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Anatomy of the small intestine
BASIC SCIENCE
Anatomy of the small intestine
The duodenum The duodenum commences at the pyloroduodenal junction and ends at the duodenojejunal flexure where it is continuous with the jejunum. The commencement of the duodenum lies somewhat to the right of the midline while its distal end is 2 cm to the left of the midline. The duodenum is the widest, shortest and least mobile of the three segments of the small intestine. Unlike the jejunum and ileum, it does not possess a mesentery. The duodenum is a C-shaped tube about 25 cm in length (Figure 1) that is moulded around the head of the pancreas. It has two bends along its length, termed the superior and inferior duodenojejunal flexures. These bends allow the subdivision of the duodenum, for descriptive purposes, into four parts (Figure 2). In succession these are the superior part (1st part), descending part (2nd part), horizontal part (3rd part) and ascending part (4th part). In an average adult these parts are, respectively, 5 cm, 7.5 cm, 10 cm and 2.5 cm in length. The initial 2 cm or so of the duodenum, like the stomach, is ensleeved in peritoneum, and consequently has a degree of mobility. The remainder of the duodenum is a retroperitoneal, sessile and immobile structure. The topographical relations of the duodenum are shown in Figure 1. The concavity of the C-shaped duodenum is intimately related to, and surrounds the head and neck of the pancreas (Figure 1). The 1st part of the duodenum lies above the head of the pancreas and passes laterally, upwards and backwards to the right of the vertebral column at the level of the first lumbar vertebra. Situated immediately behind the 1st part of the duodenum are the gastroduodenal artery and common bile duct. Lying still more posteriorly is the inferior vena cava. Anterior relations of the 1st part of the duodenum are the liver (quadrate lobe) and neck of gallbladder. Chronic cholecystitis may result in inflammatory adhesion of the gallbladder wall to that of the 1st part of the duodenum. Subsequent gradual and progressive erosion of the walls of the gallbladder and duodenum may result in gallstones dropping into the duodenal lumen and being propagated along the small intestine, with possible impaction in the terminal ileum causing small bowel obstruction (gallstone ileus). The 2nd part of the duodenum lies anterior to the right renal hilum, and immediately lateral to the head of pancreas. Crossing in front of the 2nd part of the duodenum is the commencement of the transverse colon and its mesocolon. The 2nd part of the duodenum is thus vulnerable during mobilization of the hepatic flexure and transverse colon in the course of a right hemicolectomy (Figures 1 and 4). Since the 2nd part of the duodenum is a retroperitoneal structure being plastered down by the posterior parietal peritoneum, mobilization of the 2nd part of the duodenum requires a vertical incision of the peritoneum lateral to the duodenum prior to moving the duodenum anteriorly and medially. This technique is known as Kocher’s manoeuvre. The main pancreatic duct and the common bile duct have a common opening into the posteromedial wall of the second part of the duodenum at the major duodenal papilla. As estimated on endoscopy, this papilla is usually 9e10 cm distal to the pyloroduodenal junction. Frequently, a minor duodenal papilla is seen 2e3 cm proximal to the major one. It contains the opening of the accessory pancreatic duct.
Vishy Mahadevan
Abstract The small intestine (small bowel) commences at the pyloroduodenal junction and ends at the ileocaecal junction. It comprises, successively, the duodenum, jejunum and ileum. The principal function of the small intestine is the digestion and absorption of ingested food, electrolytes and vitamins and this important functional specialization is reflected in the enormous surface area of the small intestinal mucosa which is the result of the extensive and wavy in folding of the mucosal lining along the entire length of the small intestine. As might be expected from its very active role as a secretory and absorptive viscus, the small intestine has a rich blood supply. This is derived chiefly from the superior mesenteric artery via its inferior pancreaticoduodenal, jejunal and ileal branches. Venous drainage is via corresponding tributaries that drain to the superior mesenteric vein and thence to the portal vein. These vessels are accompanied by lymphatic vessels and autonomic nerve fibres. Developmental anomalies, mechanical obstruction and inflammatory disease affect the small intestine far more commonly than do neoplastic conditions. A Meckel’s diverticulum is a relatively common (2%) congenital anomaly of the distal ileum. In this article, an account of the topographical and surgical anatomy of the duodenum is followed by that of the jejunum and ileum. Various surgically-important features are highlighted.
