Updated: 03/13/08 11:26 AM
HOME HEAL EDUCATE RESEARCH DIRECTORY OUTREACH


Authors: G. Leinbach, B.J. Reid, D.R. Saunders, and T.D. Nguyen
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"Theory helps us to bear our ignorance of facts."
George Santayana

A. Objectives
  1. Know the anatomic relationships to be able to understand pancreas divisum; jaundice and pancreatitis from a common duct stone; how patients with pancreatitis may present with retroperitoneal collections of blood, or plasma; how pancreatitis may cause duodenal or colonic ileus; how pancreatic carcinoma can cause hepatic, and pulmonary metastases.

  2. Know the intimate functional relationship between pancreatic islets and acini although histological relations appeared separated.

  3. Understand the autoregulation of pancreatic exocrine and endocrine secretion during the digestion of a meal.

  4. Learn the major causes of acute pancreatitis, theories about pathogenesis, the protean clinical manifestations, the difficulties of diagnosis, and the rationale of treatment.

  5. Be able to plan treatment for patients with pancreatic insufficiency.

  6. Know about the basic genetic defects in cystic fibrosis, and in hereditary pancreatitis.
B. Structure

Parts of pancreas:
Head
Body
Tail 		The pancreas is a 10 - 15 cm long organ of about 100 g which lies in the retroperitoneal space on the posterior wall of the abdomen. The “head" or right portion of the organ is broad and flat, and fits snugly into the loop of the duodenum. The "body" goes to the left across the spine and behind the stomach, eventually tapering into the "tail" which lies on the hilus of the spleen. Protected safely by a layer of peritoneum, and hiding behind numerous abdominal organs, it defies the attempts of the examining hand to find it under normal and often under abnormal conditions.

Endocrine and exocrine functions are interrelated The pancreas mediates both endocrine (insulin, glucagon) and exocrine (digestive enzymes) functions; it delivers its hormones into the draining venous system and its enzymes into the intestine. The hormones are essential for the regulation of carbohydrate metabolism and the enzymes facilitate the digestion of food in the intestinal lumen. Capillaries arranged in an insuloacinar portal system allow hormones from the islet cells to reach the acinar cells. Although the nature and function of its enzymes are well described, the smooth regulation of its responses after eating is not completely understood.



Figure 1
Overview of Pancreatic Secretion

1. Embryology

Body, tail, and part of head of pancreas derived from dorsal bud. Uncinate process of head derived from ventral bud. The pancreas arises as ventral (anterior) and dorsal (posterior) buds from the primitive duodenum. The dorsal bud forms the body, tail and part of the head of the pancreas. The ventral bud ultimately develops into the hepatobiliary system, but at the same time gives off a small bud from the bile duct close to the duodenum. This is the ventral pancreatic bud, which eventually forms the remainder of the head of the pancreas and its uncinate process.

Rotation of pancreatic buds As the stomach’s posteriorly located greater curvature rotates anteriorly to the left, the posterior pancreatic bud ends up along the left posterior abdominal wall and becomes retroperitoneal while the anterior pancreatic bud is carried posteriorly around to the right to end up in the C-loop of the duodenum.

Fusion of pancreatic ducts from dorsal and ventral buds The anterior and posterior pancreatic buds then fuse, and their ducts unite to form the major pancreatic excretory duct. Thus, exocrine secretions from the body and tail of the pancreas drain initially into the ductal system of the posterior pancreatic bud, and then through the duct of the anterior bud (duct of Wirsung) into the duodenum. The proximal portion of the duct originally draining the posterior pancreatic bud into the duodenum may or may not remain functional as the accessory duct of Santorini.



Figure 2

2. Developmental Anomalies

Annular pancreas If the ventral bud fails to rotate completely, a band of remaining pancreatic tissue may encircle the second portion of the duodenum and produce an annular pancreas. This anomaly may cause duodenal obstruction.

Ectopic pancreatic tissue Ectopic pancreatic tissue is a developmental anomaly in which small islands of pancreatic tissue are found in the stomach, duodenum or upper jejunum. Such ectopic tissue presumably arises from embryonic tissue similar to that which gave rise to pancreatic buds. These ectopic islands of pancreatic tissue are usually asymptomatic but may occasionally cause symptoms related to their position or size (ulceration, obstruction).

10% incidence of failure of ventral and dorsal ducts to fuse (pancreas divisum) ↑ incidence of pancreatitis In about 10% of the population, the ventral pancreatic duct fails to fuse with the dorsal pancreatic duct so that pancreatic juice from the body and tail drain via the accessory duct (Santorini) and accessory (minor) papilla while juice from the pancreatic head drains via the duct of Wirsung and the papilla of Vater. This is known as pancreas divisum. These people may have an increased incidence of pancreatitis, either in the ventral or dorsal pancreas or both.

The common bile duct and pancreatic duct are joined at the ampulla of Vater. Pancreatic tumors or pancreatitis may cause biliary obstruction and jaundice. Gallstones may cause pancreatitis. As a result of its embryological development, the terminal portion of the common bile duct, which rotated with the ventral bud posterior to the duodenum, now travels to a variable extent within the pancreatic head to empty into the duodenum either as a common channel with the pancreatic duct or separately. Therefore, tumors or inflammatory conditions of the pancreatic head can obstruct the biliary flow from the liver and can distend the gallbladder until it becomes palpable to the examining hand. At the same time, jaundice (yellowing of the skin and the sclerae of the eyes) may occur because of a failure to excrete bilirubin. If a gallstone lodges in and obstructs the distal common duct the patient will become jaundiced. If, in addition, pancreatic drainage is blocked, the pancreas may become inflamed leading to pancreatitis.

