Introduction:
The pH level in the duodenum plays a crucial role in regulating various digestive processes. The duodenum is the first part of the small intestine where the partially digested food from the stomach enters. It receives secretions from multiple organs, which are regulated by the pH level. When the pH of the duodenum decreases below 5, it triggers specific secretions to maintain the optimal conditions for digestion.

1. Secretin:
Secretin is a hormone released by the duodenal wall in response to low pH levels. When the pH drops below 5, secretin is released into the bloodstream. Secretin acts on the pancreas, stimulating the release of bicarbonate ions. Bicarbonate ions neutralize the acidic chyme coming from the stomach, raising the pH level and creating a suitable environment for digestive enzyme activity. This secretion helps to protect the duodenal lining from the corrosive effects of stomach acid.

2. Enterokinase:
Enterokinase is an enzyme secreted by the duodenal mucosa. It is released in response to low pH levels below 5. Enterokinase plays a crucial role in the activation of pancreatic enzymes. It converts trypsinogen, an inactive enzyme produced by the pancreas, into its active form trypsin. Trypsin then activates other pancreatic enzymes such as chymotrypsin, carboxypeptidase, and pancreatic amylase. These activated enzymes help break down proteins, carbohydrates, and fats for absorption.

3. Gastrin:
Gastrin is a hormone released by the stomach in response to the presence of food. However, when the pH of the duodenum decreases below 5, it inhibits the release of gastrin. This is because the low pH indicates that the stomach acid has not been effectively neutralized yet. Inhibiting gastrin release helps prevent further acid secretion from the stomach, ensuring that the duodenum is not exposed to excessively acidic conditions.

4. Cholecystokinin:
Cholecystokinin (CCK) is a hormone released by the duodenal mucosa in response to the presence of fats and proteins. It stimulates the gallbladder to contract and release bile into the duodenum. However, when the pH of the duodenum decreases below 5, the secretion of cholecystokinin is reduced. This is because the low pH indicates that the chyme is still acidic and needs neutralization before the introduction of bile. Inhibiting CCK secretion allows time for the secretions of secretin and bicarbonate ions to raise the pH level before the release of bile.

Conclusion:
When the pH of the duodenum decreases below 5, the secretion of secretin, enterokinase, gastrin, and cholecystokinin is affected. Secretin and enterokinase are released to neutralize the acidic chyme and activate pancreatic enzymes, respectively. Gastrin secretion is inhibited to prevent further acid secretion from the stomach, while the secretion of cholecystokinin is reduced to delay the release of bile until the pH level is optimal. These mechanisms work together to maintain the ideal pH and create an optimal environment for digestion and absorption in the duodenum.

If the pH of duodenum decreases below 5, then secretion of gastric acid stop.

Secretin is secreted when pH of duodenum falls significantly. After secreting, it stops secretion of gastric juice by feedback control.