax with alternating timing to pump liquid from the stomach into the duodenum in precise metered doses. This timed and coordinated squeezing action is called peristalsis, and is the mode of action of all your intestines as well as your esophagus.
Now here's the important part: notice that when you squeezed your top fist, liquid squirted out in both directions. Dynamic studies of the pylorus in action show that these two "squirts" are not the same. The forward squirt is usually smaller and contains only liquid and fine particles of food, whereas the larger backward squirt (called retropulsion) contains all of the large chunks of food that require further digestion.
So how does the pylorus do this? The answer lies largely in the shape and structure of the sphincter wall, and here's where it gets really crazy. The pyloric cylinder has two sets of muscles moving it: the big strong external rings of muscle, and a finer inner layer of muscle in the mucosal lining. When these muscles contract and relax, the inner mucosal lining forms various patterns of folds and wrinkles. When the pylorus is pumping liquid from the stomach to the small intestine, the two sets of muscles work together to shape folds and wrinkles that "grip" chunks of food, prevent them from passing, and then launch them backward, back into the stomach. The shapes of the folds is highly variable, and it appears that your pylorus adapts these intricate patterns in its lining from moment to moment based on the properties of the food in your stomach-- it's texture, viscosity, the shapes and sizes of the chunks in it, etc.
How does your stomach "know" these details about the food in it? I don't think anybody really knows yet.
In case anyone's looking for an exact source, here's the money quote from page 62 of the book:
> A certain contraction of the outer muscular tube is necessary for the formation of macroscopic mucosal folds. The variability of the fold pattern is also dependent on the hydrodynamic action of the fluid content of the submucosa, which in turn depends on the degree of filling of the blood vessels; the mass of mucous membrane, and consequently the volume of its folds, is regulated by the varying vascularity in the submucosa.
> In addition Forssell showed that the surface of the mucosa, or mucosal relief pattern, may vary from moment to moment; these movements are independent of, but co-ordinated with the contractions of the muscularis externa. Especially in the small intestine, but also in the stomach, various active contractile shapes, in some of which the mucosa "grips" particles of food, may be discerned. These consist of digestion chambers, blocking or filtering devices, re-absorption reservoirs and smooth or corrugated transporting tubes. In this way each region may best meet the varying demands placed on it from moment to moment, namely digestion, storage, absorption and transport respectively. One moment the mucosa may be occupied with one task, the next with another. Forsell called the inherent ability of the mucosa to move "mucosal autoplastik", providing a working relief pattern. It also determined to a large extent the number, position and form of the folds. While the coarser breakdown of food particles is effected by contractions of the muscularis externa, the finer dispersion occurs through changes in the relief pattern of the mucosa, which may enhance or counteract effects of contraction of the muscular walls. The special contractile organ of the mucous membrane is the muscularis mucosae; being attached to the mucosa and being incorporated in the submucosa, it is able to displace the former in different directions.
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EDIT
It's been almost twenty years since I read this book, and re-reading the quote above, I realize that this wild mechanism I described is also used in the small intestine (not just in the distal stomach and pylorus). It makes perfect sense, but it never occurred to me that this is how a whole lot of our guts work to move and process food. It's kind of another epiphany for me.
So here again, how does your stomach "know" when to stop trying to digest certain pieces of food and let them pass on to the small intestine? This topic is called gastric emptying, and it's a whole other rabbit trail. Your stomach monitors the chemistry of the food in it-- specifically, the chemistry of the digestion products generated as the food breaks down. It knows lots of details about the carbohydrate, protein, and fat being released, and it knows how quickly its digestive enzymes and acids are being used up. So it knows how much more work needs to be done, and whether or not its worth trying to get more nutrients out of a piece of unchewed nut, for example.
There are many other variables, and I think digestion is still largely shrouded in mystery. Studies have shown variation in gastric emptying rates based on the biological activity of food, infections in gut tissues (even if they're subclinical, i.e., you don't feel unwell), time of day, and even the person's emotional state. The guts are the seat of a large part of the autonomous nervous system, and science has barely scratched the surface of it.
I'm not an expert in, well, anything, so I rely a lot on Wikipedia for the basics.