The duodenum has villi. Digestive system
The small intestine begins to develop in the 5th week of embryogenesis. The epithelium of the villi, crypts and duodenal glands of the small intestine is formed from the intestinal endoderm. At the first stages of differentiation, the epithelium is single-row cubic, then it becomes double-row prismatic, and finally, at the 7-8th week, a single-layer prismatic epithelium is formed. At 8-10 weeks of development, villi and crypts appear. During the 20-24th week, circular folds form. By this time, duodenal glands also appear. The intestinal epithelial cells of a 4-week embryo are not differentiated and are characterized by high proliferative activity.
The duodenum is formed from the final section of the foregut and the initial section of the middle intestine; this rudiment grows and forms a loop. The onset of differentiation is associated with high activity (active growth) of the epithelium, which most often leads to temporary closure of the lumen in the duodenum. However, during the formation of villi, active growth and differentiation of the mesenchyme is of great importance. The first signs of crypt formation were noted in the duodenum at 8 weeks.
The jejunum and ileum are formed from the middle and posterior parts of the midgut. Between 5-10 weeks of fetal development, the loop of the growing midgut is “pushed” out of the abdominal cavity into the umbilical cord, and the mesentery grows towards the loop. By the end of this period, the loop of the intestinal tube “returns” to the abdominal cavity, it rotates (rotate by 270°) and grows in both caudal and proximal directions.
In a newborn, the histostructure of all components of the intestinal wall is incomplete. Its growth continues after birth, especially during 1 year of life, when the length of the intestine doubles. Further increase in this indicator occurs slowly. The folds and villi of the mucous membrane, the muscular layer, and the lymphoid apparatus are poorly expressed. Among the epithelial cells there are many goblet cells. Subsequently, the number of the latter decreases/Crypts are 2 times lower than those of an adult. Pannet cells are numerous. They are also found on the surface of the villi. The duodenal glands in a newborn are small in size, their histogenesis is not yet complete. By adulthood, their number decreases (there is an opinion - it increases). These glands develop most intensively in the first years of life.
The connective tissue of the mucosa and submucosa is rich in reticular elements and contains diffusely located clusters of lymphocytes. In the first days after birth, lymphopoiesis increases, single and group lymphoid nodules appear, in which reproduction centers appear. The appearance and development of follicles is associated with the penetration of microflora into the digestive tract.
(Fig. 34)
The rabbit duodenum is fixed with Zenker's mixture, transverse sections are prepared and stained with hematoxylin and eosin.
The wall of the duodenum, like the entire intestine, consists of three main membranes: mucosa (tunica mucosa), submucosa (t. submucosa) and external muscle (t. muscularis externa). From the outside, the intestine is covered by a serous membrane (t. serosa).
The intestinal mucosa is significantly different from the gastric mucosa. Digestion and absorption of food occurs in the small intestine, and therefore increasing the surface of the mucous membrane is of particular importance. This is achieved in two ways. The mucous membrane, together with the submucosa, forms numerous annular folds (Kerkring's folds), in addition, on the entire surface of the mucosa there are either long or short finger-like projections, the so-called villi, between which there are deep invaginations - crypts. In the duodenum, the villi are thick and lie closely next to each other.
The villi are covered with a single-layer epithelium, in which, under high magnification, two types of cells can be distinguished; the majority are tall prismatic cells with a regular oval nucleus. On their free surface, facing the intestinal lumen, one can discern a longitudinally striated thin cuticle, the so-called brush border. Under an electron microscope, it can be seen that it consists of finger-like outgrowths of the cytoplasm, due to which the absorption surface of the intestine increases. These cells perform the function of absorbing nutrients from the intestinal lumen.
Between the absorptive cells are cells that secrete mucus, the so-called goblet cells, which are single-celled glands.
They can be distinguished by a light secretory vacuole in the apical part of the cell. The resulting mucus pushes the nucleus into the basal part of the cell.
