The pancreas is a soft, elongated, lobulated gland located behind the stomach on the posterior abdominal wall. It lies across the upper abdomen in the epigastric and left hypochondriac regions and extends transversely from the duodenum to the spleen. 

It is divided into four main parts: the head, body, neck, and tail. The head is the widest part and lies within the C-shaped curve of the duodenum. The body extends across the abdomen, and the tail narrows toward the hilum of the spleen.

The endocrine portion consists of scattered clusters of cells called the islets of Langerhans. These cells release hormones directly into the bloodstream, including insulin, glucagon, somatostatin, and pancreatic polypeptide, which are involved in the regulation of metabolism and blood sugar levels.

Pancreatic Juice

The pancreas produces more than one liter of pancreatic juice each day. This clear, slightly alkaline fluid (pH 7.1 to 8.2) contains water, salts, sodium bicarbonate, and digestive enzymes. Sodium bicarbonate neutralizes the acidic chyme entering the duodenum from the stomach, inactivates pepsin, and creates an optimal environment for digestive enzymes to function.

Pancreatic enzymes help digest carbohydrates, proteins, fats, and nucleic acids. Amylase digests starch, lipase digests fats, and nucleases digest nucleic acids. Protein-digesting enzymes are secreted in inactive forms, including trypsinogen, chymotrypsinogen, and procarboxypeptidase, to prevent damage to the pancreas itself. In the duodenum, the enzyme enteropeptidase activates trypsin from trypsinogen, and trypsin then activates the other protein-digesting enzymes.

Regulation of Pancreatic Secretion

Pancreatic secretion is controlled by hormones and the parasympathetic nervous system. When acidic chyme enters the duodenum, it stimulates the release of the hormone secretin. Secretin causes pancreatic duct cells to release bicarbonate-rich pancreatic juice. The presence of proteins and fats in the duodenum stimulates the release of cholecystokinin (CCK), which causes the acinar cells to secrete enzyme-rich pancreatic juice and enhances the effects of secretin.

Parasympathetic stimulation, mainly through the vagus nerve, also promotes pancreatic secretion during the cephalic and gastric phases of digestion. Normally, the pancreas secretes enough bicarbonate to neutralize the hydrochloric acid produced by the stomach, helping maintain a stable blood pH as blood flows to the liver.