Although the pancreas is both an endocrine and exocrine gland, it is the exocrine secretions known as pancreatic juice that are of interest to us in relation to the digestive system. Approximately 1500 ml of pancreatic juice flows into the duodenum through the sphincter of Oddi each day.
A. PANCREATIC JUICE
Pancreatic juice is a watery solution containing high concentrations of bicarbonate ions. These bicarbonate ions contribute to the alkaline pH of pancreatic juice (pH ~ 8) and helps neutralise the highly acidic chyme that arrives from the stomach.
In addition to the bicarbonate ions there are over 20 different enzymes in pancreatic juice and these are generally categorised according to their substrates:
(i) Proteolytic Enzymes
This class of enzyme is responsible for the chemical digestion of protein when they mix with chyme in the small intestine. In order to prevent these enzymes destroying the pancreas they are usually secreted as inactive precursors.
When they reach the duodenum they are cleaved into their active forms and continue with the process of protein digestion that was initiated by the pepsins secreted by the stomach.
Some of these proteolytic enzymes are listed in the table on the right.
(ii) Pancreatic Amylase
The process of carbohydrate digestion is initiated by salivary amylase secreted by the salivary glands. This process is dramatically accelerated by the presence of the structurally-related pancreatic amylase. Pancreatic amylase is secreted in its activate form and cleaves the glycosidic bonds that hold polysaccharides together. Consequently carbohydrates are digested into smaller and smaller fragments until absorbable monosaccharides are formed.
Pancreatic amylase is responsible for the digestion of the vast majority of carbohydrate in the digestive tract.
(iii) Lipolytic Enzymes
Pancreatic juice also contains a number of enzymes that help digest lipids and are referred to as lipases. The most important of these is pancreatic lipase which is secreted in its active form and cleaves triglycerides to form fatty acids and monoglycerides.
B. CONTROL OF PANCREATIC JUICE SECRETION
Under fasting conditions a low level of pancreatic juice secretion keeps the pancreas ticking over. This is around 5% of maximal secretion rates and is known as basal release. In response to a meal, pancreatic juice secretion is significantly elevated in the usual three stages:
(i) Cephalic Phase
The sight, smell and taste of food all cause a small but significant elevation in pancreatic juice secretion. This appears to be mediated primarily by an increase in parasympathetic tone directly to the pancreas.
(ii) Gastric Phase
The presence of food in the stomach (in particular the products of protein digestion) produce a further increase in pancreatic juice secretion. This appears to be mediated primarily by the release of gastrin from the G cells of the stomach.
(iii) Intestinal Phase
The most significant enhancement of pancreatic secretion is produced by stimuli that affect the small intestine itself. This accounts for around 70% of the maximal secretion and is produced by a number of factors:
The low pH of the chyme in the duodenum causes the release of secretin from the S cells
. As well as inhibiting gastric motility (described previously) secretin enhances water and bicarbonate secretion by the duct cells of the pancreas.
The presence monoglycerides and fatty acids (and to a lesser extent peptides) in chyme stimulates the production of cholecystokinin from the I cells
The presence of monoglycerides and fatty acids (and to a lesser extent peptides) in chyme are also detected by chemosensitive sensory neurones that increase parasympathetic tone (through a long reflex) that further enhances pancreatic juice secretion
Because the magnitude of the increase in pancreatic juice secretion is dependent upon the size of the meal, these three phases ensure that an adequate supply of bicarbonate and digestive enzymes are available in the small intestine when they are required. As the food moves out of the small intestine the excitatory stimuli diminish and pancreatic juice secretion returns to its basal levels.