Gross anatomy of the pancreas
The pancreas is a retroperitoneal organ located posterior to the stomach on the posterior abdominal wall.
It is supplied arterially by the pancreaticoduodenal arteries, themselves branches of the superior mesenteric artery. Venous drainage is via the pancreaticoduodenal veins (duh) which end up in the portal vein. The splenic vein passes 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 (some books say 40% of people), the inferior mesenteric vein also joins with the splenic vein behind the pancreas (in others it simply joins with the superior mesenteric vein instead).
Function of the pancreas
To set the record straight, the pancreas is not only an endocrine organ that produces insulin, glucagon and somatostatin (the part of the pancreas that does this are the Islets of Langerhans), but is also, perhaps more importantly, an exocrine organ vital for the digestion of food.
The exocrine function of the pancreas is actually the major function of the pancreas and takes up much more of the pancreas' mass than its endocrine function. It secretes proteases to digest proteins and polypeptides, pancreatic lipase to digest fats and amylase to digest carbohydrates. The pancreas is the main source of enzymes for digesting fats (lipids) and proteins - the intestinal walls have enzymes that will digest polysaccharides. Pancreatic secretions 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 enzymes gastrin, cholecystokinin and secretin, which are enzymes secreted by cells in the stomach and duodenum, in response to distension and/or food and which cause secretion of pancreatic juices.
The two major proteases the pancreas excretes are trypsinogen and chymotrypsinogen. These 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.
In addition to these enzymes already mentioned above, the pancreas also secretes a bunch of other digestive enzymes to digest specific things including nuclear material (DNA).