Keywords Autonomic nerves; duodenum; ileum; jejunum; lymphatic drainage; Meckel’s diverticulum; mesentery; superior mesenteric artery and vein
The small intestine extends from the pylorus (which marks the distal end of the stomach) to the ileocaecal junction. It comprises three segments. These are, in succession, the duodenum, jejunum and ileum. Owing to its many distinctive anatomical features, the duodenum is usually considered a separate anatomical entity, different from the remainder of the small intestine. The distinguishing features of the duodenum include its retroperitoneal location, its immobility due to the absence of a mesentery and its numerous intimate surgically-important topographical relationships. By contrast, the jejunum and ileum have a considerably greater mobility within the abdominal cavity owing to their attachment to a suspensory mesentery. Another point of difference between the duodenum and the remainder of the small intestine is that the duodenum has a dual embryological origin, in part from the foregut and partly from the midgut, while the jejunum and ileum are derived exclusively from the midgut. In common clinical usage the term small bowel (small intestine) usually denotes just the jejunum and ileum and does not include the duodenum.
Vishy Mahadevan MBBS PhD FRCS (Ed & Eng) is the Barbers’ Company Professor of Anatomy at the Royal College of Surgeons of England, London, UK. Conflicts of interest: none.
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Ó 2014 Elsevier Ltd. All rights reserved.
BASIC SCIENCE
Inferior vena cava and hepatic veins
Oesophagus
Inferior phrenic vessels
Coeliac trunk Left gastric artery
Right suprarenal gland Spleen
Portal vein Bile duct Hepatic artery Right kidney Duodenum
Splenic artery Splenic vein Body of pancreas Inferior mesenteric vein
Uncinate process of head of pancreas
Duodenojejunal flexure
Ascending colon Descending colon Ureter Left sympathetic trunk Gonadal vessels
Superior mesenteric vein Superior mesenteric artery
Figure 1 The duodenum and its relationships.
The 3rd part of the duodenum commences at the inferior duodenal flexure and passes more or less horizontally to the left to become continuous with the 4th part of the duodenum, immediately to the left of the aorta. It is covered anteriorly by peritoneum. In its course from right to left, the 3rd part of the duodenum lies inferior to the pancreas and anterior to the ureter and gonadal vessels of the right side as these overlie the ventral surface of the right psoas major muscle. The 3rd part of the duodenum then crosses in front of the inferior vena cava and abdominal aorta and becomes continuous with the 4th part, in front of the left psoas major muscle. Where it crosses the aorta, the 3rd part of the duodenum usually overlies the origin of the inferior mesenteric artery. Crossing immediately in front of the 3rd part of the duodenum is the root of the small intestinal mesentery containing the superior mesenteric artery and vein (Figures 1 and 4). A tortuous and atherosclerosed superior mesenteric artery may compress the third part of the duodenum against the lumbar vertebral column causing intermittent obstruction of the duodenum. This is an extremely rare cause of high small-bowel obstruction and is known variously as Wilkie’s syndrome, Wilkie’s disease and superior mesenteric artery syndrome. The 4th part of the duodenum is the shortest of the four segments of the duodenum. It passes upwards on the left psoas major, lying to the left of the abdominal aorta (Figure 1). It then bends anteriorly at the duodenojejunal flexure. This flexure is typically 2 cm to the left of the median plane at the level of the 2nd lumbar vertebra. Running from the right crus of the diaphragm to gain attachment to the superior and lateral aspects of the duodenojejunal flexure is a fibrous or fibromuscular band
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termed the suspensory ligament of the duodenum (or ligament of Treitz). It is necessary to divide this ligament in order to mobilize the 4th part of duodenum prior to pancreatico-duodenectomy (Whipple’s operation) or when doing an open repair of an abdominal aortic aneurysm. Arterial supply and venous drainage of the duodenum The main arterial supply of the duodenum (and the adjacent head of pancreas) (Figures 1 and 3) is by the superior and inferior pancreaticoduodenal arteries. The former is a branch of the gastroduodenal artery (in turn a branch of the common hepatic artery) while the latter is a direct and early branch of the superior mesenteric artery. These two vessels form anterior and posterior anastomotic arterial arcades which lie along the concavity of the duodenum, in the cleft between the duodenum and head of pancreas. The veins draining the duodenum correspond to the arteries, and empty into the superior mesenteric vein and thus into the portal vein. In terms of embryological development, the duodenum develops from the distal end of the foregut and the proximal part of the midgut. The junction between the two is somewhat distal to the duodenal papilla that receives the common bile duct and main pancreatic duct. The dual origin of the duodenum is reflected in its blood supply: being derived in part from the coeliac artery (the artery of the foregut) and the superior mesenteric artery (the artery of the midgut). Since the advent of proton pump inhibitors and antimicrobials in the management of acid peptic disease, complications of acid peptic ulceration of the duodenum such as duodenal perforation and duodenal erosive haemorrhage are now rarely seen in the
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Ó 2014 Elsevier Ltd. All rights reserved.
