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The jejunum is the second part of the small intestine in humans and most higher vertebrates, including mammals, reptiles, and birds. Its lining is specialized for the absorption by enterocytes of small nutrient molecules which have been previously digested by enzymes in the duodenum .
The small intestine is about 5.5 metres (18 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.
Intestinal glands are found in the epithelia of the small intestine, namely the duodenum, jejunum, and ileum, and in the large intestine (colon), where they are sometimes called colonic crypts. Intestinal glands of the small intestine contain a base of replicating stem cells , Paneth cells of the innate immune system , and goblet cells , which ...
Jejunum: This is the midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8.2 ft) long and contains the circular folds also known as plicae circulares and villi that increase its surface area. Products of digestion (sugars, amino acids, and fatty acids) are absorbed into the bloodstream here.
In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum. [2] Its main function is to absorb vitamin B 12 , bile salts , and whatever products of digestion that were not absorbed by the jejunum .
In the lower part of the descending portion, below the point where the bile and pancreatic ducts enter the small intestine, they are very large and closely approximated. In the horizontal and ascending portions of the duodenum and upper half of the jejunum , they are large and numerous. [ 3 ]
Peyer's patches (or aggregated lymphoid nodules) are organized lymphoid follicles, named after the 17th-century Swiss anatomist Johann Conrad Peyer. [1] They are an important part of gut associated lymphoid tissue usually found in humans in the lowest portion of the small intestine, mainly in the distal jejunum and the ileum, but also could be detected in the duodenum.
Paneth cells are found throughout the small intestine and the appendix at the base of the intestinal glands. [2] There is an increase in Paneth cell numbers towards the end of the small intestine. [3] Like the other epithelial cell lineages in the small intestine, Paneth cells originate at the stem cell region near the bottom of the gland. [4]