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The jejunum and ileum are the middle and end portions of the small intestine of birds, reptiles and mammals, including humans. Both portions are in direct line with each other along the small intestine tract and share similar qualities, but do perform individual processes of digestion. There is no truly distinct terminal point where the jejunum and ileum connect, but a slight color differentiation might be apparent between the two sections.
Composing the mid-section of the small intestine is the jejunum, which is approximately 2 to 3 meters (about 6 to 10 feet) in length. In contrast the ileum composes about 2 to 4 meters (about 6 to 13 feet) of the small intestine and terminates into the cecum of the large intestine. Both the jejunum and ileum have an interior lining of mucous membrane and villi which are finger like projections that function to absorb nutrients. The overall potential Hydrogen (pH) of the jejunum and ileum are very similar with neutral to slightly alkaline values between seven and nine.
A primary function of the jejunum and ileum is the absorption of nutrients. Amino acids, water, vitamins and minerals are all absorbed into the bloodstream by the jejunum through the villi, which are small projections from the intestine wall. Sugars and fatty acids are other nutrients also transported to the bloodstream utilizing villi in the jejunum. The ileum utilizes the villi for absorption of nutrients into the bloodstream also, but mainly serves to transport vitamin B12 and various other elements that may have been missed by the jejunum.
Villi in the jejunum are longer than the villi located in the ileum. Two types of absorption methods are utilized by the villi in the jejunum and ileum, passive and active transport of nutrients. Fructose sugars are typically part of passive transport while other nutrients, water and vitamins participate in active transport to the bloodstream.
A membrane called mesentery attached to the outer portion of the jejunum and ileum hold the small intestine in place in the gut region. Smooth muscle encases the exterior of both regions to help move food down the intestinal tract through a series of minute contractions. On careful inspection the ileum might appear slightly paler in color than the jejunum, allowing the observer to visually differentiate between the two sections of intestine.
Some other differences also occur between the jejunum and ileum. The mesentery of the ileum contains more fatty deposits than what is typically seen in the mesentery of the jejunum. There are also a higher abundance of specialized lymphoid nodules in the mesentery of the ileum called Peyer’s patches. These help with the body's immunity by alerting the body to potentially illness-causing microbes. In contrast, the jejunum has relatively few of these lymphoid nodules and a larger circumference than what is seen in the ileum.