Small Intestine Anatomy

The small intestine (small bowel) lies between the stomach and the large intestine (large bowel) and includes the duodenum, jejunum, and ileum. The small intestine is so called because its lumen diameter is smaller than that of the large intestine, although it is longer in length than the large intestine.

The duodenum continues into the jejunum at the duodenojejunal junction or flexure, which lies to the left of L2 vertebra and is fixed to the retroperitoneum by a suspensory ligament of Treitz. The inferior mesenteric vein (IMV) lies to the left of the duodenojejunal junction. There are several peritoneal fossae around the duodenojejunal flexure, which may be the sites of an internal herniation of the small bowel. The rest of the small intestine is a 4-6-m long convoluted tube occupying the center of the abdomen and the pelvis, surrounded on 2 sides and above by the colon (a part of the large intestine). The ileum continues into the large intestine (cecum) at the ileocecal junction.[1, 2, 3, 4, 5]

The digestive tract anatomy is depicted in the image below.

Digestive tract, anterior view.

The small intestine is differentiated from the large intestine by the presence of a mesentery (exceptions being no mesentery in the duodenum, and presence of a mesentery in the transverse and sigmoid colons) and the absence of tenia coli and appendices epiploicae, which are present in the colon. The demarcation between the jejunum (proximal) and the ileum (distal) is not very clear.

Embryology

Embryologically, the small intestine develops mainly from the midgut, with the superior mesenteric artery (SMA) as its artery. The midgut also gives rise to the proximal large intestine (up to the proximal two thirds of the transverse colon). The proximal part of the duodenum (between the pylorus and major duodenal papilla) develops from the caudal foregut. The site of the major duodenal papilla on the medial wall of the second part of the duodenum marks the junction of embryological foregut and midgut. At an early stage of development, the midgut communicates with the yolk sac via a vitellointestinal (omphalomesenteric) duct, which disappears later.

Duodenum

The duodenum has 4 parts: superior, descending, horizontal, and ascending.

The first (superior) part, or bulb (5 cm), is connected to the undersurface of the liver (porta hepatis) by the hepatoduodenal ligament (HDL), which contains the proper hepatic artery, portal vein, and common bile duct (CBD); the quadrate lobe of the liver and gallbladder are in front, and the CBD), portal vein, and gastroduodenal artery (GDA) are behind.

The second (descending) part, or C loop (10 cm), which has an upper and a lower genu (flexure), is related to the transverse mesocolon and colon in front and the right kidney and inferior vena cava (IVC) behind; the head of the pancreas lies in the concavity of the duodenal C.

The third (horizontal) part (7.5 cm) runs from right to left in front of the inferior vena cava (IVC) and aorta, with the superior mesenteric vessels (the vein on the right and the artery on the left) in front.

The fourth (ascending) part (2.5 cm) continues as the jejunum. The duodenum continues into the jejunum at the duodenojejunal flexure.

Jejunum

The jejunum constitutes about two fifths of the proximal small intestine and the ileum makes the distal three fifths. No clear demarcation is noted between the jejunum and ileum; however, there are some features which distinguish the jejunum from the ileum. The jejunum has a thicker wall and a wider lumen than the ileum and mainly occupies the left upper and central abdomen. Mesenetric fat is less abundant in the mesentery of the jejunum and vessels in the mesentery are, therefore, well seen.

Ileum

The ileum constitutes about three fifths of the distal small intestine and the jejunum makes the proximal two fifths. No clear demarcation is noted between the ileum and jejunum; however, there are some features which distinguish the ileum from the jejunum. The ileum has a thinner wall and a smaller lumen than the jejunum and mainly occupies the central and right lower abdomen and pelvis. Mesenteric fat is abundant in the mesentery of the ileum and vessels in the mesentery are, therefore, not well seen. (In cystic fibrosis, the jejunum is where the mesentery vessels are well seen because much less mesenteric fat is present in the jejunum than in the ileum.)

