Parts of Small Intestine

The main function of the digestive system is to breakdown and assimilate food. The digestive system starts right from the mouth and includes the esophagus, stomach, small intestine and large intestine, all of which are organs of the digestive system. The words 'small intestine', is a misnomer as this is the longest part of the digestive tract, measuring about five meters. However, this part of the digestive tract is called small intestine because of the small diameter of the intestine, which comes in at about 2.5 - 3 centimeters. There are a total of three parts of small intestine, which are detailed below.

3 Parts of Small Intestine

Duodenum
The duodenum is the first part of the small intestine and it is seen in all mammals. This is a curved and the smallest portion of the small intestine which marks its starting point, when it exits the stomach. This small twelve inch long portion of the small intestine connects the stomach to the next part of the small intestine, the jejunum. However, not to be fooled by its size, this portion of the small intestine 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 the intestine, which is what happens in small intestine problems. Cholecystokinin is secreted from the duodenum and this has the function of hydrolyzing and digesting fats.

Jejunum
The jejunum comes in second in order. The passage that takes place between the duodenum and jejunum is known as the ligament of Treitz. The jejunum is not only second in order but is also second in length when talking of the small intestine, measuring around 8.2 feet. It has a pH that ranges between seven and nine. This pH is absolutely essential to carry out the function of small intestine because it is only in a slightly alkaline pH that enzymes can work properly in this part of the gastrointestinal tract. When there is a small intestine blockage, then food does not manage to pass from the jejunum to the ileum, which leads to pain and bloating in the intestine.

Ileum
The ileum (not to be confused with ilium, which is a part of the pelvic bone) is the longest and the lowest part of small intestine, coming in at eleven and a half feet. This part executes the small intestine function of absorption of vitamin B12, deficiency of which can lead to megaloblastic anemia. The ileum is slightly paler in color 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 the small intestine is slightly different in all three parts, when one speaks of the microscopic structure, he is usually referring to the ileum. The internal lining of the small intestine 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, then it would approximately be large enough to cover an entire tennis court! This is the main point where the structure of small intestine differs from the large intestine anatomy and histology. When there are small intestine diseases, then the microvilli do not manage to absorb as much food, which is what leads to indigestion and malnutrition.

The small intestine parts may vary in their names and structure but their function of absorbing food is essentially the same. The different human body systems are really interesting to study, and among them, small intestine anatomy and parts surely tops the list!