**The Hormone Secretin and Its Secretion**

The hormone secretin is primarily secreted by the duodenum, which is the first part of the small intestine. It plays a crucial role in the digestive system, specifically in the regulation of gastric acid secretion and pancreatic enzyme secretion.

**Function and Regulation of Secretin**

Secretin is released in response to the presence of acidic chyme (partially digested food) in the duodenum. The acidic chyme stimulates the cells in the duodenal lining to release secretin into the bloodstream. Once secretin is released, it acts on various target tissues to regulate their functions.

**Effects of Secretin**

- **Gastric acid secretion:** Secretin inhibits gastric acid secretion in the stomach by acting on the parietal cells, which are responsible for producing hydrochloric acid. It does so by suppressing the activity of proton pumps in these cells, reducing the secretion of acid into the stomach.

- **Pancreatic enzyme secretion:** Secretin stimulates the secretion of pancreatic enzymes, such as bicarbonate ions and digestive enzymes like lipase and amylase. These enzymes are important for the digestion and absorption of nutrients in the small intestine.

- **Bile secretion:** Secretin also stimulates the liver to secrete bile, which is stored in the gallbladder and released into the intestine. Bile aids in the digestion and absorption of fats.

- **Growth of the pancreas:** Secretin promotes the growth and development of the pancreas, ensuring its proper functioning in the digestion process.

**Summary**

In summary, the hormone secretin is secreted by the duodenum in response to the presence of acidic chyme. It regulates gastric acid secretion, pancreatic enzyme secretion, bile secretion, and promotes the growth of the pancreas. These functions are essential for the proper digestion and absorption of nutrients in the small intestine.