Title: Small intestine | anatomy | Britannica
Headings: 
Small intestine
anatomy
Consider whether the continuous band of membranous tissue around humans' intestines should be deemed an organ
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Content: Test both halves of your mind in this human anatomy quiz. Three successive regions of the small intestine are customarily distinguished: duodenum, jejunum, and ileum. These regions form one continuous tube, and, although each area exhibits certain characteristic differences, there are no distinctly marked separations between them. The first area, the duodenum, is adjacent to the stomach; it is only 23 to 28 cm (9 to 11 inches) long, has the widest diameter, and is not supported by the mesentery. Ducts from the liver, gallbladder, and pancreas enter the duodenum to provide juices that neutralize acids coming from the stomach and help digest proteins, carbohydrates, and fats. The second region, the jejunum, in the central section of the abdomen, comprises about two-fifths of the remaining tract. The colour of the jejunum is deep red because of its extensive blood supply; its peristaltic movements are rapid and vigorous, and there is little fat in the mesentery that supports this region.