The pancreas plays multiple roles in digestion, absorption, metabolism and storage of nutrients. Its diverse functions can be broadly divided into its dual glandular activity :
- Exocrine – digestive enzymes secreted into the small intestine.
- Endocrine – hormones secreted directly into the blood stream.
Pancreatic Exocrine Functions
The acinar cells of the pancreas produces enzymes for digesting food in the gut. It is then released into the ductules which empty the contents into the pancreatic duct.
These chemical catalysts are inactive until it enters the lumen of the small intestine, where it is activated by the acidity of the stomach contents or other digestive enzymes. Various enzymes act on different types of food.
The pancreatic enzymes chemically break down the food into simpler substances which can be digested further by other enzymes or absorbed through the lining of the gut.
Protein Digesting Pancreas Enzymes
Carbohydrate Digesting Pancreas Enzymes
Fat Digesting Pancreas Enzymes
- Pancreatic lipase
- Cholesterol esterase
The actions and regulation of these catalysts are discussed further below.
Pancreas Endocrine Functions
The islet cells of the pancreas (islet of Langerhans) produce hormones to regulate metabolism and digestion. It also influences the storage of excess nutrients.
The two main hormones are insulin and glucagon. Insulin increases the uptake of glucose by the cells thereby reducing the blood glucose levels and promoting storage of excess glucose. Glucagon mobilizes glucose from the glucose stores and increases the blood glucose levels. Both these hormones can influence nutrient digestion and absorption to some extent.
Somatostatin is a digestive hormone that is also produced by islets as well as the D cells in the lining of the gastrointestinal tract. It reduces the secretion of the pancreatic enzymes (exocrine) and stomach acid, inhibits insulin and decreases nutrient absorption from the gut. Somastatin therefore also influences metabolism.
What are pancreatic enzymes?
Pancreatic enzymes are the chemical catalysts secreted by the pancreas into the duodenum to digest food. This is known as the exocrine functions of the pancreas. Being a compound gland, the pancreas also has endocrine functions which is to release hormones like insulin and glucagon in response to changes in the blood sugar levels. These hormones also play a role in controlling the secretion of digestive enzymes. However, it is the enzymes itself that function in the small intestine to break down foods into simpler nutrients so that it can be easily absorbed into the blood stream. Along with water, bicarbonates ions and mucus, the total exocrine secretion from the pancreas is approximately 1 liter per day.
These digestive enzymes from the pancreas are secreted by the acini within the gland and this empties into the pancreatic duct. From here, it joins the common bile duct and empties into the duodenum of the small intestine. Apart from the enzymes for digestion, the pancreas also secretes bicarbonate ions and water. These compounds are secreted by the epithelial cells that line the pancreatic duct and help to neutralize the gastric acid from the stomach in addition to transporting the pancreatic enzymes out of the pancreas. The pancreatic enzymes do not act in isolation – digestive enzymes from other structures of the gastrointestinal tract play a role in the complete digestion of food.
Types of Pancreatic Enzymes
The pancreatic enzymes can be categorized according to their action on different types of food – carbohydrates, protein and fats.
Pancreatic Enzymes for Protein Digestion
There are three pancreatic enzymes for the digestion of proteins within the food in the small intestine. These type of enzymes are known as proteases.
Of these, trypsin is the most abundant. The pancreas do not secrete these enzymes in it active form as this may result in digestion of the pancreas itself. Instead each enzyme is secreted in an inactive form as follows :
- Trypsin is secreted as trypsinogen
- Chymotrpsin is secreted as chymotripsinogen
- Carboxypolypeptidase is secreted as procarboxypolypeptidase
- Proelastase is secreted as elastase
Once in the lumen of the duodenum, these enzymes are activated. Trypsinogen, which is the most abundant of the three pancreatic enzymes, is activated by the presence of enterokinase which is an enzyme secreted by the lining of the small intestine. Further trypsinogen is also activated by the presence of trypsin in the gut.
Chymotrypsin, procarboxypolypeptidase and proelastase are in turn activated by trypsin. The pancreas also has other protective mechanisms to ensure that these enzymes do not damage it. The cells of the pancreas secrete trypsin inhibitor which ensures that trypsin is not activated within the pancreas and it turn, neither are the other proteases.
Action of Trypsin, Chymotrypsin and Carboxypolypetidase
Trypsin and chymotrypsin breaks down protein into peptides. Carboxypolypetidase in turn breaks down some peptides into individual amino acids but it is the peptidases secreted by the enterocytes of the small intestine that digest the rest. Elastase is more specific and digests collagen, particularly the elastin that holds fibers in meat together.
Pancreatic Enzymes for Carbohydrate Digestion
Pancreatic amylase is the enzyme responsible for carbohydrate digestion.
Action of Pancreatic Amylase
Pancreatic amylase hydrolyzes most carbohydrates into disaccharides and trisaccharides. These in turn are digested into monosaccharides by the small intestine enzymes – lactase, sucrase, maltase and iso-maltase.
Pancreatic Enzymes for Lipid Digestion
There are three pancreatic enzymes responsible for lipid digestion, each of which act on the main types of lipids in the body.
- Pancreatic lipase
- Cholesterol esterase
The pancreatic enzymes for lipid digestion are dependent on the secretion of bile from the liver and gallbladder. Bile acids emulsify fats meaning that it breaks down the fat into smaller globules. This allows the lipid enzymes to act more effectively on fats and sterols in the food.
Action of Pancreatic Lipase, Cholesterol Esterase and Phospholipase
The most important and abundant of these pancreatic lipid enzymes is pancreatic lipase. It breaks down triglycerides (neutral fats) into fatty acids and glycerol. Cholesterol esterase hydrolyzes cholesterol esters in the food while phospholipase acts on phospholipids. The remaining digestive action on fats are handled by the small intestine lipase.
Control of Pancreatic Enzyme Secretion
The secretion of digestive enzymes is regulated by nerve impulses and digestive hormones. When the stomach secretions are stimulated by the vagus nerves and enteric nervous system as discussed under stomach nerves, some of these impulses also trigger the release of acetylcholine in the pancreas. Acetylcholine along with cholecystokinin (CCK), which is a digestive hormone secreted in response to food in the duodenum, triggers the release of pancreatic enzymes into the acini.
Another digestive hormone known as secretin causes the secretion of large quantities of bicarbonate ions and water within the pancreas. This washes out the pancreatic enzymes in the acini into the duodenum. Secretin is released in response to acidic chyme in the duodenum and also neutralizes the gastric acid secretions that enter the duodenum.