What Is the Small Intestine?

The small intestine is located in the abdomen, the upper end is connected to the pylorus and the stomach, and the lower end is connected to the large intestine through the appendix, which is the main place for food digestion and absorption. The small intestine is convoluted in the abdominal cavity, the upper part is connected to the stomach pylorus, and the lower part is the cecum. The total length is about 4-6 meters, and it is divided into the duodenum, jejunum and ileum. The small intestine digestion is very important because the food is basically digested after the chemical digestion of pancreatic juice, bile and small intestinal fluid in the small intestine and mechanical digestion of small intestinal movement, and the nutrients are absorbed by the small intestinal mucosa.

The small intestine is located in the abdomen, the upper end is connected to the pylorus and the stomach, and the lower end is connected to the large intestine through the appendix, which is the main place for food digestion and absorption. The small intestine is convoluted in the abdominal cavity, the upper part is connected to the stomach pylorus, and the lower part is the cecum. The total length is about 4-6 meters, and it is divided into the duodenum, jejunum and ileum. The small intestine digestion is very important because the food is basically digested after the chemical digestion of pancreatic juice, bile and small intestinal fluid in the small intestine and mechanical digestion of small intestinal movement, and the nutrients are absorbed by the small intestinal mucosa.

Chinese name

Small intestine

Foreign name

small intestine

Segment

Duodenum, jejunum and ileum

Location

Posterior upper abdominal cavity

Small intestine anatomy

The upper end of the small intestine starts from the gastric pylorus, and the lower end ends in the ileocecal valve. It is the longest part of the digestive tract. It is 5-7m long in adults. It is divided into three parts: duodenum, jejunum, and ileum. It is the main place for food digestion and absorption. The diameter of the small intestine tapers down from the duodenum (about 3-5cm), and the lumen of the terminal ileum is only 1.0-1.2cm. Foreign objects are easily incarcerated here.

Small intestine physiological function

The histological characteristics of the small intestine create good conditions for the absorption of the small intestine. The physiological function of the small intestine shows the movement, secretion, digestion and absorption of the hairy small intestine, and is closely related to drug metabolism. For example, various forms of movement of the small intestine smooth muscle can complete the mechanical digestion of the chyme by grinding, mixing, and stirring. The small intestinal fluid secreted by the small intestine gland completes the chemical digestion of the chyme with the small intestine bile and pancreatic juice. The small intestinal mucosa is secreted and dispersed. There are many endocrine cells that can secrete a variety of gastrointestinal hormones, such as secretin, cholecystokinin, gastric inhibitory peptide, and motilin, etc., which have important regulatory effects on gastrointestinal motility and secretion.

Intestinal nutrition and metabolism

Chyme enters the small intestine from the stomach and begins the digestion of the small intestine. Due to the chemical digestion of pancreatic juice, small intestinal fluid and bile and the mechanical digestion of small intestine movement, the digestion of food is basically completed in the small intestine, and the digested nutrients are also large. Part of it is absorbed in the small intestine, so the small intestine is the most important site for digestion and absorption.

Small intestinal absorption cell

Gobletcells: scattered between absorbing cells, secreting mucus, having lubricating and protective effects, goblet cells gradually increase from the duodenum to the end of the ileum.

Panethcell: It is a characteristic cell of the small intestine gland. It is located at the base of the gland and is often clustered in groups of three or five. The cells are cone-shaped, the top of the cytoplasm is filled with coarse eosinophilic particles, and contains lysozyme, which has a certain sterilizing effect.

Undifferentiated cells: located in the lower half of the small intestine gland, scattered between other cells. The cell body is small, columnar, and the cytoplasm is basophilic. Cells continue to proliferate, differentiate, and migrate upward to supplement the absorptive cells and goblet cells that shed at the top of the villi. The renewal cycle of villous epithelium is 2 to 4 days. It is generally believed that endocrine cells and Paneth cells are also derived from undifferentiated cells.

In addition to a large number of small intestinal glands in the dense connective tissue of the lamina propria, there are abundant migratory cells such as lymphocytes, plasma cells, macrophages, and eosinophils. There are 1-2 longitudinal capillary lymph vessels in the connective tissue of the villous central axis, which is called central lacteal. It starts at the blind end and passes down through the mucosal muscles into the submucosa to form the lymphatic plexus. . The central chylotube has a large lumen, wide intercellular endothelial cells, and no basement membrane, so it has high permeability. The chylomicrons released by the cells are absorbed into the central chyle duct for output. This tube is surrounded by a rich network of perforated capillaries. Water-soluble substances such as amino acids and monosaccharides absorbed by the intestinal epithelium mainly enter the blood through this. There are also a small amount of smooth muscle fibers from the mucosa in the villi, which can shrink the villi and facilitate the absorption of substances and the operation of lymph and blood.

In addition to a large number of scattered lymphocytes in the lamina propria, there are also lymph nodules. In the duodenum and jejunum, there are mostly solitary lymph nodules, and in the ileum, several lymph nodes are aggregated. They can pass through the mucosal muscles to reach the submucosa.