What causes vomiting with reflux?
That's a great question, and I will give you two ways that vomiting happens with reflux. There are other ways, but these are two of the common ones. Neither of these reasons has much to do with serotonin.
First, in very young children, vomiting happens because their stomach acts like a stomp rocket. I bring up the subject of a stomp rocket because I recently made one. I'll try to describe what the final product looked like, but if you are unfamiliar with a stomp rocket, at this point, I'd urge you to use your computer or your phone and find an image of one. This toy consists of a hollow tube that is attached to an air pump. When I stomp on the air pump, air fills the hollow tube. The air pushes against the tube until it pops off. The rocket launches. In my case, in addition to having the hollow tube as part of my stomp rocket, I also curled up a small piece of paper into a ball. I put the ball into the hollow tube of my stomp rocket. When I stomped on the air pump, air filled the hollow tube that had the ball of paper. Both the hollow tube and the ball of paper launched into the air.
Now, there are two ways that I can hold a hollow tube attached to the air pump. In the first way, I can hold one end of the hollow tube on the ground. Air cannot escape from the end of the hollow tube that is firmly held to the ground. The ground is acting like a sort of sphincter to prevent air from going that way. This is how a stomach works when the sphincter connecting the stomach to the esophagus is closed shut. Food can't go back up into the esophagus and eventually out of the mouth. This sphincter allows the food to move in one direction - further down the digestive tract into the intestines.
Now let's say that I hold the stomp rocket in a different way. I pick up the hollow tube so that one end is no longer on the ground. Both ends of the hollow tube are now in the air. Now let's say that I stomp on the air pump really hard. Air can go both ways in the stomp rocket. If I have small balls of paper inside the hollow tube, the balls can go in either direction. Both sides of the hollow tube are in the air.
It's a similar concept with young children and babies who have reflux. Their sphincters that would prevent food from going up into the tube connected to the mouth are loose. They're not firmly attached to the "ground". When their stomachs do their jobs as "air pumps", food goes both ways: either further down the digestive tract into the intestines or further up the esophageal tube and perhaps even straight out of the mouth. Got it? It takes babies a while for this sphincter to become stronger. It takes a while before the stomp rocket known as the stomach has one end firmly attached to the ground - allowing food to go in only one direction. This is a mechanical issue that's tied to physics. It's not due to how serotonin affects certain structures around the brain that trigger vomiting.
That is reason number one for how young children vomit, and it has to do with physics, stomp rockets, and loose sphincters. A loose sphincter between the stomach and the esophagus! This is something that happens to just about every baby. Just about every baby has a loose sphincter that connects the stomach to the tube that goes up to the mouth. Sometimes doctors assign a name to vomiting that happens with a lose sphincter. They call it reflux, but it's not really a disease unless it's more severe.
More severe reflux is known as gastroesophageal reflux disease. It's a condition where a lot of stomach acid flows back into the tube connecting the mouth and the stomach. It flows back so much that the tube gets an "acid burn". It gets irritated from all of that extra acid.
Now, let's talk about this irritation of the esophagus - the tube connecting the mouth and the stomach. If this tube is irritated enough, then the sphincter might not work really well. When the stomach does its air pump job, food might go both ways - not just further down the digestive tract. It might also go straight from the stomach into the esophagus and eventually perhaps up and out of the mouth. You have to think that when the esophagus is irritated, sometimes the sphincter is irritated too. Not always!
In addition, the moist tissue that lines the inside of the esophagus - the tube connecting the mouth and the stomach - can get irritated. It has nerve endings. When it's irritated enough, it acts like a young child might act if really irritated. The child might tell a parent about the mean thing that another child did. The child might want the parent to retaliate on their behalf. In the same way, the tube connecting the mouth and the stomach might say, "You know! I'm feeling really irritated and crabby. Let me lash out and tell my parent to hurt this child." The nerves in the stomach tell the brain, "Make the body vomit." Like a very complaint mother who allows her child to get everything that they want, the brain listens to the child. The body vomits. All thanks to the irritated esophagus and the direct nerve pathway between the digestive tract and the brain!
This episode is part of the