The structure of the small intestinal mucosa. Sections of the small intestine: description, structure and functions

Chinese sages said that if a person has a healthy intestine, he can overcome any disease. Delving into the work of this organ, you never cease to be amazed at how complex it is, how many degrees of protection are built into it. And how easy it is, knowing the basic principles of its work, to help the intestines maintain our health. I hope that this article, written on the basis of the latest medical research by Russian and foreign scientists, will help you understand how the small intestine works and what functions it performs.

The intestine is the longest organ of the digestive system and consists of two sections. The small intestine, or small intestine, forms a large number of loops and continues into the large intestine. The human small intestine is approximately 2.6 meters long and is a long, tapering tube. Its diameter decreases from 3-4 cm at the beginning to 2-2.5 cm at the end.

At the junction of the small and large intestines there is an ileocecal valve with a muscular sphincter. It closes the exit from the small intestine and prevents the contents of the large intestine from entering the small intestine. From 4-5 kg ​​of food gruel passing through the small intestine, 200 grams of feces are formed.

The anatomy of the small intestine has a number of features in accordance with its functions. So the inner surface consists of many semicircular folds
forms. Thanks to this, its suction surface increases 3 times.

In the upper part of the small intestine, the folds are higher and located closely to each other; as they move away from the stomach, their height decreases. They can completely
absent in the area of ​​transition to the large intestine.

Sections of the small intestine

The small intestine has 3 sections:

• jejunum
• ileum.

The initial section of the small intestine is the duodenum.
It distinguishes between upper, descending, horizontal and ascending parts. The small intestine and ileum do not have a clear boundary between themselves.

The beginning and end of the small intestine are attached to the posterior wall of the abdominal cavity. On
throughout the rest of its length it is fixed by the mesentery. The mesentery of the small intestine is the part of the peritoneum that contains blood vessels, lymphatic vessels, and nerves and allows intestinal motility.

Blood supply

The abdominal part of the aorta is divided into 3 branches, two mesenteric arteries and the celiac trunk, through which blood is supplied to the gastrointestinal tract and abdominal organs. The ends of the mesenteric arteries narrow as they move away from the mesenteric edge of the intestine. Therefore, the blood supply to the free edge of the small intestine is much worse than the mesenteric one.

The venous capillaries of the intestinal villi unite into venules, then into small veins and into the superior and inferior mesenteric veins, which enter the portal vein. Venous blood first flows through the portal vein into the liver and only then into the inferior vena cava.

Lymphatic vessels

The lymphatic vessels of the small intestine begin in the villi of the mucous membrane; upon leaving the wall of the small intestine they enter the mesentery. In the mesenteric area, they form transport vessels that are capable of contracting and pumping lymph. The vessels contain a white liquid similar to milk. That's why they are called milky. At the root of the mesentery are the central lymph nodes.

Some lymphatic vessels may empty into the thoracic stream, bypassing the lymph nodes. This explains the possibility of rapid spread of toxins and microbes through the lymphatic route.

Mucous membrane

The mucous membrane of the small intestine is lined with single-layer prismatic epithelium.

Epithelial renewal occurs in different parts of the small intestine within 3-6 days.

The cavity of the small intestine is lined with villi and microvilli. Microvilli form the so-called brush border, which provides the protective function of the small intestine. Like a sieve, it sifts out high-molecular toxic substances and does not allow them to penetrate the blood supply and lymphatic system.

Nutrients are absorbed through the epithelium of the small intestine. Through the blood capillaries located in the centers of the villi, water, carbohydrates and amino acids are absorbed. Fats are absorbed by lymphatic capillaries.

The formation of mucus lining the intestinal cavity also occurs in the small intestine. It has been proven that mucus performs a protective function and helps regulate intestinal microflora.

Functions

The small intestine performs the most important functions for the body, such as

• digestion
• immune function
• endocrine function
• barrier function.

Digestion

It is in the small intestine that the processes of food digestion occur most intensively. In humans, the digestion process practically ends in the small intestine. In response to mechanical and chemical irritations, the intestinal glands secrete up to 2.5 liters of intestinal juice per day. Intestinal juice is secreted only in those parts of the intestine in which the food lump is located. It contains 22 digestive enzymes. The environment in the small intestine is close to neutral.

Fright, angry emotions, fear and severe pain can slow down the functioning of the digestive glands.

Rare diseases - eosinophilic enteritis, common variable hypogammaglobulinemia, lymphangiectasia, tuberculosis, amyloidosis, malrotation, endocrine enteropathy, carcinoid, mesenteric ischemia, lymphoma.

