In our previous post we talked about large intestine and its great importance as one of the major segments of the digestive system. Now it is time to deal with one organ similar in function and importance – small intestine. Also, keep in mind following: just like the large intestine is called the large bowel, we call small intestine the small bowel.
The small intestine represents long and highly convoluted tube. It is part of the digestive system. Its function is absorption of about 90% of the nutrients from the food. It processes around 2 gallons of food, liquids, and digestive secretions every day.
The small intestine is made up of four layers of tissue, just like the rest of the gastrointestinal tract. These four layers are: the mucosa, the submucosa layer, the muscularis layer and the serosa. The mucosa forms the inner layer of epithelial tissue. It is specialized in absorption of nutrients from chyme. The submucosa layer provides support to the mucosa on the surface by blood vessels, lymphatic vessels, and nerves. The muscularis layer is made of several layers of smooth muscle tissue. It helps in contraction and movements of the small intestines. Lastly, the serosa, which forms the outermost layer of epithelial tissue, surrounds the intestines.
As it is a complex organ, there is a large number of possible conditions that may affect the function of the small intestine. Some are common, with almost 10% of people being affected at some point in life, while others are extremely rare.
Structure of the small intestine
The average length of the small intestine in an adult male is 6.9 m and in an adult female 7.1 m. It can vary greatly from 4.6 m to 9.8 m. This means that small intestine is almost twice the length of the large intestine. However, it is approximately 2,5-3 cm in diameter – less than half the diameter of large intestine.
The small intestine is divided into three different parts:
Duodenum is the first section of the small intestine. It represents a hollow jointed tube about 25–38 cm long that connects the stomach to the jejunum. When it comes to duodenum function, it is responsible for the breakdown of food with help of the enzymes.
Jejunum is the middle section of the small intestine – lies between the duodenum and the ileum. It measures around 1 meter in length. Function of the lining of the jejunum is the absorption of small nutrient particles which have been previously digested by enzymes in the duodenum.
Ileum is the final section of the small intestine. It is about 2–4 m long. The function of the ileum is absorption of the vitamin B12, bile salts and other products of digestion that were not absorbed by the jejunum.
Functions of the small intestine
Digestion: The small intestine is where most digestion takes place. The pancreas secretes most of the digestive enzymes that act in the small intestine. They enter the small intestine through the pancreatic duct in response to the hormone cholecystokinin. Also, the hormone secretin causes bicarbonate release into the small intestine from the pancreas, all in order to neutralize acid coming from the stomach, which could possibly be harmful.
Absorption: Digested food is after that able to pass into the blood vessels of the wall of the intestine. Most of the nutrients from ingested food are absorbed in the small intestine. To be more precise, absorption of the nutrients happens in the jejunum. Of course, there are some exceptions:
- Iron is absorbed in the duodenum,
- Vitamin B12 and bile salts are absorbed in the ileum,
- Water and lipids are absorbed throughout the small intestine by passive diffusion,
- Sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport,
- Fructose is absorbed by facilitated diffusion.
Immunological: Another important role of the small intestine is support of the body’s immune system. The presence of probiotic flora contributes to the host’s immune system.