The
exocrine pancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to...
has ducts that are arranged in clusters called
acini (singular
acinus). Pancreatic secretions are secreted into the
lumenA lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
of the acinus, and then accumulate in intralobular ducts that drain to the main
pancreatic ductThe pancreatic duct, or duct of Wirsung , is a duct joining the pancreas to the common bile duct to supply pancreatic juices which aid in digestion provided by the "exocrine pancreas"...
, which drains directly into the
duodenumThe duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
.
Control of the exocrine function of the pancreas is via the hormones
gastrinIn humans, gastrin is a hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the stomach, duodenum, and the pancreas. Its release is stimulated by gastric luminal peptides...
,
cholecystokininCholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein...
and
secretinSecretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body.Acting as an anti-Müllerian hormone it is produced in the S cells of the duodenum in the crypts of Lieberkühn. Its effect is to regulate the pH of the duodenal contents...
, which are
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
s secreted by cells in the
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
and
duodenumThe duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
, in response to distension and/or food and which cause secretion of pancreatic juices.
There are two main classes of exocrine pancreatic secretions:
Pancreatic secretions from ductal cells contain
bicarbonateIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
ions and are alkaline in order to neutralize the acidic
chymeChyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum. In other words, chyme is half-digested food.Also known as chymus, it is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum...
that the stomach churns out.
The pancreas is also the main source of enzymes for digesting
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at normal room temperature, depending on their structure and composition...
s (lipids) and proteins.
The
exocrine pancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as an exocrine gland, secreting pancreatic juice containing digestive enzymes that pass to...
has ducts that are arranged in clusters called
acini (singular
acinus). Pancreatic secretions are secreted into the
lumenA lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
of the acinus, and then accumulate in intralobular ducts that drain to the main
pancreatic ductThe pancreatic duct, or duct of Wirsung , is a duct joining the pancreas to the common bile duct to supply pancreatic juices which aid in digestion provided by the "exocrine pancreas"...
, which drains directly into the
duodenumThe duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
.
Control of the exocrine function of the pancreas is via the hormones
gastrinIn humans, gastrin is a hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the stomach, duodenum, and the pancreas. Its release is stimulated by gastric luminal peptides...
,
cholecystokininCholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein...
and
secretinSecretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body.Acting as an anti-Müllerian hormone it is produced in the S cells of the duodenum in the crypts of Lieberkühn. Its effect is to regulate the pH of the duodenal contents...
, which are
hormoneA hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms...
s secreted by cells in the
stomachIn most mammals, the stomach is a hollow, muscular organ of the gastrointestinal tract , between the esophagus and the small intestine. It is involved in the second phase of digestion, following mastication . The word stomach is derived from the Latin stomachus, which derives from the Greek word...
and
duodenumThe duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
, in response to distension and/or food and which cause secretion of pancreatic juices.
There are two main classes of exocrine pancreatic secretions:
| Secretion |
Cell producing it |
Primary signal
>-
| bicarbonateIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
ions |
Centroacinar cells In some animals spindle-shaped cells occupy the center of the alveolus of the pancreas and are known as the centroacinar cells of Langerhans. These cells are commonly known as duct cells, and secrete an aqueous bicarbonate solution under stimulation by the hormone secretin...
|
Secretin Secretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body.Acting as an anti-Müllerian hormone it is produced in the S cells of the duodenum in the crypts of Lieberkühn. Its effect is to regulate the pH of the duodenal contents...
>-
| digestive enzymes |
Basophilic cells |
CCKCholecystokinin is a peptide hormone of the gastrointestinal system responsible for stimulating the digestion of fat and protein...
|
Pancreatic secretions from ductal cells contain
bicarbonateIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
ions and are alkaline in order to neutralize the acidic
chymeChyme is the semifluid mass of partly digested food expelled by the stomach into the duodenum. In other words, chyme is half-digested food.Also known as chymus, it is the liquid substance found in the stomach before passing through the pyloric valve and entering the duodenum...
that the stomach churns out.
The pancreas is also the main source of enzymes for digesting
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at normal room temperature, depending on their structure and composition...
s (lipids) and proteins. (The enzymes that digest polysaccharides, by contrast, are primarily produced by the walls of the
intestineIn anatomy, the intestine is the segment of the alimentary canal extending from the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s.)
The cells are filled with secretory granules containing the precursor digestive enzymes. The major proteases which the pancreas secretes are
trypsinogenTrypsinogen is the precursor form of the pancreatic enzyme trypsin or a zymogen. It is found in pancreatic juice, along with amylase, lipase, and chymotrypsinogen. It is activated by enteropeptidase, which is found in the intestinal mucosa, to form trypsin. Once activated, the trypsin can activate...
and
chymotrypsinogenChymotrypsinogen is a precursor of the digestive enzyme chymotrypsin .This molecule is inactive and must be cleaved by trypsin, and then by other...
. Secreted to a lesser degree are
pancreatic lipasePancreatic lipase is an enzyme secreted from the pancreas that uses hydrolysis to break apart fat molecules. Bile salts secreted from the liver and stored in gallbladder are released into the duodenum where they coat and emulsify large fat droplets into smaller droplets, thus increasing the...
and pancreatic
amylaseAn amylase is an enzyme that breaks starch down into sugar. Amylase is present in human saliva, where it begins the chemical process of digestion. Foods that contain much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns some of their...
. The pancreas also secretes
phospholipase A2Phospholipases A2 are enzymes that releasefatty acids from the second carbon group of glycerol. This particular phospholipase specifically recognizes the sn-2 acyl bond of phospholipids and catalytically hydrolyzes the bond releasing arachidonic acid and lysophospholipids...
,
lysophospholipaseIn enzymology, a lysophospholipase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are 2-lysophosphatidylcholine and H2O, whereas its two products are glycerophosphocholine and carboxylate....
, and cholesterol esterase.
It is important to synthesize inactive enzymes in the pancreas to avoid autodegradation, which can lead to
pancreatitisPancreatitis is inflammation of the pancreas that can occur in two very different forms. Acute pancreatitis is sudden while chronic pancreatitis "is characterized by recurring or persistent abdominal pain with or without steatorrhea or diabetes mellitus."...
. These granules are termed
zymogenA zymogen is an inactive enzyme precursor. A zymogen requires a biochemical change for it to become an active enzyme. The biochemical change usually occurs in a lysosome where a specific part of the precursor enzyme is cleaved in order to activate it...
granules (the term "zymogen" referring to the inactive precursor enzymes). Trypsinogen is an inactivated forms of
trypsinTrypsin is a serine protease found in the digestive system of many vertebrates, where it hydrolyses proteins. Trypsin is produced in the pancreas as the inactive proenzyme trypsinogen. Trypsin predominantly cleaves peptide chains at the carboxyl side of the amino acids lysine or arginine, except...
, and chymotrypsinogen is an inactivated form of
chymotrypsinChymotrypsin is a digestive enzyme that can perform proteolysis. Chymotrypsin cleaves peptides at the carboxyl side of tyrosine, tryptophan, and phenylalanine because these three amino acids contain aromatic rings, which fit into a 'hydrophobic pocket' in the enzyme...
.
Once released in the intestine, the enzyme
enteropeptidaseEnteropeptidase is an enzyme involved in human digestion. It is produced by cells in the duodenum wall, and is secreted from duodenum's glands, the crypts of Lieberkühn, whenever ingested food enters the duodenum from the stomach...
(formerly, and incorrectly, called enterokinase) present in the intestinal mucosa activates trypsinogen by cleaving it to form trypsin. The free trypsin then cleaves the rest of the trypsinogen and chymotrypsinogen to their active forms.
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