pylorus

Meaning: The pylorus is the narrow final region of the stomach, leading into the duodenum. It includes the pyloric antrum and the pyloric sphincter, a ring of smooth muscle. The sphincter is usually mostly closed and opens in small bursts, so acidic chyme enters the small intestine slowly enough to be neutralized by pancreatic bicarbonate and mixed with bile and pancreatic enzymes. G cells in the antrum secrete gastrin, which stimulates parietal cells and stomach motility.

On the MCAT: Expect questions that trace food through the tract: mouth, esophagus, lower esophageal sphincter, stomach, pylorus, duodenum. A typical prediction is that fat or acid in the duodenum triggers CCK, secretin and neural reflexes, which slow gastric emptying, so less chyme crosses the pylorus. Another common question asks where gastrin is made and what it does.

Common confusion: Students mix up the pyloric sphincter with the lower esophageal (cardiac) sphincter. The lower esophageal sphincter is the stomach's entrance and helps keep acid from backing up into the esophagus. The pylorus is the exit to the duodenum. The word comes from the Greek for gatekeeper, which is a handy way to remember that it controls what leaves the stomach.