Small intestine

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CHAPTER 66 Small intestine

OVERVIEW

The small intestine consists of the duodenum, jejunum and ileum. It extends from the distal end of the pyloric canal to the ileocaecal valve, an overall length of 5 metres (3–7 metres) in the living adult. The duodenum extends up to the duodenojejunal junction, and the remaining small intestine is often referred to as the ‘small bowel’, of which the proximal two-fifths is referred to as the jejunum and the distal three-fifths as the ileum. There is no clear distinction between the two parts, but there is a gradual change in morphology from the proximal to distal ends of the small bowel. The distal 30 cm or so of the ileum is often referred to as the terminal ileum, which has some specialized physiological functions.

The duodenum lies mostly in the upper retroperitoneum. The jejunum and ileum occupy the central and lower parts of the abdominal cavity and usually lie within the boundary formed by the abdominal colon; they are attached to the posterior abdominal wall by a mesentery which allows considerable mobility of the loops of small bowel. In the supine position, loops of jejunum may be found anterior to the transverse colon, stomach and even lesser omentum, whereas in the upright position, loops of ileum may descend into the pelvis anterior to the rectum and, in women, may occupy the rectouterine pouch. The upper jejunum and some ileum are often covered anteriorly by the greater omentum. The jejunum and ileum are covered by peritoneum on all but their mesenteric borders, here the adipose connective tissue of the mesentery abuts the muscular wall. The serosal peritoneum is continuous with the peritoneum enclosing the tissues of the mesentery. Mesenteric fat covers 20% of the circumference wall of the ileum and somewhat less of the jejunum.

DUODENUM

The adult duodenum is 20–25 cm long and is the shortest, widest and most predictably placed part of the small intestine. It is only partially covered by peritoneum, although the extent of the peritoneal covering varies along its length: the proximal 2.5 cm is intraperitoneal, and the remainder is retroperitoneal. The duodenum forms an elongated ‘C’ that lies between the level of the first and third lumbar vertebrae in the supine position. The lower ‘limb’ of the C extends further to the left of the midline than the upper limb. The head and uncinate process of the pancreas lie within the concavity of the duodenum which is ‘draped’ over the prominence formed from the lumbar spine, the duodenum therefore bends in an antero-posterior direction as well as following the form of a ‘C’. The duodenum lies entirely above the level of the umbilicus and is described as having four parts (Figs 66.1, 66.2).

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Fig. 66.1 A, The four parts of the duodenum. B&C, The relations of the duodenum: B, anterior surface; C, posterior surface.

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Fig. 66.2 Contrast radiographic appearance of the duodenum showing a distended duodenal cap and the remainder of the duodenum up to the duodenojejunal flexure.

FIRST (SUPERIOR) PART

The first, and most mobile, part of the duodenum is about 5 cm long. It starts as a continuation of the duodenal end of the pylorus and ends at the superior duodenal flexure. Peritoneum covers the anterior and superior part of its posterior aspect close to the pylorus, where the duodenum forms part of the anterior wall of the epiploic foramen. Here the lesser omentum is attached to its upper border and the greater omentum to its lower border. The first 2 or 3 cm have a bland internal mucosal appearance and readily distend on insufflation during endoscopy. This part is frequently referred to as the duodenal ‘cap’. It has a triangular, homogeneous appearance during contrast radiology, shows the same pattern of internal rugae as the pylorus, and is often visible on plain radiographs of the abdomen as an isolated triangular gas shadow to the right of the first or second lumbar vertebra. The duodenum next passes superiorly, posteriorly and laterally for 5 cm before curving sharply inferiorly into the superior duodenal flexure: it rapidly becomes more retroperitoneal during this part of its course, until peritoneum only covers its anterior aspect. From the end of the duodenal cap, its internal appearance is characterized by extensive, deep mucosal folds that involve up to half of the circumference of the lumen. Even during endoscopic insufflation, these folds are pronounced (Fig. 66.3) and they are readily seen on contrast radiographs. The section from the duodenal cap to the superior duodenal flexure lies posterior and inferior to the quadrate lobe of the liver.

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Fig. 66.3 Endoscopic appearance of the duodenum: A, Duodenal cap (first part). B, Second part showing the pronounced, ‘branched’ mucosal folds. C, Major duodenal papilla. D, Third part.

The first part of the duodenum lies anterior to the gastroduodenal artery, common bile duct and portal vein and anterosuperior to the head and neck of the pancreas. It is posterior to the neck of the gallbladder. The gastroduodenal artery lies immediately posterior to the posterior wall of the duodenum: penetrating peptic ulceration in the posterior wall and subsequent erosion of the gastroduodenal artery may lead to dramatic haemorrhage. Penetrating peptic ulceration in the anterior wall may lead to free perforation into the peritoneal cavity because the anterior surface of the first part is covered only by peritoneum.

The common hepatic and hepatoduodenal lymph nodes lie close to the first part of the duodenum and can be visualized using endoscopic ultrasound. This may be important in the staging of gastric, pancreatic or bile duct tumours. The proximity of the common bile duct to the first part of the duodenum allows endoscopic ultrasound examination of the distal common bile duct and the formation of a surgical anastomosis between bile duct and duodenum (choledochoduodenostomy) when required.

