The main function of the digestive system is to breakdown, and assimilate food. The digestive system starts from the mouth, and includes the esophagus, stomach, small and large intestines. The words 'small intestine', is a misnomer as this is the longest part of the digestive tract, measuring about five meters. It gets its name because of its small diameter, which comes in at about 2.5 - 3 centimeters.
Parts of the Small Intestine
The duodenum is the first part of this organ, and it is seen in all mammals. This is the smallest portion of the intestine, and is curved, which marks its starting point, as it exits the stomach. This small 12 inch-long portion connects the stomach to the jejunum. However, not to be fooled by its size, this portion has the important function of chemical digestion. Thus, this is the region where there is breakdown of food with the help of enzymes. The duodenum contains tubular submucosal glands known as Brunner's glands. These glands are responsible for secreting mucus, which helps to lubricate the lining of the small intestine, and prevents the enzymes from acting on the cells of this organ. Cholecystokinin is secreted from the duodenum and this has the function of hydrolyzing and digesting fats.
The jejunum comes in second in order. The passage between the duodenum and jejunum is known as the ligament of Treitz. This part is second, both in order, and length, measuring around 8.2 feet. It has a pH that ranges between 7 and 9. This is absolutely essential to carry out the function of this organ because it is only in a slightly alkaline pH that enzymes can work properly in this part of the gastrointestinal tract. When there is any blockage here, food does not manage to pass from the jejunum to the ileum, which leads to pain and bloating of the intestine.
The ileum (not to be confused with ilium, which is a part of the pelvic bone) is the longest, and the lowest part of this organ, coming in at 11'6". This part executes the function of absorption of vitamin B12, deficiency of which can lead to megaloblastic anemia. The ileum is slightly pale when compared to the jejunum, and mostly absorbs fatty acids and glycerol, besides glucose and amino acids.
Microscopic Parts of Small Intestine
The membrane that connects all parts of the small intestine is known as the mesentery. This membrane is richly supplied by blood vessels in the form of small capillaries which help in absorption of food. Although the microscopic description of the internal lining of this organ is slightly different in all three parts, when one speaks of the microscopic structure, one is usually referring to the ileum. The internal lining of this organ in the ileum consists of glandular epithelium which is present in the form of highly convoluted and folded microscopic structures known as microvilli. These are present so as to increase the surface area for absorption of food. This increases the surface area to such an extent that if the inner surface of the intestine were to be completely uncoiled, it would approximately be large enough to cover an entire tennis court! This is perhaps the main structural difference between the two intestines. When there are intestinal diseases, the microvilli do not manage to absorb as much food, which leads to indigestion, and malnutrition.
The parts of this vial organ may vary in their names and structure but their function of absorbing food is essentially the same.