CHAPTER 70 Pancreas
The pancreas is the largest of the digestive glands and performs a range of both endocrine and exocrine functions. The major part of the gland is exocrine, secreting a range of enzymes involved in the digestion of lipids, carbohydrates and proteins. The endocrine function of the pancreas is derived from cells scattered throughout the substance of the gland: they take part in glucose homeostasis and are also involved in the control of upper gastrointestinal motility and function.
The pancreas is salmon pink in colour with a firm, lobulated smooth surface. The main portion is divided into four parts, head, neck, body and tail, purely on the basis of anatomical relations: there are only very minor functional or anatomical differences between each part (Figs 70.1 and 70.2). The pancreas also possesses one accessory lobe (the uncinate process), which is anatomically and embryologically distinct. In adults the pancreas measures between 12 and 15 cm long and is shaped as a flattened ‘tongue’ of tissue, thicker at its medial end (head) and thinner towards the lateral end (tail). With age, the amount of exocrine tissue tends to decline, as does the amount of fatty connective tissue within the substance of the gland, and this leads to a progressive thinning atrophy which is particularly noticeable on CT scanning. The pancreas lies within the curve of the first, second and third parts of the duodenum, and extends transversely and slightly upwards across the posterior abdominal wall to the hilum of the spleen, behind the stomach. It does not lie in one plane but is effectively ‘draped’ over the other structures in the retroperitoneum and the vertebral column and so forms a distinct shallow curve, of which the neck and medial body are the most anterior parts. Because of its flattened shape, the parts of the pancreas, particularly the body, are often referred to as having surfaces and borders.
Fig. 70.2 Coronal reformat CT of the pancreas
(by courtesy of Dr Louise Moore, Chelsea and Westminster Hospital).
HEAD
The head of the pancreas lies to the right of the midline, anterior and to the right side of the vertebral column, within the curve of the duodenum. It is the thickest and broadest part of the pancreas but is still flattened in the anteroposterior plane. Superiorly it lies adjacent to the first part of the duodenum, but close to the pylorus the duodenum is on a short mesentery, and here the duodenum lies anterior to the upper part of the head. The duodenal border of the head is flattened and slightly concave, and is firmly adherent to the second part of the duodenum; occasionally a small part of the head is actually embedded in the wall of the second part of the duodenum. The superior and inferior pancreaticoduodenal arteries lie between the head and the duodenum in this area. The inferior border lies superior to the third part of the duodenum and is continuous with the uncinate process. Close to the midline, the head is continuous with the neck. The boundary between head and neck is often marked anteriorly by a groove for the gastroduodenal artery and posteriorly by a similar but deeper deep groove that contains the union of the superior mesenteric and splenic veins as they form the portal vein.
The anterior surface of the head (Fig. 70.3) is covered in peritoneum and is related to the origin of the transverse mesocolon.
Fig. 70.3 A, Regions and anterior surfaces and borders of the pancreas. B, Anterior relations of the pancreas. Areas covered in peritoneum are shown in blue and structures overlying these areas are separated from the pancreas by peritoneal ‘spaces’. The spleen in relation to the tail lies anterior to the anterior leaf of the splenorenal ligament and not in direct contact with the pancreatic tissue. D1, first part of the duodenum; SMA, superior mesenteric artery; SMV, superior mesenteric vein.
NECK
The neck of the pancreas is approximately 2 cm wide and links the head and body. It is often the most anterior portion of the gland and is defined as the portion of the pancreas that lies anterior to the portal vein, which is closely related to the upper posterior surface (see Fig. 70.8). The lower part of the neck lies anterior to the superior mesenteric vein just before the formation of the portal vein. These are important relations during surgery for pancreatic cancer because malignant involvement of these vessels may make resection impossible. The anterior surface of the neck is covered with peritoneum and lies adjacent to the pylorus just inferior to the epiploic foramen. The gastroduodenal and anterior superior pancreaticoduodenal arteries descend in front of the gland in the region of the junction of the neck and head.
BODY
The body of the pancreas is the longest portion of the gland and runs from the left side of the neck to the tail. It is slightly triangular in cross-section, becoming progressively thinner and less broad towards the tail. The body is described as having three surfaces, anterosuperior, posterior and anteroinferior.
The anterosuperior surface of the pancreas makes up most of the anterior aspect of the gland close to the neck. Laterally, it narrows and lies slightly more superiorly to share the anterior aspect with the anteroinferior surface. It is covered by peritoneum, which runs anteroinferiorly from the surface of the gland to be continuous with the anterior, ascending layer of the greater omentum (see Fig. 64.4). The superior leaf of the transverse mesocolon is reflected and is continuous with the descending, posterior leaf of the greater omentum just above the anterior border and so forms virtually none of the covering of the anterosuperior surface. It is separated from the stomach by the lesser sac.
The posterior surface of the pancreas is devoid of peritoneum. It lies anterior to the aorta and the origin of the superior mesenteric artery, the left crus of the diaphragm, left suprarenal gland and the left kidney and renal vessels, particularly the left renal vein (Fig. 70.4). It is closely related to the splenic vein which lies between the posterior surface and the other posterior relations and runs from left to right forming anything from a shallow groove to a near enveloped ‘tunnel’ in the substance of the gland. The left kidney is also separated from the posterior surface by perirenal fascia and fat.
The anteroinferior surface of the pancreas begins as a narrow strip just to the left of the neck. As the body runs laterally, it broadens out to form more of the anterior aspect of the body. It is covered by peritoneum which is continuous with that of the posteroinferior layer of the transverse mesocolon. The fourth part of the duodenum, the duodenojejunal flexure and coils of jejunum all lie inferiorly. The lateral end of the inferior border often lies superior and posterior to the splenic flexure. The peritoneum of the anterosuperior layer of the transverse mesocolon is reflected onto the upper part of the anteroinferior surface at the line of the anterior border where it is continuous with the posterior leaf of the greater omentum. The apex of the ligament of Treitz may form an anterior relation of this surface of the body laterally.
On the right side, the superior border of the pancreas is initially blunt and somewhat flat, but as the gland is followed to the left, the surface changes to become narrower and sharper. An omental tuberosity usually projects from the right end of the superior border above the level of the lesser curvature of the stomach, in contact with the posterior surface of the lesser omentum. The superior border is related to the coeliac artery. The common hepatic artery runs to the right just above the gland. The splenic artery runs a course that is often highly tortuous to the left along the superior border; it tends to rise above the level of the superior border at several points along its course.
The anterior border of the pancreas separates the anterosuperior from the anteroinferior surfaces. The two layers of the transverse mesocolon diverge along this border. The upper is reflected inferiorly after a very short distance into the descending (posterior) leaf of the greater omentum whilst the other runs downwards and backwards over the anteroinferior surface.
The inferior border of the pancreas separates the posterior from the anteroinferior surfaces. At the medial end of the inferior border, adjacent to the neck of the pancreas, the superior mesenteric vessels emerge from behind the gland. More laterally, the inferior mesenteric vein runs under the border to join the splenic vein on the posterior surface. This is a useful site of identification of the inferior mesenteric vein during left-sided colonic resections and on CT imaging.
TAIL
The tail of the pancreas is the narrowest, most lateral portion of the gland and lies between the layers of the splenorenal ligament. It is continuous medially with the body and is between 1.5 and 3.5 cm long in adults. It may finish at the base of the splenorenal ligament or extend up nearly as far as the splenic hilum, in which case it is prone to injury at splenectomy during ligation of the splenic vessels. Posteriorly it is related to the splenic branches of the splenic artery and the splenic vein and its tributaries. The tip of the tail may lie in contact with the splenic hilum.
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