BASIC SCIENCE
a
Gastroduodenal artery
1st
Duodenal branches 2nd
Supraduodenal artery
Superior pancreaticoduodenal artery Right gastroepiploic artery
4th
Posterior branch Anterior branch
3rd
Posterior branch Anterior branch Inferior pancreaticoduodenal artery
Quadrate lobe
b
Neck of gallbladder
Duodenal branch First jejunal artery
Right end of the gastrocolic omentum
Figure 3 Arterial supply of duodenum.
Attachment of the transverse mesocolon
at the level of the 2nd lumbar vertebra), across the midline to the right iliac fossa (Figures 4 and 5) This attachment of the mesentery to the posterior abdominal wall is referred to as the root of the mesentery. It is about 15 cm in length and contains the superior mesenteric artery and vein and lymph nodes. Starting at the duodenojejunal flexure, the root of the small intestinal mesentery crosses successively, the 3rd part of the duodenum, the aorta, the inferior vena cava, the right gonadal vessels, right ureter, right psoas major and right sacroiliac joint (Figure 4). The jejunum ( jejune; Latin for ‘empty’) and ileum (Greek for ‘to twist’) have a collective length of 5e6 m. The jejunum accounts for the proximal two-fifths of this length and the ileum makes up the distal three-fifths. The small intestinal mesentery and the entire length of the jejunum and ileum are located wholly in the infracolic compartment of the peritoneal cavity; which is to say, inferior to the attachment of the transverse mesocolon (Figures 4 and 5). There is no clear, sharp anatomical demarcation between the jejunum and ileum, the transition being an insidious one. Nevertheless, the proximal parts of the jejunum can be readily distinguished from the distal parts of the ileum on the basis of the following features. The jejunum is wider and possesses a thicker wall than the ileum. The decrease in calibre is a gradual one. The narrowest part of the small bowel is usually just proximal to the ileocaecal junction. Indeed it is at this point that solid objects are most likely to be impacted (as for example in gallstone ileus). The jejunum usually appears more vascular than the ileum. The jejunum loops tend to lie in the upper left part of the infracolic compartment, while the loops of ileum tend to be found in the lower part of the abdominal cavity, mainly on the right side, and often flop into the pelvic cavity (where they may be damaged by pelvic radiotherapy). The small bowel mesentery corresponding to the ileum is usually thicker and more fat-laden than the mesentery corresponding to the jejunum. Consequently the vascular detail in the ileal mesentery is not as clearly seen as in the relatively avascular and translucent jejunal mesentery.
Superior mesenteric artery Superior mesenteric vein
c
Bile duct Portal vein Right kidney
Gastroduodenal artery Left kidney
Left renal vessels Right renal vessels Right ureter Right gonadal vein
Left ureter Aorta Left gonadal artery
Right gonadal artery Inferior vena cava
Figure 2 The four parts of the duodenum.
developed world. However these complications are still prevalent in the developing world.
The jejunum and ileum The jejunum is the direct continuation of the duodenum and commences at the duodenojejunal flexure. Unlike the duodenum, the jejunum and ileum are ensleeved by the free edge of the small intestinal mesentery and consequently, are fairly mobile within the abdominal cavity. The small intestinal mesentery is a fanshaped, double-layered sheet of peritoneum attached to the posterior abdominal wall along a line which runs obliquely from the duodenojejunal junction (which lies to the left of the midline
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BASIC SCIENCE
Bare area
Upper layer of Transverse Falciform ligament coronary ligament mesocolon Left triangular ligament Splenorenal ligament
Lower layer of coronary ligament Right triangular ligament
Left suprarenal gland
Epiploic foramen
Tail of pancreas and splenic vessels
Greater omentum
Paraduodenal recess Phrenicocolic ligament Iliohypogastric nerve
Right paracolic gutter Superior mesentric vessels
Ilioinguinal nerve
Right infracolic compartment
Lateral femoral cutaneous nerve
Root of mesentery
Left paracolic gutter Femoral nerve
Left infracolic compartment
Sigmoid mesocolon
Figure 4 Location of root of small intestinal mesentery and attachments of parietal peritoneum to the posterior abdominal wall.
