Small intestine

The small intestine, or small bowel, sits between the stomach and the large intestine. It is the primary site where nutrients from food are absorbed into the body. To help with digestion, it receives bile and pancreatic juice via the pancreatic duct. Despite being longer than the large intestine—typically about 6.5 meters (21 feet) in length and folded many times to fit inside the abdomen—it is called "small" because its diameter is narrower.

The organ has three distinct sections: the duodenum, jejunum, and ileum. The duodenum, the shortest part, is where preparation for absorption begins, aided by tiny finger-like projections called intestinal villi. The jejunum specializes in absorbing small nutrient particles that were broken down by enzymes in the duodenum; this happens through its lining cells called enterocytes. The ileum’s main job is to absorb vitamin B12, bile salts, and any remaining digestion products not taken up by the jejunum.

**Structure** **Size** The small intestine’s length varies widely, from as short as 3 meters (10 feet) to as long as 10.5 meters (34.5 feet), depending on measurement technique. In a living person, it typically measures 3–5 meters (10–16.5 feet). Length is influenced by a person’s height and whether the bowel is empty or measured after death (postmortem measurements are longer). In newborns after 35 weeks of gestation, its diameter is about 1.5 centimeters (0.6 inches); in adults, it is 2.5–3 centimeters (1–1.2 inches). On abdominal X-rays, dilation is considered abnormal beyond 3 centimeters; on CT scans, beyond 2.5 centimeters. The surface area of the mucosa, enlarged by folds, villi, and microvilli, averages 30 square meters (320 square feet).

**Parts** The duodenum is a short, C-shaped structure 20–25 centimeters (8–10 inches) long that wraps around the head of the pancreas. It receives gastric chyme from the stomach, along with digestive enzymes from the pancreas and bile from the liver. Enzymes break down proteins, while bile emulsifies fats into micelles. The duodenum also contains Brunner’s glands, which secrete a mucus-rich alkaline fluid containing bicarbonate. This secretion, together with bicarbonate from the pancreas, neutralizes stomach acids in the chyme. The jejunum, the midsection about 2.5 meters (8 feet) long, connects the duodenum to the ileum. It features circular folds and intestinal villi that increase surface area. Here, sugars, amino acids, and fatty acids are absorbed into the bloodstream. The suspensory muscle of the duodenum marks the boundary between the duodenum and jejunum. The ileum is the final section, roughly 3 meters (9.8 feet) long, with villi similar to the jejunum. It primarily absorbs vitamin B12, bile acids, and any leftover nutrients. It joins the large intestine’s cecum at the ileocecal junction. Both the jejunum and ileum are suspended in the abdominal cavity by the mesentery, a part of the peritoneum that carries arteries, veins, lymph vessels, and nerves.

**Blood supply** The small intestine receives blood from the celiac trunk and the superior mesenteric artery, both branches of the aorta. The duodenum gets blood from the celiac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery; these two arteries have anterior and posterior branches that meet and anastomose in the midline. The jejunum and ileum are supplied by the superior mesenteric artery, whose branches form a series of arches (arterial arcades) within the mesentery, sometimes several layers deep. Straight vessels called vasa recta travel from the innermost arcades to the organs.

**Microanatomy** At the microscopic level, the three sections look similar, though important differences exist. The parts of the intestine are as follows:

**Gene and protein expression** About 20,000 protein-coding genes are expressed in human cells, and 70% of these are active in the normal duodenum. Roughly 300 genes are more specifically expressed in the duodenum, with very few exclusive to the small intestine. The corresponding proteins are found in glandular cells of the mucosa, such as fatty acid binding protein FABP6. Most genes more specifically expressed in the small intestine are also expressed in the duodenum—for example, FABP2 and DEFA6, a protein in secretory granules of Paneth cells.

**Development** The small intestine develops from the midgut of the primitive gut tube. By the fifth week of embryonic life, the ileum grows rapidly, forming a U-shaped fold called the primary intestinal loop. This loop lengthens so quickly that it protrudes through the umbilicus, outgrowing the abdomen. By week 10, it retracts back inside. Between weeks six and ten, the small intestine rotates anticlockwise (as viewed from the front of the embryo), and after returning to the abdomen, it rotates another 180 degrees. This process creates the twisted shape of the large intestine.

**Function** Food enters the duodenum from the stomach through the pylorus, controlled by the pyloric sphincter muscle.

**Digestion** Most chemical digestion occurs in the small intestine. Many digestive enzymes act here, breaking down food for absorption.