Small Intestine

The small intestine is a long tube-like structure or organ that joins the stomach and large intestine together. The oesophagus, large intestine, and stomach give rise to the gastrointestinal tract in humans. It measures about 6-7 metres in length. It is highly convoluted and is present in the central and lower region of the abdominal cavity. The walls of the small intestine are covered by villi (finger-like projections) which help in absorption and a surface area of about 200 metres is present. 

It also has three parts – duodenum, jejunum, and ileum. In the small intestine, the food which is being broken by the process of chewing and enzymes in the stomach is again catabolized with the help of different enzymes. 

The small intestine or small bowel is long, narrow, or entwined (coiled). It starts from the stomach (pylorus) and terminates at the large intestine (cecum). The length of the small intestine is about 22-25 feet long, a complex structure present in the abdominal part of humans. The main purpose is to complete the full digestion of food and help in the absorption of nutrients. 

There is the thin membrane known as mesentery which gives support to the small intestine. This mesentery has a fat layer that holds the heat in the organs and also the complex network of blood vessels. The nerves lead to the small intestine in two ways of the autonomic nervous system: one is the parasympathetic nerves which initiate the contractions of the muscles and help in moving the food down the tract by peristalsis; the other is the sympathetic nerves which suppress the movements of the intestine. The malfunctioning of the small intestine can lead to an uneasy state of health and can bring in trouble like diarrhoea.

Structure of the Small Intestine 

The anatomy of the small intestine is somewhat complex. It is divided into three parts, i.e., duodenum, jejunum, and ileum. They can extend up to six metres in length. These three parts are covered by the anterior greater omentum. The parts of the small intestine are described as follows. 

1. Duodenum – The initial part of the small intestine near the stomach is the duodenum. It measures about 23-28 cm. It has the widest diameter. This is a C shaped structure that extends from the pyloric sphincter of the stomach and is covered all around the pancreas and terminates at the duodenojejunal flexure. The peritoneal fold called the suspensory muscle (a type of ligament) helps in the attachment of this part to the posterior abdominal wall. The function of the duodenum is that the ducts from different regions (gallbladder, liver, and pancreas) enter here and produce juices that will neutralise the acids entering from the stomach and help in the digestion of food.  

Further, the duodenum can be divided into four parts:

• Superior (ampulla), 
• descending, 
• horizontal, and 
• ascending: 

2.Jejunum – it is the second part of the small intestine. It originates from the 

duodenojejunal flexure and is found in the abdomen (left quadrant). This is in the central part of the abdomen. It comprises two-fifth of the total length. Because of the extensive blood supply, it appears deep red. The peristalsis movement in this section is rapid and vigorous, and there is a small part of the fat in the mesentery which supports the region. There is no clear line separating the jejunum and ileum.

• The jejunum is thicker and its lumen is wider as compared to the ileum. 
• It has more prominent Kerckring folds (circular).

3.Ileum – The last and longest part. It is located in the abdomen (right quadrant). The terminal ileum can extend up to the pelvic cavity. It ends where the cecum of the large intestine originates. The walls are narrower and also thinner so the supply of blood is limited. The peristaltic movement is slower, and the mesentery has more fatty components. 

Functions of the Small Intestine

• Digestion – It is the part where most of the chemical digestion occurs. The enzymes secreted here by pancreatic ducts are mostly from the pancreas and liver. After the process of chewing, the food is degraded with the activity of more enzymes here. 
• Absorption – The small intestine has finger-like projections known as villi which increase the surface area and help in the proper digestion of food.
• The small intestine also plays a role in the immune system of the body. The gut flora helps in this. 

Conclusion

The small intestine is a tube-like structure arising from the stomach to the large intestine. Its function is to digest the food and absorb the nutrients. The small intestine is found in the gastrointestinal tract. It is about 6.5 m. It is extended from the pylorus of the stomach and ends at the ileocaecal junction. It has three segments – the duodenum, jejunum, and ileum. The duodenum is wrapped around the pancreas and is further divided into four parts. The second part is the jejunum, which arises from the duodenojejunal flexure. And the last segment is the ileum, which is located in the lower right quadrant and has thin intestinal walls.