The structure of the wall of the small intestine is similar in all sections. It consists of the mucous membrane, submucosa, muscular and serous membranes.

Mucous membrane small intestine is characterized by characteristic relief, formed by the presence of a number of anatomical formations: circular folds, villi and intestinal glands or crypts. Thanks to these structures, the total surface area, including the absorption surface, increases, which facilitates the performance of basic biological functions by the thin section intestines :

circular folds (lat. plicae circulares) formed by the mucous membrane and submucosa of the small intestine;

intestinal villi (lat. villi intestinales) are formed by finger-shaped or leaf-shaped protrusions of the mucous membrane, freely protruding into the lumen of the small intestine. The number of villi in the small intestine is very significant: the largest number is in the duodenum and jejunum - there are from 22 to 40 villi per square millimeter of the mucous membrane. There are slightly fewer of them in the ileum - from 18 to 31 villi per square millimeter;

intestinal glands or crypts (lat. glandulae seu cryptae intestinales) are represented by tubular depressions located in the lamina propria of the mucous membrane, and their mouth open into the lumen of the small intestine between the intestinal villi. Moreover, there are up to 100 crypts per square millimeter of the surface of the mucous membrane of the small intestine, their total number exceeds 150 million intestinal glands throughout, and the total area of ​​crypts in the small intestine reaches 14 m2.

Submucosa often contains lobules adipose tissue, it contains vessels (arterial, venous, lymphatic) and submucosal nerve plexus .

Muscularis the small intestine consists of two layers muscle cells: more powerful internal (or circular) and less developed external (or longitudinal). Moreover, the direction of movement of the muscle fiber bundles in both layers is not strictly longitudinal or circular, but spiral, and the spiral curls in the outer layer are more stretched compared to the inner layer. Between the layers of the muscular lining of the small intestine there is a layer of loose fibrous connective tissue, which contains nodes of the myenteric nerve plexus and vessels. The biological significance (main function) of the muscular lining of the small intestine is to mix and push chyme along the intestine in the caudal direction. In this case, two types of muscle contractions are distinguished: contractions of a local nature, performed rhythmically with a frequency of 12-13 times per minute, caused mainly by contractions of the inner layer of the muscle membrane, and others ( peristaltic ) contractions caused by the action of the muscular elements of both layers and spreading sequentially along the entire length of the small intestine. The regulation of muscle contractions is carried out by fibers of the myenteric nerve plexus ( lat. plexus myenteriens): increased peristalsis is observed when the sympathetic nerves are excited, and weakened when excited vagus nerve .

Serosa covers the small intestine from the outside and on all sides (with the exception of the duodenum, which is covered with peritoneum only in front, and otherwise has only a connective tissue membrane), forming mesentery .

Structure of the duodenum The duodenum (duodenum) is the initial section of the small intestine, immediately following the pylorus (pylorus of the stomach). Then this section of the intestine goes from left to right and somewhat posteriorly, turns down, descends along the anterior surface of the right kidney, turns to the left and, rising obliquely upward, passes into the jejunum. The name of this section of the intestine is associated with its length, which is exactly twelve diameters of the fingers. The anatomy of the duodenum is closely related to the biliary system, as well as the pancreas. On the inner surface of the descending duodenum is the papilla of Vater (or major duodenal papilla). Here, the common bile duct, the pancreatic duct, opens through the sphincter of Oddi (in some people, the pancreatic duct may directly empty into the common bile duct). The small duodenal papilla is located 8-40 mm above the major duodenal papilla. An additional pancreatic duct opens through it. This structure is anatomically variable. The histological structure of the mucous membrane of the duodenum ensures the resistance of its epithelium to the aggressive composition of gastric juice, bile, and pancreatic enzymes. Functions of the duodenum One One of the main functions of the duodenum is to bring the pH of food gruel coming from the stomach to alkaline, which will not irritate the distal parts of the intestine and is suitable for carrying out processes of parietal digestion. It is in this section of the intestine that the processes of intestinal digestion begin. Second An important function of the duodenum is the initiation and regulation of the enzymatic activity of the pancreas and liver, which depends on the chemical composition and acidity of the incoming food gruel. Third The function of the duodenum is the regulation of the reflex opening and closing of the pylorus, depending on the acidity and chemical composition of the contents of this section of the intestine, as well as the regulation of the acidity of gastric juice due to the secretion of humoral factors that ensure the secretory activity of the stomach.

79.features of the structure of the wall of the colon. consists of the cecum, colon and rectum. Absorption of nutrients and water ends there, and feces are formed.

