The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the pancreatic duct to aid in digestion. The small intestine is about 6.5 metres (21 feet) long and folds many times to fit in the abdomen. Although it is longer than the large intestine, it is called the small intestine because it is narrower in diameter. The small intestine has three distinct regions – the duodenum, jejunum, and ileum. The duodenum, the shortest, is where preparation for absorption through small finger-like protrusions called intestinal villi begins. The jejunum is specialized for the absorption through its lining by enterocytes of small nutrient particles which have been previously digested by enzymes in the duodenum. The main function of the ileum is to absorb vitamin B12, bile salts, and whatever products of digestion were not absorbed by the jejunum.
Structure
Size The length of the small intestine can vary greatly, from as short as 3 metres (10 feet) to as long as 10.5 m (34+1⁄2 ft), also depending on the measuring technique used. The typical length in a living person is 3–5 m (10–16+1⁄2 ft). The length depends both on how tall the person is and how the length is measured. Taller people generally have a longer small intestine and measurements are generally longer after death and when the bowel is empty.
It is approximately 1.5 centimetres (5⁄8 inch) in diameter in newborns after 35 weeks of gestational age, and 2.5–3 cm (1–1+1⁄8 in) in diameter in adults. On abdominal X-rays, the small intestine is considered to be abnormally dilated when the diameter exceeds 3 cm. On CT scans, a diameter of over 2.5 cm is considered abnormally dilated. The surface area of the human small intestinal mucosa, due to enlargement caused by folds, villi and microvilli, averages 30 square metres (320 sq ft).
Parts
The small intestine is divided into three structural parts.
The duodenum is a short structure ranging from 20–25 cm (8–10 in) in length, and shaped like a "C". It surrounds the head of the pancreas. It receives gastric chyme from the stomach, together with digestive juices from the pancreas (digestive enzymes) and the liver (bile). The digestive enzymes break down proteins and bile emulsifies fats into micelles. The duodenum contains Brunner's glands, which produce a mucus-rich alkaline secretion containing bicarbonate. These secretions, in combination with bicarbonate from the pancreas, neutralize the stomach acids contained in gastric chyme. The jejunum is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8 ft) long, and contains the circular folds, and intestinal villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here. The suspensory muscle of duodenum marks the division between the duodenum and the jejunum. The ileum: The final section of the small intestine. It is about 3 m (9.8 feet) long, and contains villi similar to the jejunum. It absorbs mainly vitamin B12 and bile acids, as well as any other remaining nutrients. The ileum joins to the cecum of the large intestine at the ileocecal junction. The jejunum and ileum are suspended in the abdominal cavity by mesentery. The mesentery is part of the peritoneum. Arteries, veins, lymph vessels and nerves travel within the mesentery.
Blood supply The small intestine receives a blood supply from the celiac trunk and the superior mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the celiac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery via the inferior pancreaticoduodenal artery. These two arteries both have anterior and posterior branches that meet in the midline and anastomose. The jejunum and ileum receive blood from the superior mesenteric artery. Branches of the superior mesenteric artery form a series of arches within the mesentery known as arterial arcades, which may be several layers deep. Straight blood vessels known as vasa recta travel from the arcades closest to the ileum and jejunum to the organs themselves.
Microanatomy
The three sections of the small intestine look similar to each other at a microscopic level, but there are some important differences. 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 genes are expressed in the normal duodenum. Some 300 of these genes are more specifically expressed in the duodenum with very few genes expressed only in the small intestine. The corresponding specific proteins are expressed in glandular cells of the mucosa, such as fatty acid binding protein FABP6. Most of the more specifically expressed genes in the small intestine are also expressed in the duodenum, for example FABP2 and the DEFA6 protein expressed in secretory granules of Paneth cells.
Development
The small intestine develops from the midgut of the primitive gut tube. By the fifth week of embryological life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the primary intestinal loop. The loop grows so fast in length that it outgrows the abdomen and protrudes through the umbilicus. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of the large intestine.
Function Food from the stomach is allowed into the duodenum through the pylorus by a muscle called the pyloric sphincter.
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