The small intestine (intestinum tenue) is the part of the digestive tract located between the stomach and the large intestine. The small intestine together with the thick intestine forms the longest part of the digestive system. In the small intestine distinguish the duodenum, skinny, ileum. In the small intestine, the chyme (food gruel), processed with saliva and gastric juice, is exposed to the action of intestinal and pancreatic juice, as well as bile. In the lumen of the small intestine, with the mixing of chyme, its final digestion and absorption of the products of its cleavage take place. The remains of food move into the large intestine. The endocrine function of the small intestine is important. Endocrinocytes of its integumentary epithelium and glands produce biologically active substances (secretin, serotonin, motilin, etc.).
The small intestine begins at the border of the bodies of the XII thoracic and I lumbar vertebrae, ends in the right ileal fossa, is located in the region of the belly (middle abdomen), reaching the entrance to the small pelvis. The length of the small intestine in an adult is 5-6 m. In men, the gut is longer than in females, while in a human, the small intestine is shorter than a corpse that lacks muscle tone. The length of the duodenum is 25-30 cm; about 2/3 the length of the small intestine (2-2.5 m) is thin and about 2.5-3.5 m - the ileum. The diameter of the small intestine is 3-5 cm, it decreases in the direction of the large intestine. The duodenum has no mesentery, unlike the lean and ileum, which is called the mesenteric part of the small intestine.
The jejunum and ileum form the mesenteric part of the small intestine. Most of them are located in the umbilical region, forming 14-16 loops. A part of the loops descends into the small pelvis. The loins of the jejunum mainly lie in the left upper, and the ileum - in the right lower part of the abdominal cavity. Strict anatomical boundary between the jejunum and ileum is absent. Anterior to the intestinal loops is a large omentum, behind - the parietal peritoneum, lining the right and left mesenteric sinuses. The lean and ileum intestines are connected with the back wall of the abdominal cavity with the help of a mesentery. The mesentery root ends in the right ileal fossa.
The walls of the small intestine are formed by the following layers: the mucosa with the submucosa, the muscular and outer membranes.
The mucosa (tunica mucosa) of the small intestine has circular (kerkringovy) folds (plicae circularis). Their total number reaches 600-700. The folds are formed with the participation of the submucous base of the intestine, their dimensions decrease towards the colon. The average height of folds is 8 mm. The presence of folds increases the surface area of the mucous membrane by more than 3 times. In addition to circular folds, the duodenum is characterized by longitudinal folds. They are present in the upper and downward parts of the duodenum. The most pronounced longitudinal crease is located on the medial wall of the descending part. In its lower part there is an elevation of the mucous membrane - a large papilla of the duodenum (papilla duodeni major), or pectoral papillae. Here the common bile opens the common bile duct and the pancreatic duct. Above this papilla on the longitudinal fold there is a small papilla of the duodenum (papilla duodeni minor), where an additional duct of the pancreas opens.
The mucosa of the small intestine has numerous outgrowths - intestinal villi (villi intestinales), there are about 4-5 million. On the area of 1 mm 2 of the mucous membrane of the duodenum and jejunum there are 22-40 villi, the ileum - 18-31 villi. The average length of the villi is 0.7 mm. The size of the villi decreases towards the ileum. Isolate leaf, lingual, finger-like villi. The first two species are always oriented across the axis of the intestinal tube. The longest villi (about 1 mm) are mostly leaf-shaped. At the beginning of the jejunum villi usually have the shape of a tongue. The distal form of villi becomes finger-shaped, their length decreases to 0.5 mm. The distance between the villi is 1-3 μm. The nares are formed by a loose connective tissue covered with epithelium. In the thick of the villi are many smooth myoites, reticular fibers, lymphocytes, plasma cells, eosinophils. In the center of the villi is a lymphatic capillary (milky sinus) around which blood vessels (capillaries) are located.
From the surface, intestinal villi are covered with a single-layered high cylindrical epithelium located on the basal membrane. The majority of epithelial cells (about 90%) are columnar epithelial cells with a striated brush border. The border is formed by microvilli of the apical plasma membrane. On the surface of the microvilli is a glycocalyx, represented by lipoproteins and glycosaminoglycans. The main function of columnar epithelial cells is absorption. The composition of the integument epithelium includes many goblet cells - unicellular glands secreting mucus. On average, 0.5% of the cells of the cover epithelium are in the endocrine cells. In the thickness of the epithelium there are also lymphocytes penetrating from the stroma of the villi through the basal membrane.
In the lumens between the villi, the intestinal glands (glandulae intestinales) or crypts open onto the surface of the epithelium of the entire small intestine. In the duodenum there are also mucous duodenal (Brunner's) glands of complex tubular form located mainly in the submucosa, where they form lobules 0.5-1 mm in size. The intestinal (liberic) glands of the small intestine have a simple tubular shape, they occupy a place in their own plate of the mucous membrane. The length of the tubular glands is 0.25-0.5 mm, the diameter is 0.07 mm. On the area of 1 mm 2 of the mucosa of the small intestine there are 80-100 intestinal glands, their walls are formed by a single layer of epitheliocytes. In total, there are more than 150 million glands in the small intestine (crypt). Epithelial gland cells distinguish columnar epitheliocytes with striated edges, goblet cells, intestinal endocrinocytes, beskamchatchatye cylindrical (stem) cells and Panet cells. Stem cells are a source of regeneration of the intestinal epithelium. Endocrinocytes produce serotonin, cholecystokinin, secretin, etc. Panet cells secrete erepsin.
For the own plate of the mucous membrane of the small intestine is characterized by a large number of reticular fibers, forming a dense network. In its own plate there are always lymphocytes, plasma cells, eosinophils, a large number of single lymphoid nodules (in children - 3-5 thousand).
In the mesenteric part of the small intestine, especially in the iliac, there are 40-80 lymphoid, or peyer, plaques (noduli lymfoidei aggregati), which are clusters of single lymphoid nodules that are organs of the immune system. Plaques are located mainly on the anti-brazier margin of the intestine, they have an oval shape.
The muscular lamina of the mucous membrane (lamina muscularis mucosae) has a thickness of up to 40 μm. It has an internal circular and an outer longitudinal layer. From the muscular plate into the thickness of its own plate of the mucous membrane and into the submucosa base separate smooth myocytes separate.
The submucosa (tela submucosa) of the small intestine is formed by a loose fibrous connective tissue. In its thickness there are branches of the blood and lymph vessels and nerves, various cellular elements. 6 submucosal base of the duodenum are the secretory sections of the duodenal (Bruner's) glands.
The muscular membrane (tunica muscularis) of the small intestine consists of two layers. The inner layer (circular) of the thickness of the outer (longitudinal) layer. The direction of the bundles of myocytes is not strictly circular or longitudinal, but has a spiral course. In the outer layer, the coils of the spiral are more stretched than the inner layer. Between the muscle layers in the loose connective tissue are the nerve plexus and vessels.
The serosa (tunica serosa) is located on a subserous base. It covers the small intestine from all sides, except for the duodenum, covered with the peritoneum only partially (in front), and in the remaining parts - by adventitia.
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