Rice. 34. Rabbit duodenum, transverse section (magnification approx. 5, volume 8):
U-mucous membrane, 2-submucous membrane, 3 - muscular layer, 4 - serous membrane, - 5 - villus, 6 - single-layer villous epithelium, 7 - tunica propria, 8 - crypt, 9 - muscular layer, 10 - Brunner's glands , 11 - ring layer of the outer muscular layer, /2 - longitudinal layer of the outer muscular layer, US - nerve cells of the Auerbach plexus, 14 - blood vessel
Under the epithelium of the villi there is connective tissue of the tunic propria (t. propria), containing a large number of reticular elements. It contains white blood cells, mainly lymphocytes, and wandering elements. They can be distinguished by their small, round, very dark cores.
Blood capillaries pass through the connective tissue of the villi, and in the center there is a lymphatic capillary. After absorption through the epithelium, mainly proteins and carbohydrates enter the blood capillaries, and mainly fats enter the lymphatic capillaries. Between the connective tissue cells lie elongated smooth muscle cells, singly or in groups. By their long narrow nuclei they can be easily distinguished from connective tissue cells with oval nuclei.
Smooth muscle cells originate from t. muscularis mucosae. Their contraction causes shortening of the villi, which helps push nutrients from the capillaries into the blood and lymphatic channels.
The crypts are lined with the same epithelial cells as the villi. Only at their base are the so-called Paiet cells, in the apical part of which acidophilic secretory granules are visible.
In the depths of the crypts, cells can multiply by means of mitosis, pictures of which can often be found in these places of the preparation; the resulting cells move towards the villus, where they gradually replace the dying and exfoliating cells of the villous epithelium, which is not capable of division.
T. muscularis mucosae is formed by two thin layers of smooth muscle: internal circular and external longitudinal.
The entire thickness of the submucosal membrane is occupied by Brunner's glands. They are present only in the duodenum; there are no glands in the wall of other parts of the intestine.
The highly branched tubular secretory sections of the glands are cut in different directions in the specimen. They are lined with light cubic and prismatic cells secreting a mucous-protein secretion; a lumen is clearly visible in each terminal section. The excretory ducts of Brunner's glands usually open at the bottom of the crypts. In the rabbit, the brunier glands, in addition to the tubular terminal secretory sections, also have alveolar glands lined with dark cells; Apparently, a protein secretion is formed in them.
The connective tissue of the submucosal membrane is reduced to thin layers between the lobules of the glands; blood vessels and sometimes separate groups of nerve cells of the Meissner plexus are visible in it.
The outer muscular layer consists of an inner circular and outer longitudinal layer of smooth muscle. In the connective tissue separating these layers, one should consider the nerve cells of the Auerbach plexus.
The serous membrane has a normal structure.
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In front, the duodenum is covered by the stomach, the right lobe of the liver and the mesentery of the transverse colon; it itself covers the head of the pancreas. In newborns, the duodenum is usually ring-shaped; in adults, it is V-shaped, C-shaped, folded or irregular. Its length in an adult is 27-30 cm, capacity - 150-250 ml.
The duodenum has 4 parts. The top part is the shortest; it has a round shape, length up to 3-4 cm; starts from the stomach and goes to the right and back along the right surface of the spine, passing in the area of the upper bend into the descending part. The initial section of the upper part of the D. is known clinically as the bulb. The descending part, 9-12 cm long, descends almost vertically and ends at the lower bend. The common bile duct and the pancreatic duct open into the lumen of the duodenum in this part, forming the major duodenal papilla (papilla of Vater) on the mucous membrane.
Above it is sometimes located the minor duodenal papilla, into which the accessory duct of the pancreas opens. The horizontal (lower) part, having a length of 1 to 9 cm, passes at the level of the III and IV lumbar vertebrae, below the mesentery of the transverse colon, partially behind the root of the mesentery of the small intestine. The ascending part, 6-13 cm long, passes directly into the jejunum, forming a bend at the transition site. The upper part of the duodenum is covered on three sides by peritoneum. The descending and horizontal parts are located retroperitoneally, the ascending part gradually again takes an intraperitoneal position. The duodenum is connected to the pancreas by smooth muscles, excretory ducts of the gland and common blood vessels, and to the liver by the hepatoduodenal ligament.