Greater omentum Transverse colon Transverse mesocolon
Root of mesocolon
Duodenum
Duodenojejunal flexure
Right infracolic compartment
Root of mesentery
Figure 5 Infracolic compartment of peritoneal cavity showing jejunum, ileum and small intestinal mesentery. SURGERY 32:8
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BASIC SCIENCE
The distal part of the ileum often presents, on its antimesenteric border, elongated, pale plaques. These are aggregations of lymphoid follicles in the ileal mucosa known as Peyer’s patches. They are visible through the muscle in the ileal wall. Very characteristically, the jejunal arteries form just one or two tiers of arcades within the mesentery, with relatively long and fewer straight vessels (vasa recta) passing to the gut wall. The arterial arcades in the ileal mesentery, by contrast, are multi-tiered with the tier nearest the bowel wall giving rise to numerous short, straight vessels which sink into the bowel wall (Figure 6).
process of the pancreas, with the vein joining the splenic vein behind the neck of the pancreas to form the portal vein (Figure 1). Lymphatic drainage and innervation of jejunum and ileum Lymphatic vessels accompany the blood vessels and drain into a series of mesenteric lymph nodes lying between the two layers of the small bowel mesentery. These nodes are easily palpable, and even visible, in the mesenteries of individuals without much body fat. These mesenteric nodes eventually drain into the preaortic lymph nodes situated around the origin of the superior mesenteric artery. It is because lymphatics become distended with milky fluid after a fatty meal, that they were initially named lacteals. The small bowel is innervated by efferent sympathetic fibres which pass from the mid-thoracic segments of the grey matter of the spinal cord via the splanchnic nerves to synapse in the superior mesenteric ganglia. These nerves are vasoconstrictor to the blood vessels and inhibitory to the small bowel musculature. Parasympathetic fibres from the vagus nerve are secretomotor to the mucosa and motor to the musculature of the gut wall. Afferent sympathetic fibres from the mid-gut pass centrally in the splanchnic nerves and carry sensations from the small bowel. The natural mobility of the jejunum and ileum predisposes these segments of the small intestine (especially the ileum) to enter the sacs of groin hernias and pelvic herniations. A tight neck of the hernial sac may result in the herniated segment of bowel becoming incarcerated or worse, strangulated. The latter being a potentially serious complication, is invariably a surgical emergency. The mobility of the small intestine may also result in a loop of bowel being constricted by a peritoneal adhesion resulting in small bowel obstruction.
Arterial supply and venous drainage of jejunum and ileum The jejunum and ileum, being derivatives of the embryological midgut, are supplied exclusively by branches of the superior mesenteric artery (Figure 1). From the left side of the curved superior mesenteric artery arise 15 to 20 branches. The proximal seven or eight are fairly large and are the jejunal arteries while the distal 10 to 12 branches are smaller in calibre and are the ileal branches. The jejunal and ileal arteries run between the two layers of the small intestinal mesentery towards the bowel wall. These vessels give off side-branches which anastomose with similar branches from adjacent vessels to form a series of vascular arches. The terminal branches from these arches are relatively straight and short vessels and are termed vasa recta. The corresponding veins (jejunal and ileal) drain into the superior mesenteric vein which lies alongside the artery in the root of the mesentery. Together, the superior mesenteric artery and vein cross the third part of the duodenum and uncinate
Meckel’s diverticulum: this relatively common congenital anomaly, occurring in about 2% of subjects, represents the remains of the embryonic vitello-intestinal duct, the embryonic communication between the mid-gut and the yolk sac. It is therefore found on the anti-mesenteric border of the lower ileum, about 2 feet, or 60 cm, from the ileocaecal junction. Its mucosa may contain islands of acid-secreting, oxyntic cells which may result in peptic ulceration with haemorrhage or perforation in children or young adults. As well as its more usual form, the duct may persist as a fistula or band connecting the ileum to the umbilicus (where it may serve as the axis for a small bowel volvulus), as a cyst attached to the anti-mesenteric border of the ileum or as a mass of red mucosa (‘raspberry tumour’) at the umbilicus. A
a
FURTHER READING 1 Langman’s medical embryology. 12th edn. TW Sadler Wolters Kluwer/Lippincott Williams & Wilkins, 2012; 208e28. 2 Moore KL, Dalley AF, Agur AMR. Clinically oriented anatomy. 6th edn. Wolters Kluwer/Lippincott Williams & Wilkins, 2010; 239e46. 3 Ellis H, Mahadevan V. Clinical anatomy. 13th edn. WILEY Blackwell, 2013; 83e5. 4 Bailey and Love’s short practice of surgery. In: Williams NS, Bulstrode CJK, O’Connell PR, eds. 25th edn. Hodder Arnold, 2008; 1154e203.
b Figure 6 Arterial arcades in mesentery of jejunum (a) and ileum (b).