The junction of the first and second parts of the duodenum lies posterior to the neck of the gallbladder.

SECOND (DESCENDING) PART

The second part of the duodenum is 8–10 cm long. It starts at the superior duodenal flexure and runs inferiorly in a gentle curve, convex to the right side of the vertebral column and extending to the lower border of the third lumbar vertebral body. It then turns sharply medially into the inferior duodenal flexure which marks its junction with the third part of the duodenum. It is covered by peritoneum only on its upper anterior surface, lies posterior to the neck of the gallbladder and the right lobe of the liver at its start, and is crossed anteriorly by the transverse colon. The medial end of the gastrocolic omentum and the origin of the transverse mesocolon are attached to the anterior surface of the duodenum by loose connective tissue. Below the attachment of the transverse mesocolon, the connective tissue and vessels forming the mesentery of the upper ascending colon and hepatic flexure are loosely attached to its anterior surface. This section of duodenum is at risk of injury during surgical mobilization of the ascending colon.

The second part lies anterior to the hilum of the right kidney, the right renal vessels, the edge of the inferior vena cava and the right psoas major. The head of the pancreas and the common bile duct are medial and the hepatic flexure is above and lateral. A small part of the pancreatic head is sometimes embedded in the medial duodenal wall, and pancreatic ‘rests’ in the duodenal wall may produce small filling defects on contrast radiology. The internal appearance is similar to that of the distal portion of the first part of the duodenum, with pronounced mucosal folds (Fig. 66.3). The common bile duct and pancreatic duct enter the medial wall obliquely and usually unite to form the common hepatopancreatic ampulla. The narrow distal end opens on the summit of the major duodenal papilla (ampulla of Vater), which is situated on the posteromedial wall of the second part, 8–10 cm distal to the pylorus. There are variants of the ductal anatomy. The most common is a second, accessory pancreatic duct which may open 2 cm above the major papilla on a minor duodenal papilla. Peptic ulceration of the second part of the duodenum is less common than that of the first part, and tends to occur on the anterior or lateral wall.

Duodenal diverticula

The duodenum is the most common site for a diverticulum in the small intestine. Diverticula are congenital and usually solitary. They almost always arise in the medial wall of the second part of the duodenum, intimately related to the head of the pancreas, such that the major duodenal papilla may frequently be found either on the mucosal fold at the mouth of a diverticulum or arising from the mucosa within the body of a diverticulum, particularly on the superomedial wall. This may complicate interpretation of contrast radiographs of the duodenum or biliary system, and may cause difficulties during attempted endoscopic cannulation of the papilla because the diverticulum may restrict access. If the papilla is located on or close to the anterior wall of a diverticulum, the procedure of opening the ampulla with an electrocautery current during attempted cannulation (sphincterotomy) is made more hazardous because the wall is thin at this point and so there is a risk of free perforation into the peritoneal cavity.

THIRD (HORIZONTAL) PART

The third part of the duodenum starts at the inferior duodenal flexure and is approximately 10 cm long. It runs from the right side of the lower border of the third lumbar vertebra, angled slightly superiorly, across to the left, anterior to the inferior vena cava, becoming continuous with the fourth part in front of the abdominal aorta. It lies posterior to the transverse mesocolon, the origin of the small bowel mesentery and the superior mesenteric vessels. The lower portion of its anterior aspect is covered by peritoneum which is reflected anteriorly to form the posterior layer of the origin of the small bowel mesentery. The anterior surface of the left lateral end, close to the junction with the fourth part, is also covered with peritoneum.

The third part is anterior to the right ureter, right psoas major, right gonadal vessels, inferior vena cava and abdominal aorta (at the origin of the inferior mesenteric artery), and inferior to the head of the pancreas. Anteroinferiorly, loops of jejunum lie in the right and left infracolic compartments. The mid portion of the third part lies in the angle between the superior mesenteric vessels, just below their origin, anteriorly, and the abdominal aorta posteriorly: very rarely, this arrangement is associated with intermittent symptoms of obstruction of the duodenum at this point.

FOURTH (ASCENDING) PART

The fourth part of the duodenum is 2.5 cm long. It starts just to the left of the aorta, runs superiorly and laterally to the level of the upper border of the second lumbar vertebra, then turns sharply anteroinferiorly at the duodenojejunal flexure to become continuous with the jejunum. The main trunk of the inferior mesenteric vein lies either posterior to the duodenojejunal flexure or beneath the adjacent peritoneal fold. (The duodenojejunal flexure is a useful landmark to locate the vein radiologically or surgically.) The aorta, left sympathetic trunk, left psoas major, left renal and left gonadal vessels are all posterior relations, the left kidney and left ureter are posterolateral, and the upper part of the root of the small bowel mesentery, the left lateral transverse mesocolon and transverse colon, which separate it from the stomach, are anterior. The peritoneum of the root of the small bowel mesentery continues over its anterior surface. The lower border of the body of the pancreas is superior.