Mesentery

The mesentery is a double fold of peritoneum attached to the posterior abdominal wall. It is fan-shaped with a root of about 15 cm extending obliquely from the left L2 transverse process level to the right sacroiliac joint and crossing the third part of the duodenum, aorta and inferior vena cava (IVC) and right ureter, and a 4- to 6-m periphery, which covers the entire length of the jejunum and ileum. Between the 2 leaves of the mesentery are the mesenteric vessels and lymph nodes.

Normal versus obstructed intestine

In adults, normal (nonobstructed) small intestine is empty or filled with a small amount of fluid only and is, therefore, not seen on plain abdominal radiographs; in children a few (2-3) air-filled small intestine loops may be normally seen.

In intestinal obstruction, the small intestine is dilated and gets filled with air and fluid. Plain radiographs of the abdomen show dilated loops (in the supine position) and air fluid levels (in the erect position).

Dilated jejunum has a stack-of-coins appearance because of plicae circulares or valvulae conniventes (mucosal folds), while a dilated ileum has the appearance of a cylindrical tube ("characterless").

Blood supply

The superior mesenteric artery (SMA) is the artery of the midgut and, therefore, of the small intestine; it comes off as the second branch (the inferior mesenteric artery is its third branch) from the anterior surface of the abdominal aorta about 1 cm below the origin of the celiac trunk, at the level of L1 behind the neck of the pancreas. From there, it descends in front of the uncinate process of the pancreas and the third (horizontal) part of the duodenum to enter the small intestine mesentery.

Multiple jejunal and ileal branches arise from the left side of the SMA. They anastomose with each other to form a series of loops or arcades from which arise the terminal (end) branches, called vasa recta, which supply the jejunum and ileum and lie between the 2 leaves of the small intestine mesentery. Jejunum has fewer (2-3) series of arcades, and the vasa recta are longer. The ileum has more (4-5) series of arcades, and the vasa recta are shorter.

From the right side of the SMA arise ileocolic, right colic, and middle colic arteries. The ileocolic artery or one of its branches gives off the appendicular artery. The ileal branch of the ileocolic artery anastomoses with the terminal ileal branch of the SMA. The left branch of the middle colic artery anastomoses with the ascending branch of the left colic artery (which in itself is a branch of the inferior mesenteric artery). The arc of Riolan connects the middle colic artery (or its left branch) to the left colic artery (or its ascending branch).

Jejunal, ileal, ileocolic, right colic, and middle colic arteries are accompanied by the same named veins, which drain into the SMV.

The superior mesenteric vein (SMV) lies to the right of the SMA in front of the uncinate process of the pancreas and the third part of the duodenum. Union of the vertical SMV and the horizontal splenic vein forms the portal vein (PV) behind the neck of the pancreas. The inferior mesenteric vein (IMV) lies to the immediate left of the duodenojejunal (DJ) flexure and joins the junction of the splenic vein (SV) and SMV. The PV runs up (superiorly) behind the first part of the duodenum in the hepatoduodenal ligament (HDL) behind (posterior to) the bile duct on the right and the proper hepatic artery (HA) on the left. The portal venous system (SV, SMV, and PV) has no valves.

The small intestine contains the standard 4 layers present in most parts of the gastrointestinal tract: the mucosa, submucosa, muscularis propria, and serosa.

The mucosa includes a columnar epithelium with glands called crypts of Lieberkuhn; mucus-secreting goblet cells; Paneth cells, which secrete lysozymes; enteroendocrine cells, which secrete hormones; fingerlike (leaflike) projections called villi, which increase its absorptive surface area several times; lamina propria (connective tissue); and muscularis mucosa.

The ileum has subepithelial aggregates of lymphoid tissue along the antimesenteric border; these are called Peyer patches. Mucosa is much thicker in the jejunum than in the ileum and is arranged in spiral folds called plicae circulares, which appear as valvulae conniventes on plain abdominal radiographs.

