Page 11 - 86 human physiology part-2

Chapter 12

and the gallbladder stores the bile until it is needed. The enzymes and bile travel through special
channels called ducts and into the small intestine where they help break down the food.

Pancreas

The pancreas is located posterior to the stomach and in close association with the duodenum.

In humans, the pancreas is a 6-10 inch elongated organ in the abdomen located retro peritoneal. It
is often described as having three regions: a head, body and tail. The pancreatic head abuts the second
part of the duodenum while the tail extends towards the spleen. The pancreatic duct runs the length of
the pancreas and empties into the second part of the duodenum at the ampulla of Vater. The common
bile duct commonly joins the pancreatic duct at or near this point.

The pancreas is supplied arterially by the pancreaticoduodenal arteries, themselves branches of the
superior mesenteric artery of the hepatic artery (branch of celiac trunk from the abdominal aorta). The
superior mesenteric artery provides the inferior pancreaticoduodenal arteries while the gastroduodenal
artery (one of the terminal branches of the hepatic artery) provides the superior pancreaticoduodenal
artery. Venous drainage is via the pancreatic duodenal veins which end up in the portal vein. The
splenic vein passed posterior to the pancreas but is said to not drain the pancreas itself. The portal vein
is formed by the union of the superior mesenteric vein and splenic vein posterior to the body of the
pancreas. In some people (as many as 40%) the inferior mesenteric vein also joins with the splenic vein
behind the pancreas, in others it simply joins with the superior mesenteric vein instead.

The function of the pancreas is to produce enzymes that break down all categories of digestible
foods (exocrine pancreas) and secrete hormones that affect carbohydrates metabolism (endocrine
pancreas).

• Exocrine

The pancreas is composed of pancreatic exocrine cells, whose ducts are arranged in clusters called
acini (singular acinus). The cells are filled with secretory granules containing the precursor digestive
enzymes (mainly trypsinogen, chymotrypsinogen, pancreatic lipase, and amylase) that are secreted into
the lumen of the acinus. These granules are termed zymogen granules (zymogen referring to the
inactive precursor enzymes.) It is important to synthesize inactive enzymes in the pancreas to avoid
auto degradation, which can lead to pancreatitis.

The pancreas is near the liver, and is the main source of enzymes for digesting fats (lipids) and
proteins - the intestinal walls have enzymes that will digest polysaccharides. Pancreatic secretions from
ductal cells contain bicarbonate ions and are alkaline in order to neutralize the acidic chyme that the
stomach churns out. Control of the exocrine function of the pancreas are via the hormone gastrin,
cholecystokinin and secretin, which are hormones secreted by cells in the stomach and duodenum, in
response to distension and/or food and which causes secretion of pancreatic juices.

The two major proteases which the pancreas are trypsinogen and chymotrypsinogen. These
zymogens are inactivated forms of trypsin and chymotrypsin. Once released in the intestine, the
enzyme 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.

226 | Human Physiology