The blood supply to the duodenum is carried out from the distal and anterior superior, as well as inferior pancreaticoduodenal arteries - branches of the gastroduodenal and superior mesenteric arteries, which, anastomosing with each other, will please the anterior and posterior arches. Venous blood flows into the portal vein system. The outflow of lymph from the D. to. is carried out into the pancreaticoduodenal, upper mesenteric, celiac, and lumbar lymph nodes.
The sources of innervation of the duodenum are the vagus nerves (parasympathetic nervous system), celiac (solar), superior mesenteric, hepatic and gastroduodenal plexuses (sympathetic nervous system). There are two main nerve plexuses in the intestinal wall - the most developed intermuscular (Auerbach) and submucosal (Meissner).
The wall of the duodenum consists of serous, muscular and mucous membranes, as well as a submucosa, separated from the mucous membrane by a muscular plate. On the inner surface of the duodenum there are intestinal villi, covered with high prismatic bordered epithelium, thanks to the microvilli of which the absorption capacity of the cell increases tens of times. The bordered epithelium is interspersed with goblet enterocytes that produce glycosaminoglycans and glycoproteins. There are also cells (Paneth cells and intestinal endocrinocytes) that synthesize various gastrointestinal hormones - secretin, gastrin, enteroglucagon, etc.
The lamina propria of the mucous membrane is moderately infiltrated with lymphocytes and plasma cells, and lymphatic follicles are also found. In the submucosa there are mucous duodenal (Brunner's) glands, the excretory ducts of which open at the base or on the side walls of the intestinal crypts - tubular depressions of the epithelium in the lamina propria of the mucous membrane. The muscular lining of the duodenum is a continuation of the muscular lining of the stomach; it is formed by bundles of smooth (non-striated) muscle cells arranged in two layers. In the outer layer they are located longitudinally, in the inner layer - circularly. The serous membrane covers the duodenum only partially; the remaining sections are covered with adventitia, formed by loose fibrous connective tissue containing a large number of vessels and nerves.
The duodenum occupies one of the main places in the implementation of the secretory, motor and evacuation functions of the digestive tract. The secretion of the duodenum itself is produced by goblet enterocytes and duodenal glands. In addition, pancreatic juice and bile enter the cavity of the duodenum, providing further hydrolysis of nutrients that began in the stomach.
The duodenum is characterized by tonic, peristaltic, pendular contractions and rhythmic segmentation. The latter play a role in mixing and moving the chyme and are carried out through contractions of the longitudinal and circular layers of muscles. The motor activity of the duodenum depends on the physical and chemical properties of food and is regulated by neurohumoral mechanisms. The frequency of intestinal contractions decreases with systematic loss of bile, hypo- or hyperthyroidism.
Inhibition of intestinal motor activity occurs under the influence of adrenaline, norepinephrine, and irritation of the sympathetic nerves. When acetylcholine acts in large doses, the excitation of motor activity is replaced by its inhibition. Serotonin, gastrin, bradykinin, angiotensin, cholecystokinin, as well as irritation of the parasympathetic nerves stimulate the contractile activity of the duodenum. Prostaglandins have a variety of effects.
