The pancreas is positioned behind the stomach and performs a dual role, classified as both endocrine and exocrine. The endocrine component comprises the Islets of Langerhans, which release hormones like insulin and glucagon directly into the bloodstream to manage blood sugar levels. The majority of the pancreas is dedicated to its exocrine function, synthesizing and secreting substances necessary for digestion into the gastrointestinal tract.
Pancreatic Juice: Overview and Delivery
The substances secreted by the exocrine pancreas are collectively known as pancreatic juice. This secretion is produced and modified by acinar cells, which synthesize and release digestive enzymes, and duct cells, which contribute the bulk of the fluid and bicarbonate. The pancreas secretes a substantial volume of this juice, typically ranging from one to two liters per day. This fluid leaves the pancreas through a network of ducts that merge into the main pancreatic duct. The main duct then delivers the pancreatic juice into the duodenum, the first section of the small intestine, where it mixes with partially digested food arriving from the stomach.
Digestive Enzymes Secreted
The acinar cells secrete a suite of digestive enzymes capable of breaking down proteins, fats, and carbohydrates.
Enzymes for protein digestion, known as proteases, are secreted in an inactive form called zymogens to prevent the pancreas from digesting itself. For example, trypsinogen and chymotrypsinogen are released into the duodenum. There, an intestinal enzyme called enterokinase activates trypsinogen into trypsin. The resulting active trypsin then initiates a cascade, activating the remaining zymogens like chymotrypsinogen and procarboxypeptidase.
The digestion of fats relies on pancreatic lipase, which is secreted in its active form. This enzyme hydrolyzes triglycerides, breaking them down into fatty acids and monoglycerides. For starches and other complex carbohydrates, the pancreas secretes pancreatic amylase. This enzyme continues the digestive process started in the mouth, breaking down starches into simpler sugars like maltose.
Neutralizing Acidity: The Role of Bicarbonate
A second component of the exocrine secretion is the bicarbonate ions (\(\text{HCO}_3^{-}\)), which are secreted by the duct cells. Bicarbonate is dissolved in the fluid, making the overall secretion highly alkaline, with a pH that can reach 8.0 to 8.3. The primary purpose of this alkalinity is to neutralize the acidic chyme entering the duodenum from the stomach. The bicarbonate acts as a buffer, raising the pH of the chyme to a safe, slightly alkaline range (typically between pH 7 and 8). This neutralization is necessary both to protect the duodenal wall and to create the optimal environment for the pancreatic enzymes to function, as they operate most efficiently at this neutral to mildly alkaline pH.
Regulatory Signals for Secretion
The release of pancreatic juice is controlled by signals originating in the duodenum. Two primary hormones released from the intestinal lining regulate this process: Secretin and Cholecystokinin (CCK).
The presence of acidic chyme in the duodenum triggers the release of Secretin into the bloodstream. Secretin stimulates the pancreatic duct cells to secrete a large volume of fluid rich in bicarbonate.
The presence of partially digested fats and proteins stimulates the release of CCK. CCK primarily acts on the acinar cells, instructing them to release the enzyme-rich component of the pancreatic juice. These two hormones work in a coordinated fashion to provide the necessary enzymes and alkaline fluid for efficient digestion.