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The pancreas has portions : exocrine and endocrine. In this section, the exocrine portion is discussed.

The exocrine portion secretes the pancreatic juice which contains enzymes important in digestion.

A. Functional anatomy

The exocrine cells have abundant rough endoplasmic reticulum and zymogen granules at the apexes of the cells. The zymogen granules contain the enzymes. Secretion from them is emptied (by exocytosis) into small ducts; these small ducts join to form the pancreatic duct of Wirsung. The duct of Wirsung joins the common bile duct to form the ampulla of Vater. The ampulla of Vater opens into the duodenum through the duodenal papilla. The opening of ampulla of Vater is surrounded by a sphincter called the sphincter of Oddi. Some individuals have an accessory pancreatic duct (called the duct of Santorini); the duct of Santorini opens into the duodenum proximal to the opening of the ampulla of Vater. 

B. Pancreatic juice

It is a colourless, odourless watery secretion isotonic with plasma. It is alkaline with a high bicarbonate content of about 113 meq/L. (plasma bicarbonate content is about 24 meq/L). About 1500 mL of pancreatic juice is secreted per day. The alkaline pancreatic juice, along with bile and intestinal juice (which are also neutral or alkaline), helps in neutralizing the gastric acid and raising the pH of the duodenal contents to about 6.0 to 7.0.

Thus, by the time the chyme reaches the jejunum, its reaction is nearly neutral, but the intestinal contents are rarely alkaline.

Composition of the pancreatic juice

Inorganic constituents :

a. Cations : Na+, K+, Ca2+, Mg2+

b. Anions : HCO3-, Cl-, SO42-, HPO42-


2. Organic constituents

a. mucous

b. enzymes (refer chapter)

c. trypsin inhibitor

Conversion of the inactive proenzymes into active enzymes

The enzymes of the pancreatic juice are secreted as inactive proenzymes as follows :


3. Trypsinogen to tryspin

Trypsinogen is converted to the active enzyme trypsin by the brush border enzyme enteropeptidase (enterokinase) when the pancreatic juice enters the duodenum. Enteropeptidase has a high polysaccharide content (of about 40%); this high polysaccharide content apparently prevents it from being digested itself before it can exert its effect.


4. Conversation of other inactive pancreatic proteases into active forms

a. All the other inactive forms (e.g. chymotrypsinogens, proelastase, procarboxypeptidases) are converted into

b. their active forms by trypsin. Trypsin can also activate trypsinogen; therefore, once some trypsin is formed,

c. there is an auto-catalytic chain reaction.

d. Enteropeptidase deficiency occurs as a congenital abnormality and leads to protein malnutrition.

5. Trypsin inhibitor

This is secreted by the pancreas in the pancreatic juice. It is a polypeptide with a molecular weight of about

5000 to 6000.

Action : It inhibits both trypsin and chymotrypsin.


6. Function

Trypsin is a very powerful enzyme by itself and also converts the other inactive pancreatic enzymes into their powerful active forms. The conversion of trypsinogen to trypsin occurs in the duodenum by enteropeptidase. 

However, it is possible that some trypsin may get activated within the pancreas itself. This would lead to autodigestion of the pancreas. Trypsin inhibitor prevents this autodigestion by trypsin.


7. Trypsin and acute pancreatitis

  • One of the enzymes activated by trypsin is phospholipase A2. Phospholipase A2 converts lecithin to lyso-lecithin (by removing a fatty acid). Lysolecithin damages cell membranes.
  • Lecithin is a normal constituent of bile. It is possible that acute pancreatitis, phospholipase A2 is activated in the pancreatic ducts; this in turn forms lysolecithin from lecithin. Lysolecithin damages the pancreatic tissue.
  • Small amounts of pancreatic digestive enzymes normally leak into the circulation, but in acute pancreatitis, the circulating levels of the digestive enzymes rise markedly. Measurement of the plasma amylase or lipase concentration is therefore of value in diagnosing the disease.

Regulation of pancreatic secretion

Secretion of pancreatic juice is primarily under the hormonal control of secretin and CCK-PZ.

  1. Actions of secretin :
a. Acts on the pancreatic ducts to cause copious secretion of a very alkaline pancreatic juice that is rich in HC03 and poor in enzymes; its effect is mediated by cAMP.
b. Stimulates bile secretion.
  1. Actions of CCK-PZ
a. Acts on the acinar cells and causes discharge of the zymogen granules;  it causes low volume secretion of pancreatic juice that is rich in enzymes; its effect is mediated by phospholipase C

Other hormones/factors

  1. Gastrin

This plays a minor role in pancreatic secretion. It acts in two ways :

  • Direct action : it stimulates the pancreatic acinar cells directly and increases acinar secretion
  • Indirect action : it stimulates the parietal cells and increases HCl secretion; HCl in turn enters the duodenum and releases both secretin and CCK-PZ. This causes increase in enzymes, bicarbonate and water output.
  1. VIP

VIP stimulates pancreatic secretion mainly rich in enzymes. It also stimulates intestinal secretion of electrolytes and water.

  1. Vagal stimulation

This causes secretion of a small amount of pancreatic juice rich in enzymes. The acetylcholine released acts directly on the acinar cells to cause discharge of the zymogen granules; like CCK, acetylcholine acts on via phospholipase-C.  There is evidence for vagally mediated conditioned reflex secretion of pancreatic juice in response to the sight or smell of food.

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