Digestive System

The pancreas is a banana-shaped organ that lies directly below and behind the stomach.  The tissue that makes up the pancreas has a spongy feel to it. The pancreas consists of 3 parts called the head, the body, and the tail. The head, which is the widest part of the pancreas, is located to the right of the body of the pancreas. The tail, which has a blunt tapered edge, is located to the left of the body of the pancreas.

The pancreas is considered to be both an exocrine gland (a gland with ducts) and an endocrine gland (a gland without ducts).  The portion of the pancreas with ducts has cells called acini. These cells secrete pancreatic juice through a series of small ducts that eventually join together to form the pancreatic duct. The pancreatic duct runs on the inside of the pancreas from one end to the other like a midrib in a leaf. The pancreatic duct branches out to join the common bile duct, which leads to the sphincter of Oddi and the duodenum. When the sphincter of Oddi is open, bile and pancreatic juice are secreted into the duodenum at the same time.

Even though the common bile duct is the main means for the pancreas to secrete pancreatic juice into the duodenum, the pancreas also has a another duct that can bypass the sphincter of Oddi. This duct is a called the accessory pancreatic duct. The accessory pancreatic duct branches off at the main pancreatic duct and goes directly into the duodenum. Thus, this duct allows the pancreas to secrete pancreatic juice into the duodenum regardless of whether the liver and the gallbladder are secreting bile.

The pancreatic juice secreted through these ducts contains a mixture of water, electrolytes, sodium bicarbonate, enzymes, and molecules called zymogens. When the chyme is in the duodenum, the sodium bicarbonate in the pancreatic juice mixes with the chyme and neutralizes the acid from the stomach. Thus, the sodium bicarbonate helps to reduce the effect of the stomach acid in the chyme, so that the acid does not burn a hole in the duodenum.

The enzymes in the pancreatic juice consist of 4 different enzymes called pancreatic amylase, pancreatic lipase, ribonuclease, and deoxyribonuclease. After these enzymes are secreted into the duodenum, they come into contact with the bile and the ions within the cavity of the small intestine. Then they become active and help to break down the structures of the nutrients in the chyme. Each enzyme has a specific function. Like the amylase in saliva, pancreatic amylase helps to break down starch into more simple sugars. Pancreatic lipase helps to break down fat. Nucleases, such as deoxyuribonuclease and ribonuclease, help to break down the DNA and RNA structures of nutrients in the chyme.

Pancreatic juice also contains tiny molecules called zymogens.  These zymogens are called trypsinogen, chymotrypsinogen, and procarboxypeptidase. Unlike enzymes, zymogens are not in a form in which they can break down nutrients when they are first secreted into the duodenum.  Instead, they change into enzymes when they reach the small intestine. For instance, when trypsinogen reaches the small intestine, it comes into contact with enterokinase, an enzyme on the surface of the cells in the wall of the intestine. Enterokinase converts trypsinogen into trypsin. Trypsin then converts chymotrypsinogen into  chymotrypsin and procarboxypeptidase into carboxypeptidase. Each of these enzymes help to break down proteins in the chyme.

The portion of the pancreas that does not have ducts has cells called Islets of Langerhans.  When an increased amount of glucose is found in the blood, the Islets of Langerhans will secrete insulin into the blood, which prevents glucose from being released. However, if decreased amounts of glucose are found in the blood, the Islets of Langerhans will secrete glucagon into the blood.  This glucagon will allow glucose to be released into the blood.