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Anatomy of the small intestine
The duodenum The duodenum commences at the pyloroduodenal junction and ends at the duodenojejunal flexure where it is continuous with the jejunum. The commencement of the duodenum lies somewhat to the right of the midline while its distal end is 2 cm to the left of the midline. The duodenum is the widest, shortest and least mobile of the three segments of the small intestine. Unlike the jejunum and ileum, it does not possess a mesentery. The duodenum is a C-shaped tube about 25 cm in length (Figure 1) that is moulded around the head of the pancreas. It has two bends along its length, termed the superior and inferior duodenojejunal flexures. These bends allow the subdivision of the duodenum, for descriptive purposes, into four parts (Figure 2). In succession these are the superior part (1st part), descending part (2nd part), horizontal part (3rd part) and ascending part (4th part). In an average adult these parts are, respectively, 5 cm, 7.5 cm, 10 cm and 2.5 cm in length. The initial 2 cm or so of the duodenum, like the stomach, is ensleeved in peritoneum, and consequently has a degree of mobility. The remainder of the duodenum is a retroperitoneal, sessile and immobile structure. The topographical relations of the duodenum are shown in Figure 1. The concavity of the C-shaped duodenum is intimately related to, and surrounds the head and neck of the pancreas (Figure 1). The 1st part of the duodenum lies above the head of the pancreas and passes laterally, upwards and backwards to the right of the vertebral column at the level of the first lumbar vertebra. Situated immediately behind the 1st part of the duodenum are the gastroduodenal artery and common bile duct. Lying still more posteriorly is the inferior vena cava. Anterior relations of the 1st part of the duodenum are the liver (quadrate lobe) and neck of gallbladder. Chronic cholecystitis may result in inflammatory adhesion of the gallbladder wall to that of the 1st part of the duodenum. Subsequent gradual and progressive erosion of the walls of the gallbladder and duodenum may result in gallstones dropping into the duodenal lumen and being propagated along the small intestine, with possible impaction in the terminal ileum causing small bowel obstruction (gallstone ileus). The 2nd part of the duodenum lies anterior to the right renal hilum, and immediately lateral to the head of pancreas. Crossing in front of the 2nd part of the duodenum is the commencement of the transverse colon and its mesocolon. The 2nd part of the duodenum is thus vulnerable during mobilization of the hepatic flexure and transverse colon in the course of a right hemicolectomy (Figures 1 and 4). Since the 2nd part of the duodenum is a retroperitoneal structure being plastered down by the posterior parietal peritoneum, mobilization of the 2nd part of the duodenum requires a vertical incision of the peritoneum lateral to the duodenum prior to moving the duodenum anteriorly and medially. This technique is known as Kocher’s manoeuvre. The main pancreatic duct and the common bile duct have a common opening into the posteromedial wall of the second part of the duodenum at the major duodenal papilla. As estimated on endoscopy, this papilla is usually 9e10 cm distal to the pyloroduodenal junction. Frequently, a minor duodenal papilla is seen 2e3 cm proximal to the major one. It contains the opening of the accessory pancreatic duct.
Vishy Mahadevan
Abstract The small intestine (small bowel) commences at the pyloroduodenal junction and ends at the ileocaecal junction. It comprises, successively, the duodenum, jejunum and ileum. The principal function of the small intestine is the digestion and absorption of ingested food, electrolytes and vitamins and this important functional specialization is reflected in the enormous surface area of the small intestinal mucosa which is the result of the extensive and wavy in folding of the mucosal lining along the entire length of the small intestine. As might be expected from its very active role as a secretory and absorptive viscus, the small intestine has a rich blood supply. This is derived chiefly from the superior mesenteric artery via its inferior pancreaticoduodenal, jejunal and ileal branches. Venous drainage is via corresponding tributaries that drain to the superior mesenteric vein and thence to the portal vein. These vessels are accompanied by lymphatic vessels and autonomic nerve fibres. Developmental anomalies, mechanical obstruction and inflammatory disease affect the small intestine far more commonly than do neoplastic conditions. A Meckel’s diverticulum is a relatively common (2%) congenital anomaly of the distal ileum. In this article, an account of the topographical and surgical anatomy of the duodenum is followed by that of the jejunum and ileum. Various surgically-important features are highlighted.
