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.
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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.