The structure of the wall of the large intestine

Mucous membrane

The mucous membrane, unlike the mucosa of the small intestines, is devoid of circular folds and villi, and the lymphoid tissue forms only single follicles in it. However, the intestinal crypts are deeper, and among the cells of the single-layer columnar epithelium there are a lot of goblet cells, the number of which increases towards the rectum (see Atl.). Therefore, a lot of mucus, devoid of enzymes, is secreted in the large intestines, which facilitates the passage of undigested food debris. The surface of the cells of the integumentary epithelium, as in the small intestine, is covered with microvilli. In addition, enteroendocrine cells are found in the epithelium. Migration of cells from the depths of the crypts to the surface of the epithelium occurs in the same way as in the small intestine.

The part of the rectum adjacent to the anus (anorectum) is devoid of crypts and covered with stratified squamous epithelium. It smoothly passes into the epidermis of the skin. The mucous membrane of the anorectal canal forms longitudinal folds or columns. In this area, the muscular plate of the mucosa gradually disappears. The venous plexus is well developed here. When these small convoluted veins expand, the mucous membrane protrudes into the intestinal lumen, causing a disease - hemorrhoids.

Muscularis

The muscular layer consists of two layers - internal (circular) and external (longitudinal), which is developed unevenly. Most muscle cells are concentrated in three narrow muscle bands(see Atl.). The sections of the intestine between the ribbons form protrusions - haustrae, separated by transverse grooves, which on the inside correspond to semilunar folds. The latter are formed by all the membranes of the wall, and not just the mucous membrane, as in the small intestine

In the rectum, the longitudinal muscle layer is located evenly throughout the entire wall, and there are no bands or protrusions. Circular muscle cells in the anal canal form internal sphincter.

Serosa

The serous membrane covers the cecum, transverse colon and upper part of the rectum on all sides, and the ascending and descending colon on three sides. Sometimes the serous membrane moves away from the surface of the intestine, forming outgrowths filled with adipose tissue.

The small intestine (intestinum tenue) begins from the pylorus of the stomach. This is the longest part of the digestive tube, reaching 5-6 m. The small intestine is divided into three parts: duodenum (duodenum), jejunum (intestinum jejunum) and ileum (intestinum ileum). The wall of the small intestine consists of three membranes. The outer layer is either adventitial or serous. The middle layer is smooth muscle and consists of an outer longitudinal and inner circular layer, the muscle fibers of which are evenly distributed. The inner lining - the mucosa - forms numerous circular folds throughout almost the entire length of the small intestine, which are permanent. In the upper parts of the intestine, these folds are highest, and as they approach the colon they become lower. The surface of the mucosa has a velvety appearance, which depends on the many outgrowths, or villi. In some parts of the intestine they have a cylindrical shape, in others (for example, in the duodenum) they rather resemble a flattened cone. Their height ranges from 0.5 to 1.5 mm. The number of villi is very large: in an adult there are up to 4 million of them. A huge number of villi increases the surface of the small intestine by 24 times, which is important for the process of absorption of nutrients. Villi are protrusions of the epithelium and the lamina propria, which forms their skeleton. In the center of the villus there is a lymphatic vessel, on the sides of which smooth muscle cells lie in small bundles. The villus includes an artery that breaks up into capillaries, which are located under the epithelium in the form of a network. The capillaries, gathering into one stem, form a vein. Thanks to the presence of muscle cells, the villi can contract. At the height of suction, 4-6 contractions of the villi occur per minute, which helps the circulation of lymph and blood in the vessels, which quickly fill during the period of vigorous absorption of food. Fats are transported into the body through lymphatic vessels, and proteins and carbohydrates through blood vessels. In addition to villi, there are protrusions on the surface of the mucosa, or, as they are called, crypts. They protrude into the lamina propria and resemble tubular glands. The glandular epithelium of the crypts secretes intestinal juice. Crypts serve as a site for reproduction and restoration of the intestinal epithelium. The surface of the mucous membrane of the small intestine, i.e., villi and crypts, is covered with a single-layer cylindrical bordered epithelium. The bordered, or intestinal, epithelium bears a border, or cuticle, on its surface. Its significance is twofold: firstly, it performs a protective function, and secondly, it plays a role in the absorption of nutrients due to one-sided and selective permeability, i.e. only certain substances penetrate through this border. On the surface of the villi in the marginal epithelium there are special glandular cells resembling the shape of glasses (goblet cells). They also have a protective function by covering the surface of the epithelium with a layer of mucus. In crypts, on the contrary, goblet cells are much less common. Throughout the small intestine, lymphoid tissue forms small nodules (1 mm) in the mucous membrane - single follicles. In addition, there are accumulations of lymphoid tissue in the form of lymphatic Peyer's patches (20-30). The submucosal layer in all parts of the intestine consists of loose fibrous connective tissue. Thin arterial and venous networks of vessels branch out in it and there is a submucosal nerve plexus (Meisner's). The second nerve plexus is located in the muscular layer, between two layers of smooth muscle and is called intermuscular (Auerbach). The duodenum is the shortest (30 cm), fixed part of the small intestine. Although it is covered with adnocytes, that is, it does not have a mesentery and is not attached to the posterior wall of the abdominal cavity, the duodenum is well fixed between the stomach and the mesenteric part of the small intestine and is not able to change its position. It is located in front and to the right of the lumbar part of the diaphragm under the quadrate lobe of the liver. Its initial part is located at the level of the 1st lumbar vertebra, and the transition to the jejunum is at the level of the 2nd lumbar vertebra. It starts from the pylorus of the stomach and, bending like a horseshoe, covers the head of the pancreas. In the duodenum there are three main parts: the shortest - upper, longer - descending and lower; the lower one passes into the jejunum. At the site of the last transition, a pronounced duodenum-jejunum bend is formed. In the mucous membrane of the descending part of the duodenum there is a longitudinal fold, at the top of which there is a small elevation in the form of a papilla. The bile duct and pancreatic duct open on this papilla. There are no circular folds of the mucous membrane in the upper part of the duodenum; they begin to appear in the descending part, and in the lower part they are already well expressed. The rest, most of the small intestine, without any particular border, is divided into the initial part - jejunal 2/5 of the length, and the final part - ileum 3/5 of the length, which passes into the large intestine. Throughout their entire length, these parts of the small intestine are completely covered with a serous membrane, suspended on the mesentery to the posterior abdominal wall and form numerous intestinal loops. In the right iliac fossa, the ileum becomes the colon. At this point, the ileocecal valve is formed from the mucous membrane, consisting of two folds - the upper and lower lips, which protrude into the lumen of the cecum. Thanks to these formations, the contents of the small intestine freely penetrate into the cecum, but the contents of the cecum do not move back into the small intestine.

