<P> It is approximately 1.5 cm in diameter in newborns after 35 weeks of gestational age, and 2.5--3 cm (1 inch) 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 meters . </P> <P> The small intestine is divided into three structural parts . </P> <Ul> <Li> The duodenum is a short structure ranging from 20 cm (7.9 inches) to 25 cm (9.8 inches) 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 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, neutralize the stomach acids contained in gastric chyme . </Li> <Li> 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 . </Li> <Li> 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 . </Li> </Ul> <Li> The duodenum is a short structure ranging from 20 cm (7.9 inches) to 25 cm (9.8 inches) 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 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, neutralize the stomach acids contained in gastric chyme . </Li>

Where does most digestion take place in the small intestine