The submucosa contains the blood vessels and the Meissner nerve plexus; the muscularis propria contains inner circular and outer longitudinal muscles and myenteric (Auerbach) nerve plexus; and the serosa covering the organs of the peritoneal cavity is called the visceral peritoneum.

From the point of view of absorption of important nutrients, the proximal jejunum and distal ileum are more important; the distal jejunum and proximal ileum (mid-small bowel) can be more easily sacrificed without much disturbance of absorption. Massive resection of small bowel (eg, in mesenteric vascular disease) or repeated resections (eg, in Crohn disease) may result in short bowel syndrome.

The vitellointestinal duct may persist as the following:

• A patent duct: This is small bowel communicating with the umbilicus.

• Meckel diverticulum: This is present in about 2% of people and is a true (containing all layers) diverticulum, from the antimesenteric border of the distal ileum (about 2 ft proximal to the ileocecal junction). It is a remnant of the intestinal end of the vitelline duct. The mucosa of the diverticulum may contain ectopic gastric tissue, which may secrete acid and cause ulceration in the adjacent intestinal mucosa. The tip of the diverticulum may be attached to the umbilicus by a band around which volvulus (rotation) of ileum may occur, causing intestinal obstruction and strangulation.

• Vitelline sinus at the umbilicus: This presents as a raspberry tumor or adenoma.

• Fibrous band between ileum and umbilicus, around which torsion of a small bowel loop may occur

Malrotation

Malrotation of the gut results in the location of the small intestine on the right side and the large intestine on the left side of the abdomen. The duodenojejunal flexure comes to lie to the right (instead of the normal left) of the midline and cecum in epigastrium or right hypochondrium (instead of the normal right iliac fossa); a band (of Ladd) runs across the duodenum from right to left, and the narrow base of the small bowel mesentery predisposes it to volvulus.

Atresia

Atresia (duodenal, jejunal, and ileal), narrowing, or even complete obliteration of the lumen resulting in neonatal intestinal obstruction may be present; it may even cause polyhydramnios (excessive amniotic fluid) in utero.

Diverticulae and duplication cysts can occur in any part of the small intestine. 

The small intestine is evaluated radiologically by performing an upper GI series using barium or Gastrografin follow-through. The patient drinks the radiological contrast medium, which is then tracked by fluoroscopy. The proximal (jejunal) small bowel loops lie in the left upper quadrant whereas the distal (ileal) small bowel loops lie in the right lower quadrant. The small intestine is also evaluated by enteroclysis, in which contrast is introduced directly into the proximal jejunum thorough a nasojejunal tube. CT scanning can also be combined with enteroclysis.

Capsule endoscopy, as depicted in the video below, is a new device that contains a capsule-shaped video camera that is swallowed by the patient; as it passes through the gastrointestinal tract, it keeps transmitting digital images of the mucosa that are captured by a receiver device strapped to the patient. These images are then read by a computer. The capsule is passed in the feces.

The small intestine is usually inaccessible to conventional endoscopy (upper GI esophagogastroduodenoscopy and lower GI colonoscopy). With newer balloon-tipped upper GI endoscopes, even antegrade (per-oral) enteroscopy of the proximal jejunum is now possible; however, it is technically difficult. An expert endoscopist can view the terminal ileum through a colonoscope passed retrogradely through the ileocecal opening into the ileum (ie, retrograde enteroscopy). Capsule endoscopy can examine the entire small intestine. During surgery, an enterotomy can be made in the mid small bowel, and a flexible fiberoptic endoscope can be passed both proximally and distally to evaluate the entire small bowel (intraoperative enteroscopy); for example, in a patient with obscure GI bleeding.

Massive (as in acute mesenteric ischemia) or repeated (as in Crohn disease) small bowel resection can result in short bowel syndrome if the residual length of the small bowel is less than 180 cm.