The small intestine is the longest part of the gastrointestinal tract. It extends from the pyloric orifice to the ileocecal junction, and has an overall average length of 6 to 7 meters with a narrowing diameter from beginning to end.
The small intestine is the primary site for nutrient absorption from ingested materials. This hollow tube consists of three parts: the duodenum, the jejunum and the ileum.
The duodenum is the first and shortest part of the small intestine with a length of 20 to 25 cm. It extends from the pylorus of the stomach on the right side, to the duodenojejunal flexure on the left side. The duodenum is a C-shaped structure with the widest lumen present in all of the small intestine. It lies adjacent to the head of the pancreas, above the level of the umbilicus, and is retroperitoneal for the most part, except for its beginning, where it is connected to the liver by the hepatoduodenal ligament.
The duodenum is described as having four parts: superior, descending, inferior, and ascending.
The jejunum is the second part of the small intestine; it represents the proximal two-fifths. It extends from the duodenojejunal flexure to the ileum, though there is no distinct external demarcation between the two. The jejunum, unlike the duodenum, is intraperitoneal (present within the peritoneal cavity) and lies in the in the left upper quadrant of the abdomen. It has thicker walls than the ileum, and an extensive blood supply that makes it appear redder. Moreover, the inner mucosal lining of the jejunum has characteristic prominent folds termed the plicae circulares, which circle the lumen.
The ileum is the third part of the small intestine, and makes up the distal three-fifths. It extends from the jejunum to the ileocecal junction, and together with the jejunum is 6 to 7 meters long. The ileum is also intraperitoneal, and lies mostly in the right lower quadrant of the abdomen with its terminal portion often lying in the pelvis. It has thinner walls, more mesenteric fat, and less obvious plicae circulares compared to the jejunum.
The ileum opens into the large intestine at the cecum (the first part of the large intestine), and invaginates it. It forms the ileocecal valve, which plays a role in regulating passage from the ileum to the cecum and preventing reflux.
The arterial supply of the duodenum is provided by the gastroduodenal artery originating from a branch of the celiac trunk, and the inferior pancreaticoduodenal artery originating from the superior mesenteric artery.
The duodenal veins follow their major arteries and drain into the hepatic portal vein. The jejunum and ileum receive arterial blood from the superior mesenteric artery, which arises from the aorta at the level of the first lumbar vertebra (L1).
The superior mesenteric artery passes between layers of mesentery to split into approximately twenty branches that anastomose to form loops termed arcades. These arcades give rise to long and straight arteries, the vasa recta. The venous drainage of the jejunum and ileum flows through the superior mesenteric vein, which joins the splenic vein to form the hepatic portal vein.
The small intestine is innervated by both autonomic and sensory fibers originating from the celiac and superior mesenteric plexuses.