When Highly Acidic Chyme Enters The Duodenum, Which Hormone Stimulates The Release Of Pancreatic Secretions That Contains Very High Bicarbonate Ion Content?

a. Gastrin
b. Secretin
c. Cholecystokinin

d. Histamine

The answer is Secretin.

Explain your answer.

250 words and TWO references

The duodenum is the first segment of the small intestine, where the chyme from the stomach mixes with the digestive secretions from the pancreas and liver. Chyme is highly acidic due to the hydrochloric acid secreted by the parietal cells of the stomach. When chyme enters the duodenum, the acidic environment can damage the intestinal epithelium and inhibit the activity of digestive enzymes. To counteract this effect, the duodenum releases the hormone secretin, which stimulates the release of pancreatic secretions containing high bicarbonate ion content.

Secretin is a peptide hormone secreted by the S cells in the duodenal mucosa. When the pH of the duodenal lumen decreases due to the arrival of acidic chyme, secretin is released into the bloodstream and travels to the pancreas. In the pancreas, secretin binds to receptors on the acinar cells, which are responsible for producing and releasing pancreatic juice. Secretin stimulates the acinar cells to secrete a bicarbonate-rich fluid that neutralizes the acidic chyme and raises the pH to an optimal range for digestive enzymes.

The bicarbonate ions in the pancreatic juice react with the hydrogen ions in the chyme to form carbonic acid, which is then converted to water and carbon dioxide by the enzyme carbonic anhydrase. This process generates a net increase in bicarbonate ions in the duodenal lumen, which effectively neutralizes the acidic chyme. The pancreatic juice also contains digestive enzymes such as lipase, amylase, and proteases, which are activated by the alkaline pH of the duodenum.

In conclusion, the hormone secretin plays a crucial role in regulating the pH of the duodenal lumen by stimulating the release of bicarbonate-rich pancreatic secretions. This mechanism ensures that the digestive enzymes can function optimally and prevent damage to the intestinal mucosa. Understanding the physiology of the duodenum and the hormonal regulation of pancreatic secretion is essential for diagnosing and treating digestive disorders.

References:

1. Wibisono, R. P., & Mohammed, A. (2021). Secretin. In StatPearls [Internet]. StatPearls Publishing.
2. Farthing, M. J. (2016). Physiology of the digestive system. Surgery (Oxford), 34(1), 8-13.