Keywords Autonomic nerves; duodenum; ileum; jejunum; lymphatic drainage; Meckel’s diverticulum; mesentery; superior mesenteric artery and vein
The small intestine extends from the pylorus (which marks the distal end of the stomach) to the ileocaecal junction. It comprises three segments. These are, in succession, the duodenum, jejunum and ileum. Owing to its many distinctive anatomical features, the duodenum is usually considered a separate anatomical entity, different from the remainder of the small intestine. The distinguishing features of the duodenum include its retroperitoneal location, its immobility due to the absence of a mesentery and its numerous intimate surgically-important topographical relationships. By contrast, the jejunum and ileum have a considerably greater mobility within the abdominal cavity owing to their attachment to a suspensory mesentery. Another point of difference between the duodenum and the remainder of the small intestine is that the duodenum has a dual embryological origin, in part from the foregut and partly from the midgut, while the jejunum and ileum are derived exclusively from the midgut. In common clinical usage the term small bowel (small intestine) usually denotes just the jejunum and ileum and does not include the duodenum.
Vishy Mahadevan MBBS PhD FRCS (Ed & Eng) is the Barbers’ Company Professor of Anatomy at the Royal College of Surgeons of England, London, UK. Conflicts of interest: none.
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Ó 2014 Elsevier Ltd. All rights reserved.
BASIC SCIENCE
Inferior vena cava and hepatic veins
Oesophagus
Inferior phrenic vessels
Coeliac trunk Left gastric artery
Right suprarenal gland Spleen
Portal vein Bile duct Hepatic artery Right kidney Duodenum
Splenic artery Splenic vein Body of pancreas Inferior mesenteric vein
Uncinate process of head of pancreas
Duodenojejunal flexure
Ascending colon Descending colon Ureter Left sympathetic trunk Gonadal vessels
Superior mesenteric vein Superior mesenteric artery
Figure 1 The duodenum and its relationships.
The 3rd part of the duodenum commences at the inferior duodenal flexure and passes more or less horizontally to the left to become continuous with the 4th part of the duodenum, immediately to the left of the aorta. It is covered anteriorly by peritoneum. In its course from right to left, the 3rd part of the duodenum lies inferior to the pancreas and anterior to the ureter and gonadal vessels of the right side as these overlie the ventral surface of the right psoas major muscle. The 3rd part of the duodenum then crosses in front of the inferior vena cava and abdominal aorta and becomes continuous with the 4th part, in front of the left psoas major muscle. Where it crosses the aorta, the 3rd part of the duodenum usually overlies the origin of the inferior mesenteric artery. Crossing immediately in front of the 3rd part of the duodenum is the root of the small intestinal mesentery containing the superior mesenteric artery and vein (Figures 1 and 4). A tortuous and atherosclerosed superior mesenteric artery may compress the third part of the duodenum against the lumbar vertebral column causing intermittent obstruction of the duodenum. This is an extremely rare cause of high small-bowel obstruction and is known variously as Wilkie’s syndrome, Wilkie’s disease and superior mesenteric artery syndrome. The 4th part of the duodenum is the shortest of the four segments of the duodenum. It passes upwards on the left psoas major, lying to the left of the abdominal aorta (Figure 1). It then bends anteriorly at the duodenojejunal flexure. This flexure is typically 2 cm to the left of the median plane at the level of the 2nd lumbar vertebra. Running from the right crus of the diaphragm to gain attachment to the superior and lateral aspects of the duodenojejunal flexure is a fibrous or fibromuscular band
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termed the suspensory ligament of the duodenum (or ligament of Treitz). It is necessary to divide this ligament in order to mobilize the 4th part of duodenum prior to pancreatico-duodenectomy (Whipple’s operation) or when doing an open repair of an abdominal aortic aneurysm. Arterial supply and venous drainage of the duodenum The main arterial supply of the duodenum (and the adjacent head of pancreas) (Figures 1 and 3) is by the superior and inferior pancreaticoduodenal arteries. The former is a branch of the gastroduodenal artery (in turn a branch of the common hepatic artery) while the latter is a direct and early branch of the superior mesenteric artery. These two vessels form anterior and posterior anastomotic arterial arcades which lie along the concavity of the duodenum, in the cleft between the duodenum and head of pancreas. The veins draining the duodenum correspond to the arteries, and empty into the superior mesenteric vein and thus into the portal vein. In terms of embryological development, the duodenum develops from the distal end of the foregut and the proximal part of the midgut. The junction between the two is somewhat distal to the duodenal papilla that receives the common bile duct and main pancreatic duct. The dual origin of the duodenum is reflected in its blood supply: being derived in part from the coeliac artery (the artery of the foregut) and the superior mesenteric artery (the artery of the midgut). Since the advent of proton pump inhibitors and antimicrobials in the management of acid peptic disease, complications of acid peptic ulceration of the duodenum such as duodenal perforation and duodenal erosive haemorrhage are now rarely seen in the
392
Ó 2014 Elsevier Ltd. All rights reserved.
