[Biology Class Notes] on Small Intestine Pdf for Exam

The small intestine, the primary gastric organ in the human body, is a part of the digestive system. Most of the digestive activities occur in this organ. So, what is the small intestine can be answered as – it is a tubular element within the abdominal cavity which carries food from stomach to colon. The intestine includes the small intestine, the large intestine, and the rectum. Many of the main functions, including most of the absorption of water and nutrients, happen in the intestines. The small intestines are the location where 90% of the digestion and absorption actions occur, whereas the other 10% occur in the stomach and large intestines. 

Structure and Components 

The small intestine, also known as the small bowel, is a long, continuous tube that runs from the stomach to the anus. It is about 20 feet long and almost an inch in diameter. Absorption is helpful in the small intestine as it contains a large surface area on the inside. The task of the small intestines is to break down and disintegrate food from the stomach and absorb almost all of the nutrients from the food. 

There are three parts in order of the small intestines. The first part is named as the duodenum, the middle part is called the jejunum, and the last part is known as the ileum. The duodenum’s purpose is to complete the first phase of digestion. 

Roles of the Small Intestines 

Most of the digestion and absorption of food occurs in the small intestine, and the remaining takes place in stomach and large intestine.

The small intestines help in churning and mixing the consumed food, creating it into chyme, and moving it all along the length till the colon. It combines the ingested food with the mucus to make it easier to move and receive the digesting enzymes from the pancreas and the liver through the pancreatic and common ducts of bile. It breaks down food and water with the digestive enzymes into a thinner form which can be absorbed and then incorporate the nutrients to the tissues of the body, which in return get stronger. All the chemical digestion takes place in the small intestines, therefore making it one of the most significant parts of the digestive system. 

What Happens to Food in the Small Intestine?

The following account describes the process of digestion in the small intestine. 

 

The digestion of proteins and carbohydrates starts in the stomach and completes in the small intestine with the help of pancreatic and intestinal juice. However, lipids reach the intestine undigested. 

 

The combination of pancreatic juice and intestinal juice creates a liquid medium that helps in absorption. The digestive enzymes are secreted from the plasma membrane of microvilli.

 

Chemical digestion requires chyme and its mixing with pancreatic juice and bile. It helps in an upward adjustment of the low pH of chyme. 

Functions of the Small Intestines 

Since the small intestines have a lot of significance, it also has a lot of functions it needs to carry. 

1. Digestion is Majorly Done by the Small Intestines. 

The duodenum absorbs the half-digested food, along with other digestive juices from the liver, pancreas, and its walls. Bile juices that convert fat into easily absorbable liquid are secreted by the liver. The pancreas creates pancreatic juice that helps break down fats into fatty acids and glycerol. Furthermore, the walls of small intestines produce intestinal juice which breaks down starch and carbohydrates into a simple form of sugar, which is also known as glucose, and proteins into amino acids. 

2. Small intestines absorb the digested food. 

The blood vessels absorb the digested food into the intestines. This material is then carried to different parts of the body, providing strength. Glucose is broken down to transform oxygen and carbon dioxide, which are crucial components in respiration. 

Each component of the small intestines has different functions it needs to attend to. For example: 

The duodenum absorbs iron (Fe).

The jejunum absorbs most of the nutrients. 

The terminal or ending part of the ileum absorbs vitamin B12 and bile salts.

After all this, material that has not been broken down or disintegrated in the small intestines then passes on to the large intestine, starting another process all over again.