Methods for examining the duodenum:
Research methods include history taking, examination and palpation. Determining the nature of the pain, the time of its onset, duration, irradiation, identifying changes in the shape of the abdomen, bloating, as well as pain on palpation and increased skin sensitivity in the area of the projection of the duodenum make it possible to diagnose diseases such as peptic ulcer, duodenitis, etc. with a high degree of probability. X-ray examination, which is carried out in direct, oblique and lateral projections, is of great importance.In case of severe deformation of the duodenal bulb or the presence of another reason that does not allow identifying pathological changes in the organ, relaxation duodenography is indicated. Endoscopic examination is a valuable diagnostic method. To clarify the nature of the lesion, it is supplemented with a biopsy of the mucous membrane of the duodenum, followed by histological and histochemical studies of the obtained material. Duodenal intubation has important diagnostic value, especially for identifying concomitant pathologies (diseases of the biliary tract and pancreas, protozoal diseases, such as giardiasis).
Pathology of the duodenum:
The most common symptom of diseases of the duodenum is pain, which is localized mainly in the epigastric region and often spreads to the entire epigastric region. Signs of the disease are heartburn, belching, nausea, less often bitterness or dry mouth, and stool disorders. Due to the fact that diseases of the duodenum are often accompanied by pathological changes in other organs of the duodenohepatopancreatic zone, in some patients the symptoms of concomitant diseases come to the fore, for example gastritis, cholecystitis, colitis.Malformations include atresia, stenosis, duodenal duplication, congenital dilatation (primary megaduodenum) of the duodenum, and diverticula.
Atresia and stenosis:
Atresia and stenosis are clinically manifested mainly by symptoms of high intestinal obstruction (repeated vomiting, belching, hiccups) and can lead to expansion of the intestine above the obstruction site (secondary megaduodenum).Duplication of the duodenum:
Duplication of the duodenum, which most often occurs in the upper and descending parts of the intestine, occurs in three forms - cystic, diverticulum and tubular. It is manifested by symptoms of partial intestinal obstruction (regurgitation, vomiting), weight loss, and dehydration. When the pancreas or common bile duct is compressed, symptoms of pancreatitis and jaundice may appear. On palpation, a double duodenum may resemble a tumor-like formation in the abdominal cavity. Gastrointestinal bleeding often occurs in children.Congenital dilatation of the duodenum:
Congenital dilatation of the duodenum is extremely rare. This defect is based on disorders of the innervation of the duodenum at various levels. The expansion is usually accompanied by organ hypertrophy. Clinically, the defect is manifested by regurgitation, vomiting (vomit contains an admixture of bile, “greens”, a large amount of mucus), weight loss, and symptoms of dehydration. Patients experience swelling in the epigastric region, a “splashing noise”, caused by the accumulation of contents in the stomach and duodenum.The diagnosis of developmental defects is based on clinical findings. The main diagnostic methods are x-ray and endoscopic examination. Surgical treatment is the imposition of an anastomosis between the stomach and the jejunum (for atresia, stenosis and dilatation of the duodenum), removal of the duplication or the imposition of an anastomosis between the duplication and the duodenum or jejunum (for doubling of the organ). The prognosis is favorable.
Congenital duodenal diverticula:
Congenital diverticula of the duodenum are sac-like protrusions of its wall that arise in places of congenital underdevelopment of the muscular membrane. Duodenal diverticula can also occur as a consequence of periduodenitis, cholecystitis (acquired diverticula). Diverticula are often asymptomatic and discovered incidentally during X-ray examination. Typically, symptoms are caused by inflammation of the diverticulum - diverticulitis, which occurs as a result of stagnation of intestinal contents in it.Foreign bodies are often retained in the area of transition from the descending to the horizontal part of the duodenum. There are no symptoms, and foreign bodies, including sharp and large ones, enveloped in food masses, come out naturally without hindrance. When a foreign body is fixed or the intestinal wall is damaged, a feeling of heaviness, pain, and sometimes gastrointestinal bleeding occurs. If the duodenal wall is perforated, peritonitis may develop.
In diagnosis, the leading role belongs to x-ray and endoscopic examinations. The spontaneous release of a foreign body is facilitated by foods rich in fiber, as well as mucous porridges. Indications for intervention include fixation of a foreign body, its presence in the duodenum for more than 3 days, increased abdominal pain, signs of intestinal obstruction or peritonitis. In a significant number of cases, foreign bodies are removed using an endoscope, and sometimes laparotomy is used.
