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The small intestine or small bowel is the part of the gastrointestinal tract between the stomach and the large intestine, and is where much of the digestion and absorption of food takes place. The small intestine has three distinct regions – the duodenum, jejunum, and ileum. It receives bile and pancreatic juice through the pancreatic duct, controlled by the sphincter of Oddi. The primary function of the small intestine is the absorption of nutrients and minerals from food.〔http://www.britannica.com/EBchecked/topic/275485/human-body〕 This article is primarily about the human gastrointestinal tract. The information about its processes is directly applicable to most placental mammals. A major exception to this is the digestive process in ruminants – notably cows. In invertebrates such as worms, the term "large intestine" is often used to describe the entire gastrointestinal tract. ==Structure== The average length of the small intestine in an adult human male is , and in an adult female. It can vary greatly, from as short as to as long as .〔(【引用サイトリンク】 year = 1928 )〕 Recent studies indicate that small intestine may be shorter, around , and that the length is less affected by age after childhood than expected〔 〕 It is approximately 2.5–3 cm in diameter. The surface area of the human small intestinal mucosa averages 30 square meters. The small intestine is divided into three structural parts. The duodenum is a short structure (about 20–25 cm long) continuous with the stomach 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 gall bladder (bile). The digestive enzymes break down proteins and bile and emulsify 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, neutralizes 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 long, and contains the plicae circulares, and 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 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. The small intestine receives a blood supply from the coeliac trunk and the superior mesenteric artery. These are both branches of the aorta. The duodenum receives blood from the coeliac trunk via the superior pancreaticoduodenal artery and from the superior mesenteric artery visa the inferior pancreaticoduodenal artery. These two arteries both have anterior and posterior branches that meet in the midline and anastaomose. 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「small intestine」の詳細全文を読む スポンサード リンク
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