Why is the small intestine said to be the main site of digestion and absorption?

The food that the human body eats every day, after being chewed by the teeth, will enter the gastrointestinal tract. After being digested in the stomach, the small intestine will further absorb and utilize some substances that are beneficial to the human body, and then excrete the remaining waste through the large intestine. Therefore, the small intestine is the main part of digestion and absorption. Below, I will introduce some of the roles of the small intestine in the digestion and absorption of food.

After digestion in the small intestine, food has been broken down into small molecules that can be absorbed. Food stays in the small intestine for a long time, generally 3 to 8 hours, which provides sufficient time for absorption. The small intestine is the longest part of the digestive tract. The human small intestine is about 4 meters long. The small intestinal mucosa forms many annular folds and a large number of villi protruding into the intestinal cavity. The surface of each villus is a layer of columnar epithelial cells, and the cell membrane at the top of the columnar epithelial cells forms many tiny protrusions, called microvilli. The presence of annular folds, villi and microvilli increases the surface area of ​​the small intestinal mucosa by 600 times to about 200m2. This gives the small intestine a large absorption area.

The small intestinal mucosa, especially the jejunum, has many annular folds and villi, which greatly expand the surface area of ​​the mucosa and facilitate the digestion and absorption of nutrients. In the submucosal layer there are intestinal glands formed by the depression of the surface epithelium, which open on the surface of the mucosa and secrete intestinal fluid. Pancreatic juice and intestinal juice contain a variety of digestive enzymes that break down proteins, sugars and fats. Bile aids in the digestion and absorption of fats. Proteins, sugars and fats must be broken down into substances with simple structures before they can enter the blood and lymph through the columnar epithelial cells of the intestinal villi, and can also enter the capillaries and lymphatic capillaries through the gaps between epithelial cells.

The small intestine is a site for nutrient absorption, not a digestive organ. The reasons why the small intestine is the absorption site are: 1. The absorption area of ​​the small intestine is large; 2. Food stays in the small intestine for a long time, which is conducive to full absorption by the small intestine; 3. Food has been digested into absorbable components in the small intestine; 4. The small intestine contains capillaries and lymphatic capillaries, which are conducive to absorption; 5. The small intestine has endocrine functions and can mix with many endocrine substances to facilitate absorption.

The main function of the small intestine is to absorb nutrients. Therefore, if certain diseases occur in the small intestine, it will affect the absorption and utilization of nutrients. If this continues for a long time, it may cause malnutrition or the body to lack certain nutrients and cause some diseases. Therefore, we must protect the small intestine at ordinary times.