Damage (open and closed):
Injuries (open and closed) are the result of penetrating wounds of the abdomen (gunshot or inflicted with a bladed weapon), blunt trauma and are often combined with damage to other abdominal organs. With intraperitoneal injuries, the contents of the duodenum spill into the abdominal cavity, which leads to the development of peritonitis. Percussion in victims is determined by the absence of hepatic dullness, which occurs as a result of the release of gas into the abdominal cavity and its accumulation in the upper abdomen; with x-ray examination, free gas in the abdominal cavity is determined.With retroperitoneal injuries, the contents of the duodenum spill into the retroperitoneal tissue, causing phlegmon and then peritonitis. In the early stages after injury, the victim complains of pain in the right lumbar region, aggravated by palpation and pressure (false Pasternatsky's symptom), radiating to the right groin area and right thigh; muscle rigidity and pasty subcutaneous tissue in the lumbar region may occur. X-ray examination of the gastrointestinal tract is of great diagnostic importance, which can reveal leaks of contrast material into the retroperitoneal space; Plain radiographs of the chest and abdominal cavity reveal emphysema.
Treatment is surgical. For intraperitoneal injuries, which are easily identified, the edges of the duodenal defect are sparingly excised and double-row sutures are applied; for retroperitoneal injuries, the identification of which is difficult, the posterior layer of the parietal peritoneum is dissected, the posterior wall of the duodenum is mobilized, after identifying the defect, the edges of the wound are excised and sutured with double-row sutures . If the duodenum is completely ruptured, the edges of the intestine are excised and an end-to-end or side-to-side anastomosis is performed. A thin probe is inserted through the nose into the duodenum and used for 3-5 days. aspiration of intestinal contents is carried out. The retroperitoneal tissue is drained. The prognosis for injuries to the duodenum is serious and depends on the timing of surgery.
Duodenal fistulas:
Duodenal fistulas can be internal or external. Internal fistulas arise as a result of a pathological process in the wall of the duodenum with its subsequent spread to another organ or the transition of a pathological process from any organ to the duodenum. Most often, the D.'s cavity communicates with the cavity of the gallbladder or the common bile duct, less often with the cavity of the large or small intestine. Internal fistulas are manifested by pain in the corresponding part of the abdomen, symptoms of peritoneal irritation. When the duodenum communicates with the bile ducts, symptoms of ascending cholangitis (fever, chills, jaundice, leukocytosis, etc.) may occur; when communicating with other parts of the intestine, symptoms of colitis may occur.External fistulas usually form after injury to the abdominal cavity or surgical interventions. Their development is accompanied by the loss of bile, pancreatic enzymes, duodenal contents with an admixture of food masses, which leads to rapid exhaustion of the patient, disruption of all types of metabolism, anemia and causes the development of severe dermatitis.
The diagnosis is established based on the results of an X-ray examination of the duodenum, stomach, intestines, and biliary tract. For external fistulas, fistulography is indicated. Treatment is usually surgical (see Biliary fistulas, Intestinal fistulas).
Functional disorders (dyskinesia) are most often represented by duodenostasis, which in most cases accompanies other diseases, such as peptic ulcer, pancreatitis, duodenitis. There is a feeling of heaviness and periodic dull pain in the epigastric region and right hypochondrium, occurring soon after eating, belching, nausea, and sometimes regurgitation and vomiting, which bring relief. X-ray examination is of greatest importance for diagnosis. A delay of the contrast mass in any part of the duodenum for more than 35-40 s is regarded as a manifestation of duodenostasis. Less commonly, functional motor disorders are manifested by increased peristalsis and accelerated evacuation of intestinal contents, which is manifested by weakness, drowsiness, sweating, palpitations and other signs of dumping syndrome.