What are the features of the work of the presented parts of the digestive tract? What role do parts of the small intestine play in the absorption of nutrients? We will try to answer these and other questions in the material presented.

Parts of the human small intestine

The following sections of the small intestine are distinguished:

1. The duodenum connects to the transverse zone of the stomach. This initial portion of the small intestine forms a horseshoe-shaped loop around the pancreas. almost completely located in the retroperitoneal cavity. Only its small extension, the ampulla, extends beyond the boundaries of this space.
2. forms the upper part of the small intestine. Presented in the form of seven loops that lie on the left side of the peritoneum.
3. located in the lower right region of the abdominal cavity. Its ending in the form of loops passes into the pelvic area. The ileum connects to the rectum and is in close proximity to the bladder and uterus (in women).

Physical parameters

The above sections of the small intestine have an uneven diameter in different areas. In the distal zone the indicator is 2-3 cm, in the proximal zone - 4-6. The thickness of the walls of the small intestine is 2-3 mm, and in the case of tissue contraction it reaches 4-5. The length of the small intestine as a whole can be 5-6 meters. Moreover, its weight in an adult is approximately 650 g.

Small intestine: sections, functions

The most important digestive processes occur in the mucous membrane of local tissues that produce a huge amount of active enzymes. They process hummus, a food mush created by stomach juices. Here, useful elements are absorbed into the lymphatic and blood capillaries, which ensure their transportation to the tissues of organs and systems. Let's consider what functions the parts of the small intestine perform:

• Duodenum - hydrolysis of proteins, carbohydrates, fats. It ensures the active production of digestive enzymes. Processes undigested food particles with bile and transports stomach contents.
• Jejunum - motor, absorption, hormonal function, hydrolysis of polymers.
• The iliac zone is a transport-motor function. Provides absorption of substances formed as a result of hydrolysis. Processes bile acids.

The ability of cells to produce hormones

The production of hormones is a special function of local tissues. The sections of the small intestine are not only part of the digestive tract, but also part of the endocrine system. A wide range of hormones are produced here that regulate the transport-motor and digestive activities of the intestines.

The following set of endocrine cells is concentrated in the small intestine:

• I-cells - produce cholecystokinin;
• D-cells - somatostatin;
• M cells - motilin;
• G cells - gastrin;
• K-cells - insulinotropic glucose-dependent polypeptide;
• S-cells - secretin.

