Pancreas

Exocrine glands within the pancreas secrete an aqueous fluid referred to as pancreatic juice. This fluid is alkaline and contains a high concentration of bicarbonate ion; it is transported to the duodenum by the pancreatic duct.

Pancreatic juice neutralizes the acidic chyme entering the duodenum from the stomach. Neutralization not only prevents damage to the duodenal mucosa, but also creates a neutral or slightly alkaline environment optimal for the function of pancreatic enzymes. The pancreas also secretes several enzymes involved in the digestion of carbohydrates, proteins, and lipids. The three major phases of pancreatic secretion are:

• Cephalic phase: approximately 20% of pancreatic secretory response to a meal

• Gastric phase: 5 to 10% of pancreatic secretory response to a meal

• Intestinal phase: approximately 80% of pancreatic secretory response to a meal

As with gastric secretion, nervous stimulation and hormones regulate secretion from the pancreas. During the cephalic phase and gastric phase, the pancreas secretes a low-volume, enzyme-rich fluid mediated by the vagus nerve.

Most pancreatic secretion takes place during the intestinal phase. The intestinal hormone secretin stimulates release of a large volume of pancreatic juice with a high concentration of bicarbonate ion. Secretin is released in response to acidic chyme in the duodenum (maximal release at pH < 3.0). The intestinal hormone cholecystokinin is released in response to the presence of the products of protein and lipid digestion. Cholecystokinin then stimulates the release of digestive enzymes from the pancreas.

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