The human pancreas is a gland situated behind the stomach that performs two related roles. Its endocrine function involves releasing hormones like insulin into the bloodstream to regulate blood sugar. The pancreas also has an exocrine function, which involves producing and secreting substances through a duct system into the small intestine to aid digestion. The vast majority of the pancreas, about 99%, is dedicated to this exocrine purpose.
Acinar and Ductal Cells
The exocrine portion of the pancreas is comprised of two principal cell types: acinar cells and ductal cells. Acinar cells form the bulk of the pancreas and are arranged in clusters called acini. These cells are highly specialized for their secretory role, containing the cellular machinery to produce large quantities of proteins. They are responsible for synthesizing the digestive enzymes that the pancreas provides to the digestive system.
Ductal cells line the network of tubes that transport fluids from the acini. These cells form ducts that channel secretions toward the main pancreatic duct, which connects to the duodenum, the first part of the small intestine. While acinar cells produce the enzymes, ductal cells are responsible for modifying the fluid that carries them.
Digestive Enzyme Production
The primary function of acinar cells is the synthesis and secretion of digestive enzymes for breaking down food. These enzymes are stored within the cells in small packages called zymogen granules before they are released. To break down a meal, acinar cells produce distinct enzymes. They secrete pancreatic amylase for carbohydrates and pancreatic lipase for fats. The digestion of proteins is handled by proteases, such as trypsinogen and chymotrypsinogen.
Proteases are secreted in an inactive form, known as zymogens, as a protective measure for the pancreas. If the proteases were active within the pancreatic cells, they would begin to digest the organ itself. Once they reach the small intestine, other enzymes convert them into their active forms, like trypsin and chymotrypsin, where they can safely break down dietary proteins.
Neutralizing and Transporting Secretions
The function of ductal cells is the secretion of a watery, bicarbonate-rich fluid. These cells contain an enzyme called carbonic anhydrase, which helps produce bicarbonate. This secretion serves two purposes. It acts as a vehicle, flushing the enzymes from the acinar cells through the duct system and into the duodenum.
The bicarbonate component is also needed to create the proper chemical environment in the small intestine. Chyme, the partially digested food from the stomach, is highly acidic. The bicarbonate fluid neutralizes this stomach acid, which is necessary because pancreatic enzymes only function effectively in an alkaline (non-acidic) environment.
Regulation of Pancreatic Secretions
The release of pancreatic juices is regulated by hormones in response to food arriving in the small intestine. This ensures enzymes and bicarbonate are delivered when needed for digestion. The control system relies on two main hormones from the duodenum: cholecystokinin (CCK) and secretin.
When fats and proteins enter the duodenum, they trigger the release of CCK. CCK travels to the pancreas and stimulates the acinar cells to release their digestive enzymes. The presence of stomach acid in the duodenum prompts the release of secretin. Secretin’s main target is the pancreatic ductal cells, which it stimulates to secrete the bicarbonate-rich fluid to neutralize the acid.
Conditions Related to Exocrine Cell Dysfunction
When the exocrine cells of the pancreas do not function correctly, it can lead to health problems. Pancreatitis is a condition characterized by inflammation of the pancreas. It can occur if digestive enzymes become activated prematurely within the pancreas instead of the small intestine. This causes the enzymes to begin digesting the pancreatic tissue, leading to inflammation, pain, and damage.
Another condition is Exocrine Pancreatic Insufficiency (EPI), where the pancreas fails to produce or secrete enough digestive enzymes to break down food properly. This leads to maldigestion, where nutrients from food are not absorbed effectively. Symptoms can include diarrhea, weight loss, and nutritional deficiencies. Genetic disorders, such as cystic fibrosis, can also impact exocrine pancreatic function by altering secretions and blocking the pancreatic ducts.