Lower esophageal sphincter

The lower esophageal sphincter is the door at the point where the esophagus, the tube that carries food down, meets the stomach. It is not a ring you can see as a separate piece: it is a high-pressure zone, meaning a stretch that stays squeezed shut and therefore blocks the way. Its job is to protect the esophagus from reflux, the return of stomach contents back up.

Two things produce that seal at the same time. On the inside is the intrinsic component: smooth muscle of the esophagus itself, the kind that works on its own without being told to, and it stays contracted. On the outside is the extrinsic component: the crura of the diaphragm, the big muscle of breathing, which wrap around that zone, plus the phrenoesophageal ligament that anchors it.

The interesting part is how it opens. It has no dilator muscles, meaning there is nothing to pull its edges outward. It opens only when the smooth muscle gets the signal to relax and lets go, and then the bolus goes through, the portion of food that has already been chewed and swallowed. It is like a rubber band around the neck of a bag: you do not open it by pulling on it, you open it by loosening it.