Overview
The pancreas is an accessory digestive gland with exocrine and endocrine
functions. It is an elongated pinkish
gland, generally between 12 and 15cm in length.
It is a largely retroperitoneal structure which is closely related to
the stomach, spleen and duodenum. Disorders
of the pancreas are frequently encountered in clinical practice.
Gross Anatomy
Development
The
pancreas begins to develop at 5 weeks in-utero.
It arises as two separate outpouchings of the duodenum, a dorsal and
ventral pancreatic bud. As the organs
begin to enlarge, the ventral bud begins to rotate to fuse with the dorsal bud
to form one pancreas. The ductal system
initially begins as two separate outflow systems, each contained in the ventral
and dorsal bud. As the ventral bud and
the dorsal bud begin to join, the corresponding ducts fuse to form the major
pancreatic duct. The pancreatic duct
will eventually join the bile duct, which will empty at the ampulla of Vater into
the second (descending) part of the duodenum.
In the adult pancreas, the uncinate process is derived from the ventral
bud and the head, body and tail from the dorsal bud.
Surface Anatomy
The
pancreas lies deep within the abdominal cavity and is therefore
impalpable. The majority sits in the epigastrium
with the tail advancing into the left hypochondrium. The surface anatomy of the pancreas is best
described in relation to the transpyloric plane. The neck of the pancreas lies anterior to the
L1 and L2 vertebrae along the transpyloric plane. The head is to the right and inferior to
this, the body and tail run to the left and superiorly.
Macro-Anatomy (e.g. lobes, surfaces,
impressions etc)
The
pancreas is anatomically divided into five regions: head, uncinate process,
neck, body and tail. The head sits in
close relation to the C-shaped curvature of the duodenum. The uncinate process is an extension of the
inferior portion of the head, the superior mesenteric artery lies anterior to
the uncinate process. The neck overlies
the superior mesenteric vessels, which sit in a groove on the posterior aspect
of the neck. The neck continues to form
the body of the pancreas which is to the left of the superior mesenteric
vessels. The tail is closely related to
the hilum of the spleen and the left colic flexure and is the most mobile
portion of the pancreas. It passes through the layers of the splenorenal
ligament.
The ductal system of the pancreas starts with the small
intercalated ducts that drain into the intralobular collecting ducts then into
the main pancreatic duct to transport pancreatic secretions. The main pancreatic duct traverses the
pancreas and joins the common bile duct.
The ampulla of Vater is an enlargement where the two ducts join, and
open into the duodenum. The sphincter of
Oddi controls the secretions into the duodenum by acting as a valve.
Micro-Anatomy
A capsule
of connective tissue covers the pancreas and partitions the gland into lobules
by way of septa. The majority of the
pancreas is composed of exocrine tissue, with only 1% of the total volume taken
up by the endocrine portion. The
endocrine pancreas consists of the Islets of Langerhans arranged in
clusters. The exocrine portion is made
up of acini which are cells responsible for the secretion of digestive enzymes
into the intralobular ducts which eventually join the main pancreatic duct.
Blood Supply
The
majority of the arterial blood supply is derived from the splenic artery, the
largest branch of the coeliac trunk. The
splenic artery traverses the posterior surface of the pancreas to supply the
tail before reaching the splenic hilum. The
inferior and superior pancreaticoduodenal arteries (branches of the superior
mesenteric and gastroduodenal arteries respectively) mainly supply the head and
body of the pancreas. The body and neck
of pancreas drain into the splenic vein, and the head into the superior
mesenteric vein and portal vein. This
means that the exocrine hormones the pancreas produces arrive directly at the
liver.
Lymph
The
lymphatics follow the major arteries supplying the pancreas, with drainage to
the coeliac and superior mesenteric lymph nodes.
Nerve Supply
Both
parasympathetic and sympathetic nerves fibres pass alongside the arteries of
the coeliac and superior mesenteric plexuses.
Parasympathetic innervation is via the posterior vagus nerve and stimulates
pancreatic secretions. Autonomic
sympathetic nerve supply is derived from the coeliac, superior mesenteric and
hepatic plexi.
Physiology
The
pancreas has both exocrine and endocrine functions and is involved in
metabolism and glucose homeostasis. The
exocrine pancreas secretes enzymes to aid digestion and bicarbonate and water
to neutralise the acidity of gastric secretions.
The
exocrine pancreas is responsible for the release of:
- Trypsinogen: converted to the active form
trypsin in the small bowel, and is a protease.
- Amylase: breaks down starch to maltose.
- Lipase: hydrolyses triglycerides to aid later
digestion of fats.
- Bicarbonate ions: secreted from the ductal
epithelial cells.
The control
of exocrine secretion is mediated by three main hormones. Cholecystokinin,
secreted by the duodenum, triggers pancreatic juice secretion. Secretin is released in response to acid being
released into the duodenum and this triggers bicarbonate and water release from
the ductal pancreatic cells. Gastrin is
secreted by the stomach in response to gastric dilatation and causes the
secretion of pancreatic enzymes. The
exocrine secretions are drained via the pancreatic duct into the duodenum.
Endocrine
The primary
function of the endocrine portion of the pancreas is glucose homeostasis. The differing cell types within the Islets of
Langerhans are responsible for hormone release.
The hormones involved are:
- Glucagon: released by alpha cells, converts
glycogen to glucose.
- Insulin: from beta cells, converts glucose to
glycogen.
- Somatostatins: from delta cells, reduces
glucagons, insulin and growth hormone.
Clinical Anatomy
Quick Anatomy
Key Facts
Development:
foregut, as dorsal and ventral outpouchings of the duodenal endoderm.
Artery:
splenic, inferior and superior pancreaticoduodenal arteries.
Vein: into the
portal vein via inferior and superior pancreaticoduodenal arteries.
Lymph:
coeliac and superior mesenteric lymph nodes.
Nerve:
parasympathetic and sympathetic innervation from the vagus.
Aide-Memoire
Summary
The pancreas is a retroperitoneal gland with exocrine and endocrine
functions. It has a rich blood supply
and is related to the stomach, spleen and duodenum. Pancreatic enzyme secretion is triggered by
hormone release in response to food in the stomach. Pancreatitis and pancreatic cancer are
commonly encountered in surgical practice and an understanding of the anatomy
and physiology is useful to predict the symptoms and complications of both.
References
Essential
Clinical Anatomy. 3rd edition. Moore and Agur.
http://www.embryology.ch/anglais/sdigestive/pankreas01.html
http://www.cancer.gov/types/pancreatic/patient/pnet-treatment-pdq#section/_24