Secretin is secreted by
**Core Concept**
Secretin is a hormone involved in the regulation of digestion, particularly in the stimulation of pancreatic bicarbonate secretion and the inhibition of gastric acid secretion. It plays a crucial role in maintaining the optimal pH environment for intestinal enzyme activity.
**Why the Correct Answer is Right**
Secretin is secreted by the S cells of the duodenal mucosa in response to the presence of acidic chyme in the duodenum. This reflex is mediated by the release of gastrin-releasing peptide (GRP) from the duodenal mucosa, which in turn stimulates the S cells to release secretin. Secretin then acts on the pancreas to stimulate the release of bicarbonate-rich pancreatic juice, neutralizing the acidity of the chyme and creating an optimal environment for enzyme activity.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because secretin is not secreted by the stomach. While the stomach does secrete gastrin, which stimulates the release of secretin, the hormone itself is produced in the duodenum.
**Option B:** This is incorrect because gastrin is a hormone secreted by the G cells of the stomach, which stimulates gastric acid secretion. While gastrin does stimulate the release of secretin, it is not the source of the hormone.
**Option C:** This is incorrect because vasoactive intestinal peptide (VIP) is a hormone secreted by the L cells of the ileum, which stimulates intestinal secretion and relaxation of smooth muscle. While VIP does have some effects on pancreatic secretion, it is not responsible for the release of secretin.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the release of secretin is a key component of the duodenal reflex, which helps to regulate the pH environment in the small intestine. This reflex is critical for maintaining optimal enzyme activity and preventing damage to the intestinal mucosa.
**Correct Answer:** D. S cells of the duodenal mucosa