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The gastroesophageal sphincter is the valve at the top of the stomach that allows food to enter the stomach from the esophagus, and then keeps the food and digestive juice from leaking back up into the mouth. Theoretically. In practice, the valve tends to be a little or a lot leaky. I call it the burping and spitting valve because that's what it controls in babies.
The valve is positioned more or less at the top of the stomach, although there is often a potential air pocket above it. This is the reason a baby should be held upright for feedings and upright and leaning slightly forward for burping. That way the bubble is right under the valve. All that's necessary is to get the valve to open, which after a feeding happens normally from the alternate waves of contraction and relaxation as the stomach churns the food-digestive juice mixture.
If the valve is abnormally leaky, the baby may experience frequent spitting up or vomiting (gastroesophageal reflux), or may develop reflux esophagitis, a painful condition sometimes confused with colic.