BASIC SCIENCE
a
Gastroduodenal artery
1st
Duodenal branches 2nd
Supraduodenal artery
Superior pancreaticoduodenal artery Right gastroepiploic artery
4th
Posterior branch Anterior branch
3rd
Posterior branch Anterior branch Inferior pancreaticoduodenal artery
Quadrate lobe
b
Neck of gallbladder
Duodenal branch First jejunal artery
Right end of the gastrocolic omentum
Figure 3 Arterial supply of duodenum.
Attachment of the transverse mesocolon
at the level of the 2nd lumbar vertebra), across the midline to the right iliac fossa (Figures 4 and 5) This attachment of the mesentery to the posterior abdominal wall is referred to as the root of the mesentery. It is about 15 cm in length and contains the superior mesenteric artery and vein and lymph nodes. Starting at the duodenojejunal flexure, the root of the small intestinal mesentery crosses successively, the 3rd part of the duodenum, the aorta, the inferior vena cava, the right gonadal vessels, right ureter, right psoas major and right sacroiliac joint (Figure 4). The jejunum ( jejune; Latin for ‘empty’) and ileum (Greek for ‘to twist’) have a collective length of 5e6 m. The jejunum accounts for the proximal two-fifths of this length and the ileum makes up the distal three-fifths. The small intestinal mesentery and the entire length of the jejunum and ileum are located wholly in the infracolic compartment of the peritoneal cavity; which is to say, inferior to the attachment of the transverse mesocolon (Figures 4 and 5). There is no clear, sharp anatomical demarcation between the jejunum and ileum, the transition being an insidious one. Nevertheless, the proximal parts of the jejunum can be readily distinguished from the distal parts of the ileum on the basis of the following features. The jejunum is wider and possesses a thicker wall than the ileum. The decrease in calibre is a gradual one. The narrowest part of the small bowel is usually just proximal to the ileocaecal junction. Indeed it is at this point that solid objects are most likely to be impacted (as for example in gallstone ileus). The jejunum usually appears more vascular than the ileum. The jejunum loops tend to lie in the upper left part of the infracolic compartment, while the loops of ileum tend to be found in the lower part of the abdominal cavity, mainly on the right side, and often flop into the pelvic cavity (where they may be damaged by pelvic radiotherapy). The small bowel mesentery corresponding to the ileum is usually thicker and more fat-laden than the mesentery corresponding to the jejunum. Consequently the vascular detail in the ileal mesentery is not as clearly seen as in the relatively avascular and translucent jejunal mesentery.
Superior mesenteric artery Superior mesenteric vein
c
Bile duct Portal vein Right kidney
Gastroduodenal artery Left kidney
Left renal vessels Right renal vessels Right ureter Right gonadal vein
Left ureter Aorta Left gonadal artery
Right gonadal artery Inferior vena cava
Figure 2 The four parts of the duodenum.
developed world. However these complications are still prevalent in the developing world.
The jejunum and ileum The jejunum is the direct continuation of the duodenum and commences at the duodenojejunal flexure. Unlike the duodenum, the jejunum and ileum are ensleeved by the free edge of the small intestinal mesentery and consequently, are fairly mobile within the abdominal cavity. The small intestinal mesentery is a fanshaped, double-layered sheet of peritoneum attached to the posterior abdominal wall along a line which runs obliquely from the duodenojejunal junction (which lies to the left of the midline
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BASIC SCIENCE
Bare area
Upper layer of Transverse Falciform ligament coronary ligament mesocolon Left triangular ligament Splenorenal ligament
Lower layer of coronary ligament Right triangular ligament
Left suprarenal gland
Epiploic foramen
Tail of pancreas and splenic vessels
Greater omentum
Paraduodenal recess Phrenicocolic ligament Iliohypogastric nerve
Right paracolic gutter Superior mesentric vessels
Ilioinguinal nerve
Right infracolic compartment
Lateral femoral cutaneous nerve
Root of mesentery
Left paracolic gutter Femoral nerve
Left infracolic compartment
Sigmoid mesocolon
Figure 4 Location of root of small intestinal mesentery and attachments of parietal peritoneum to the posterior abdominal wall.
