Normal structure and function
The pancreas is a relatively small (70 to 110 gm) but versatile organ containing two specific and seemingly independent components: (a) the endocrine pancreas and (b) the exocrine pancreas. The endocrine pancreas consists of the islets of Langerhans, packets of endocrine cells peppered randomly throughout the pancreas that secrete insulin, glucagon and other polypeptide hormones. The cells of the exocrine pancreas cluster into acini that are further grouped in lobules. Acinar cells are drained by ductules that converge into ducts of increasing size, terminating in the duct of Wirsung, which drains through the sphincter of Oddi and the papilla of Vater into the second portion of the duodenum. The head of the pancreas lies within the curvature of the duodenum and the body and tail extend for about 12 to 15 cm retroperitoneally toward the hilum of the spleen (Figure 1). The head of the pancreas is in close anatomical relationship with a number of vital structures, including the common bile duct, the inferior vena cava, the aorta and the origin of the superior mesenteric artery, the splenic artery and vein, and the right adrenal gland and kidney.
Figure 1. Connections of the ducts of the gallbladder, Liver and pancreas.
The normal pancreas secretes a large volume (up to 1500 ml per day) of a distinctive fluid.
Acinar cells secrete fluid that resembles extracellular fluid: ductular cells progressively add a bicarbonate-rich liquid so that the relative amount of bicarbonate increases and may reach as high as 120 mEq/L. Pancreatic bicarbonate is an important factor in neutralizing gastric acid in the duodenal bulb.
Pancreatic fluid is protein-rich, more than 90 per cent of which represents enzymes or proenzymes secreted by acinar cells: (1) active form-lipase, amylase and ribonuclease and (2) inactive form-protease and phospholipase. The inactive proenzymes are activated in cascade fashion in the gut: enterokinase converts trypsinogen to trypsin; trypsin then activates all of the other proenzymes. Other proteins in pancreatic juice are colipase, which binds to and enhances lipase activity and trypsin inhibitors.
Control of Secretion. The small basal secretion of the exocrine pancreas is simulated by a variety of factors, the relative importance of which is unknown. Most pancreatic secretion occurs postparandially in response to one or more of three stimuli:
(1) Hormones. Two hormones seem most important and potentiate each other: secretin, which simulates ductular cells to increase water and bicarbonate; cholecystokinin (CCK), which simulates acinar cells to secrete enzymes and proenzymes.
(2) Cephalic simulation. Vagal, cholinergic pathway simulate an increase in enzyme-rich fluid.
(3) Enteropancreatic cholinergic reflex pathway.
Dr. Afsaneh Jeddi