At its left lateral end, the fourth part becomes progressively covered in peritoneum on its superior and inferior surfaces, such that it is suspended from the retroperitoneum by a double fold of peritoneum, the ligament of Treitz, at the start of the duodenojejunal flexure. The ligament of Treitz may contain a small slip of muscle, the suspensory muscle of the duodenum. When present, the suspensory muscle contains skeletal muscle fibres that run from the left crus of the diaphragm to connective tissue around the coeliac axis, and smooth muscle fibres that run from the coeliac axis: its function is unknown. It is an important landmark in the radiological diagnosis of incomplete rotation and malrotation of the small intestine (see Ch. 73). The ligament of Treitz does not form part of the duodenal mesentery because the vascular supply to the fourth part of the duodenum continues to enter its wall from the posteromedial aspect.

VASCULAR SUPPLY AND LYMPHATIC DRAINAGE

Arteries

The main vessels supplying the duodenum are the superior and inferior pancreaticoduodenal arteries. The first and second parts also receive contributions from several sources including the right gastric, supraduodenal, right gastroepiploic, hepatic and gastroduodenal arteries. Branches of the superior pancreaticoduodenal artery may contribute to the supply of the pyloric canal, with a minor degree of anastomosis to the gastric arteries in the muscular layer across the pyloroduodenal junction (Fig. 66.4).

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Fig. 66.4 The arterial supply of the duodenum. Only representative branches of the small vessels, which may be multiple, are shown.

Gastroduodenal artery

The gastroduodenal artery usually arises from the common hepatic artery behind, or sometimes above, the first part of the duodenum. It descends between the first part of the duodenum and the neck of the pancreas, immediately to the right of the peritoneal reflection from the posterior surface of the duodenum. It usually lies to the left of the common bile duct but occasionally is anterior. At the lower border of the first part of the duodenum it divides into the right gastroepiploic and superior pancreaticoduodenal arteries. Before its division, the lowest part of the artery gives rise to small branches that supply the pyloric end of the stomach and the pancreas, and retroduodenal branches that supply the first part and the proximal portion of the second part of the duodenum directly. Although the gastroduodenal artery usually branches from the common hepatic artery, it may also arise as a trifurcation with the right and left hepatic arteries, or from the superior mesenteric artery or the left hepatic artery (occasionally), or from the coeliac axis or right hepatic artery (rarely).

The supraduodenal artery often arises from the gastroduodenal artery behind the upper border of the first part of the duodenum and supplies the superior aspect of the first part; it may occasionally arise from the common hepatic artery or right gastric artery. Because of its close relationship to the posteromedial wall of the second part of the duodenum, deeply penetrating ulcers or tumours of the duodenum may cause torrential bleeding from the gastroduodenal ‘artery of haemorrhage’, not least because of the profuse arterial network in the region ensures a high flow rate in the vessel.

Superior pancreaticoduodenal arteries

The superior pancreaticoduodenal artery is usually double. The anterior artery is a terminal branch of the gastroduodenal artery and descends in the anterior groove between the second part of the duodenum and the head of the pancreas. It supplies branches to the first and second parts of the duodenum and to the head of the pancreas, and anastomoses with the anterior division of the inferior pancreaticoduodenal artery. The posterior artery is usually a separate branch of the gastroduodenal artery and is given off at the upper border of the first part of the duodenum. It descends to the right, anterior to the portal vein and common bile duct as the latter lies behind the first part of the duodenum. It then runs behind the head of the pancreas, crosses posterior to the common bile duct (which is embedded in the head of the pancreas), enters the duodenal wall and anastomoses with the posterior division of the inferior pancreaticoduodenal artery. The posterior artery supplies branches to the head of the pancreas, the first and second parts of the duodenum, and several branches to the lowest part of the common bile duct.

Inferior pancreaticoduodenal artery

The inferior pancreaticoduodenal artery usually arises from the superior mesenteric artery or its first jejunal branch, near the superior border of the third part of the duodenum. It crosses behind the superior mesenteric vein and divides directly into anterior and posterior branches. The anterior branch passes to the right, anterior to the lower border of the head of the pancreas, and runs superiorly to anastomose with the anterior superior pancreaticoduodenal artery. The posterior branch runs posteriorly and superiorly to the right, lying posterior to the lower border of the head of the pancreas, and anastomoses with the posterior superior pancreaticoduodenal artery. Both branches supply the pancreatic head, its uncinate process and the second and third parts of the duodenum. Occasionally the anterior and posterior branches arise separately from the superior mesenteric or first jejunal artery.

Jejunal artery branches

Branches from the first jejunal branch of the superior mesenteric artery supply the fourth part of the duodenum and frequently form an anastomosis with the terminal branch of the anterior inferior pancreaticoduodenal artery (which arises from the superior mesenteric artery). The fourth part of the duodenum therefore receives a potential collateral supply from the coeliac axis and superior mesenteric artery, which means that it is not commonly affected by ischaemia.

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