Greater omentum Transverse colon Transverse mesocolon
Root of mesocolon
Duodenum
Duodenojejunal flexure
Right infracolic compartment
Root of mesentery
Figure 5 Infracolic compartment of peritoneal cavity showing jejunum, ileum and small intestinal mesentery. SURGERY 32:8
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BASIC SCIENCE
The distal part of the ileum often presents, on its antimesenteric border, elongated, pale plaques. These are aggregations of lymphoid follicles in the ileal mucosa known as Peyer’s patches. They are visible through the muscle in the ileal wall. Very characteristically, the jejunal arteries form just one or two tiers of arcades within the mesentery, with relatively long and fewer straight vessels (vasa recta) passing to the gut wall. The arterial arcades in the ileal mesentery, by contrast, are multi-tiered with the tier nearest the bowel wall giving rise to numerous short, straight vessels which sink into the bowel wall (Figure 6).
process of the pancreas, with the vein joining the splenic vein behind the neck of the pancreas to form the portal vein (Figure 1). Lymphatic drainage and innervation of jejunum and ileum Lymphatic vessels accompany the blood vessels and drain into a series of mesenteric lymph nodes lying between the two layers of the small bowel mesentery. These nodes are easily palpable, and even visible, in the mesenteries of individuals without much body fat. These mesenteric nodes eventually drain into the preaortic lymph nodes situated around the origin of the superior mesenteric artery. It is because lymphatics become distended with milky fluid after a fatty meal, that they were initially named lacteals. The small bowel is innervated by efferent sympathetic fibres which pass from the mid-thoracic segments of the grey matter of the spinal cord via the splanchnic nerves to synapse in the superior mesenteric ganglia. These nerves are vasoconstrictor to the blood vessels and inhibitory to the small bowel musculature. Parasympathetic fibres from the vagus nerve are secretomotor to the mucosa and motor to the musculature of the gut wall. Afferent sympathetic fibres from the mid-gut pass centrally in the splanchnic nerves and carry sensations from the small bowel. The natural mobility of the jejunum and ileum predisposes these segments of the small intestine (especially the ileum) to enter the sacs of groin hernias and pelvic herniations. A tight neck of the hernial sac may result in the herniated segment of bowel becoming incarcerated or worse, strangulated. The latter being a potentially serious complication, is invariably a surgical emergency. The mobility of the small intestine may also result in a loop of bowel being constricted by a peritoneal adhesion resulting in small bowel obstruction.
Arterial supply and venous drainage of jejunum and ileum The jejunum and ileum, being derivatives of the embryological midgut, are supplied exclusively by branches of the superior mesenteric artery (Figure 1). From the left side of the curved superior mesenteric artery arise 15 to 20 branches. The proximal seven or eight are fairly large and are the jejunal arteries while the distal 10 to 12 branches are smaller in calibre and are the ileal branches. The jejunal and ileal arteries run between the two layers of the small intestinal mesentery towards the bowel wall. These vessels give off side-branches which anastomose with similar branches from adjacent vessels to form a series of vascular arches. The terminal branches from these arches are relatively straight and short vessels and are termed vasa recta. The corresponding veins (jejunal and ileal) drain into the superior mesenteric vein which lies alongside the artery in the root of the mesentery. Together, the superior mesenteric artery and vein cross the third part of the duodenum and uncinate
Meckel’s diverticulum: this relatively common congenital anomaly, occurring in about 2% of subjects, represents the remains of the embryonic vitello-intestinal duct, the embryonic communication between the mid-gut and the yolk sac. It is therefore found on the anti-mesenteric border of the lower ileum, about 2 feet, or 60 cm, from the ileocaecal junction. Its mucosa may contain islands of acid-secreting, oxyntic cells which may result in peptic ulceration with haemorrhage or perforation in children or young adults. As well as its more usual form, the duct may persist as a fistula or band connecting the ileum to the umbilicus (where it may serve as the axis for a small bowel volvulus), as a cyst attached to the anti-mesenteric border of the ileum or as a mass of red mucosa (‘raspberry tumour’) at the umbilicus. A
a
FURTHER READING 1 Langman’s medical embryology. 12th edn. TW Sadler Wolters Kluwer/Lippincott Williams & Wilkins, 2012; 208e28. 2 Moore KL, Dalley AF, Agur AMR. Clinically oriented anatomy. 6th edn. Wolters Kluwer/Lippincott Williams & Wilkins, 2010; 239e46. 3 Ellis H, Mahadevan V. Clinical anatomy. 13th edn. WILEY Blackwell, 2013; 83e5. 4 Bailey and Love’s short practice of surgery. In: Williams NS, Bulstrode CJK, O’Connell PR, eds. 25th edn. Hodder Arnold, 2008; 1154e203.
b Figure 6 Arterial arcades in mesentery of jejunum (a) and ileum (b).
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