3. Anatomic Relations

Lesser omental sac and relations. As the gut rotates, it and its mesenteries form the lesser omental sac. The lesser omental sac is bounded posteriorly by the pancreas, anteriorly by the stomach, and mesenteric connections between the stomach and liver, inferiorly by the transverse colon and mesocolon, on the left by mesenteric connections between the diaphragm, spleen and greater curvature of the stomach, and on the right by the liver and the epiploic foramen by which it communicates with the general peritoneal cavity.



Figure 3

Perforation of lesser sac → few symptoms; however, if pus enters peritoneal cavity, → peritonitis. Perforations into the lesser sac by a posterior gastric ulcer or a pancreatic abscess usually produce few abdominal physical findings if the perforation remains confined to the area. But if pus spills through the foramen into the general peritoneal cavity, the dramatic findings of peritonitis ensue (painful abdomen with board-like rigidity).

It's hard to detect masses (blood, serum, tumor, cyst) in or around pancreas. Because of its location, a severely inflamed pancreas may cause large unsuspected volumes of blood or serum to be lost into the retroperitoneal space. These fluid losses may only become apparent when the patient goes into shock from depletion of circulating blood volume. Tumors and cysts may grow quite large before they are detectable by physical examination.

Head: At L2-L3 in "C" of duodenum. Anterior to renal veins, rt renal artery, rt kidney, and IVC.

Body: Anterior to aorta

Tail: hilus of spleen

Effects on contiguous organs 		The head of the pancreas lies in the bend of the second portion of the duodenum at approximately the level of the second and third lumbar vertebrae. Behind it are the renal veins, the right kidney and renal artery and the vena cava. The pancreatic body crosses the aorta, portal vein, superior mesenteric vessels, splenic vein, left kidney and left adrenal. The pancreatic tail lies at the hilus of the spleen. Anterior to the pancreas lie the liver, transverse colon, duodenal bulb, gastric antrum and loops of proximal small bowel. Enlarging lesions in the head of the pancreas may widen the normal duodenal loop. Posterior antral or duodenal ulcers may penetrate into the pancreas. Inflammation of the pancreas may result in inflammation and stasis of a contiguous loop of bowel producing a local segment of dilated gas-filled intestine (sentinel loop) detectable on X-ray of the abdomen. Contiguous inflammation of the kidneys may be reflected as proteinuria or hematuria. In motor vehicle accidents, sudden deceleration may crush the pancreas against the vertebral column, resulting in pancreatic injury such as pancreatic duct rupture.

4. Vascular Supply and Lymphatics

The arterial supply of the pancreas comes from the aorta via the celiac and superior mesenteric arteries and their branches. The gastroduodenal artery, a branch of the hepatic branch of the celiac artery, courses down behind the first portion of the duodenal bulb and anastomoses there with branches of the superior mesenteric artery to form a rich arcade of vessels supplying the head of the pancreas and the duodenum. A duodenal ulcer which erodes into this vasculature can cause massive bleeding. The splenic artery (from the celiac) and its branches supply the principal portion of the body and tail. Blood from the pancreas drains entirely into the portal vein, either by the superior mesenteric or splenic veins. Thus, a usual site of metastases from pancreatic carcinoma is the liver (where the portal system ends).

Lymph drains from the pancreas into local nodes around the gland, into nodes near the hili of the liver and spleen, and generally follows the vascular supply, ultimately draining by way of the superior mesenteric and celiac nodes into the thoracic duct which drains into the systemic venous system. This drainage pathway is responsible for lymph node metastases from pancreatic carcinoma that may be felt in the left supraclavicular fossa where the thoracic duct joins the venous system. This lymph node is called the sentinel node or Virchow's node. Because venous blood is then pumped into the lungs, one may see multiple lung metastases from pancreatic cancer.

5. Nerve Supply

Sympathetic → splanchnic → innervates vessels & acini.

Parasym → vagus → acinar & islet cells and ductal smooth muscle

Somatic nerves of peritoneum and post abdominal wall account for back pain associated with pancreatitis 		The pancreas is supplied by both sympathetic splanchnic nerves and parasympathetic vagus nerves. Sympathetic nerves enter the pancreas with the arteries to innervate blood vessels and acini. Vagal fibers innervate acinar and islet cells and ductal smooth muscle. The exact functions of all these nerves are not precisely understood, but clearly vagal stimulation in animals increases pancreatic secretion of a "juice" relatively low in volume but rich in enzymes. Vagal innervation also potentiates the secretory effects of CCK and secretin. Afferent splanchnic nerves of the pancreas mediate pain and account for the constant severe pain felt in the upper two-thirds of the abdomen during attacks of pancreatic inflammation. Both inhibitory and stimulatory sympathetic fibers are probably present, but the functional significance of these is not known. The parietal peritoneum anterior to the pancreas and the posterior abdominal wall behind the pancreas are supplied by somatic nerves from spinal cord levels thoracic-10 to lumbar 2, and account for the back pain commonly seen with pancreatitis.

vagal stimulation →
1) ↑ pancreatic secretions
2) ↑ potentiation of
  1. CCK → stimulates output of enzymes, stimulates gallbladder contraction.
  2. Secretin → stimulates pancreatic HCO3; Also, stimulates bile HCO3-.
Next Section (C): Histologic Structure and Function »



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