The bulk of hormone-producing cells are located in the jejunum and duodenum. A small part of them is in the ileum.

How does digestion occur in the small intestine?

Digestion in the small intestine is performed as follows. The gruel coming from the stomach, pre-treated with saliva and gastric juice, has an acidic reaction. In the small intestine, the presented mass is exposed to alkaline effects. This creates optimal conditions for the processing of nutrients by enzymes. The breakdown of the protein components of food gruel occurs under the influence of the following elements of intestinal juices:

1. The enzymes enterokinase, kinaseogen, and trypsin process simple proteins.
2. Erepsin breaks down peptides into amino acids.
3. Nuclease separates complex molecules of protein origin, known as nucleoproteins, into microelements.
4. The enzymes maltase, phosphatase, amylase and lactase break down carbohydrates.
5. Lipase processes fats.

After the synthesis of useful substances from food gruel through enzyme treatment, carbohydrate and protein components are absorbed by the villi of the small intestine. Next, the microelements enter through the venous capillaries into the liver tissue. In turn, the fats are sent to the lymphatic system.

Diseases of the small intestine

The most common ailments that affect parts of the small intestine are diarrhea and fecal retention in the tract. Defecation disorders are often accompanied by the development of pain syndromes in the peritoneum. Quite often, with poisoning and disorders of the small intestine, excessive gas formation is observed. In this case, the pain is short-lived, moderate in nature and is not the main factor of discomfort.

A common symptom of the development of malfunctions in the small intestine is rumbling in the peritoneum, a feeling of atypical movement in the abdomen. Most often, such manifestations are a consequence of abundant gas formation as a result of consumption of legumes, cabbage, potatoes, and rye bread. These symptoms may intensify significantly at night.

Failures in the production of enzymes and the breakdown of food gruel into microelements lead to more serious consequences. If food absorption, due to the absorption of substances into the blood and lymphatic vessels, does not occur properly, this can lead to weight loss and weakening of bone and muscle tissue. The consequences of digestive disorders are often hair loss, dry skin, and swelling in the extremities.

There are several main conditions that lead to the development of pathologies in the small intestine:

• Malabsorption is a violation of the absorption of nutrients.
• Maldigestion - low digestive activity.

If we talk about insufficiently high-quality processing of food gruel, such phenomena arise against the background of a low content of enzymes in intestinal juices. Low fermentation can be either acquired or genetic. Typically, pathologies of this type are a consequence of chronic inflammation, endocrine diseases, and surgical interventions.

Diagnostics

To diagnose the development of diseases of the small intestine, specialists resort to the following research methods:

• capsule examination;
• colonoscopy;
• endoscopy;
• fiberoscopy;
• radiography.

As for analyses, standard procedures are provided here. The patient provides a stool sample and blood is drawn. The stool is examined for the presence of helminths. When studying blood, the speed of movement of red blood cells is taken into account. Additionally, diagnostics are performed, which allows one to evaluate the functioning of the liver and thyroid gland.

Treatment

Therapy aimed at restoring the functions of the small intestine involves, first of all, eliminating the underlying disease. If there is a lack of enzymes in intestinal juices, medications containing synthetic substitutes are taken. In case of weight loss, tissue products are prescribed. The latter contain fat emulsions, amino acids, protein hydrolysates, and concentrated glucose.

If the problems are caused by intestinal dysbiosis, antibiotics are prescribed. The latter can provoke partial or complete destruction of beneficial flora. For this reason, after therapy, the patient is prescribed to take Bifikol, Lactobacterin or Colibacterin - biological drugs that have a positive effect on the restoration of the intestinal biocenosis.

Quite often, patients who suffer from disorders of the small intestine are prescribed medications that cause hardening of the stool. These include medications with a high content of calcium and bismuth. If the formation of liquid feces is caused by insufficient adhesion of fatty acids, they resort to the use of activated carbon to eliminate the problem. All of the above negative manifestations require prior consultation with a doctor. To bring the small intestine back to normal, it is important to abandon self-medication, make timely diagnoses and resort to adequate therapy developed by a specialist.

Finally

So we looked at what the small intestine is, its sections, and the structure of the represented part of the digestive tract. As you can see, local tissues are directly involved in the processing of food, its breakdown into individual microelements. The small intestine produces enzymes, vitamins, hormones, and substances that help increase the body's protective functions. At the same time, the occurrence of a deficiency of beneficial bacteria that live on its walls always leads to the development of pathological conditions.