Diseases of the duodenum:
Diseases of the duodenum can be inflammatory or non-inflammatory. The most common inflammatory disease is duodenitis; Tuberculosis of the duodenum is rare, accounting for 3-4% of all cases of intestinal tuberculosis, as well as actinomycosis, which usually occurs when a specific process passes to the duodenum from other organs. One of the leading places in the pathology of the duodenum is occupied by peptic ulcer.Tumors of the duodenum:
Tumors are rare. They are benign and malignant. Benign tumors (adenomas, fibroadenomas, fibroids, papillomas, lipomas, neurofibromas, schwannomas) can be single or multiple. They are asymptomatic for a long time; when they reach large sizes, they usually manifest themselves as intestinal obstruction or (if the tumor disintegrates) intestinal bleeding.When the tumor is localized in the area of the major duodenal papilla, one of the first symptoms may be jaundice. A large tumor may be palpable. The main diagnostic methods are relaxation duodenography and duodenoscopy with targeted biopsy. Surgical treatment is tumor excision, duodenal resection or duodenectomy. Small polyp-like tumors of the duodenum are removed during duodenoscopy. The prognosis is usually favorable.
Of the malignant tumors, cancer is the most common, and sarcoma is extremely rare. Duodenal cancer is most often localized in the descending colon. Macroscopically, it usually has the appearance of a polyp or resembles cauliflower; sometimes an infiltrating form with a tendency to circular growth is observed. Histologically, it is an adenocarcinoma or columnar cell tumor; it metastasizes relatively late, mainly to regional lymph nodes, the porta hepatis, and the pancreas; grows into the pancreas, transverse colon.
Patients experience pain in the epigastric region, occurring 4-5 hours after eating, radiating to the right hypochondrium, nausea, vomiting (sometimes mixed with blood), which brings relief, signs of intestinal bleeding (tarry stools, decreased blood pressure). Characterized by progressive weight loss, anemia, anorexia, general malaise, weakness, fatigue, and so-called gastric discomfort. With infiltration of the major duodenal papilla, one of the most typical symptoms is jaundice.
In diagnosis, relaxation duodenography is of greatest importance (filling defect, circular narrowing of the lumen and suprastenotic dilatation of the intestine, in case of tumor ulceration - barium depot). Early detection of a tumor is possible with duodenoscopy and targeted biopsy. In some cases, a cytological examination of duodenal contents is performed. Differential diagnosis is made with cancer of the head of the pancreas. Treatment is surgical. The extent of the operation depends on the location and spread of the tumor: resection of the duodenum, duodenectomy, palliative operations such as gastroenterostomy with cholecystoenterostomy, etc. The prognosis is unfavorable.
Operations on the duodenum:
Operations on the duodenum are carried out for the purpose of its revision (for example, in case of abdominal trauma and peritonitis), as well as for therapeutic purposes for various pathological processes (ulcers, diverticulum, bleeding, foreign bodies, duodenal fistulas, obstruction, damage, malformations, tumors).Duodenotomy - opening the lumen of the duodenum is used to examine the internal surface and cavity of the intestine, and is also an integral part of other operations. It can be carried out in the transverse (along the anterior peritoneal wall) and longitudinal directions. In both cases, the intestine is sutured in a transverse direction to prevent narrowing of its lumen.
Papillectomy - excision of the major duodenal papilla; carried out for benign tumors (for example, papillomas), as well as in the early stages of malignant lesions in this area. After duodenotomy around the circumference of the major duodenal papilla, the mucous membrane is opened and separated. The major papilla with the common bile duct and the pancreatic duct flowing into it is removed through the duodenotomy opening, the ducts are isolated, crossed and sutured to the mucous membrane of the duodenum.
Papillotomy - dissection of the mouth of the major duodenal papilla; carried out with the aim of removing stones pinched in it. After duodenotomy, the mucous membrane is dissected longitudinally in the area of the mouth of the major duodenal papilla, then the strangulated stone is easily removed. The edges of the dissected mucous membrane are sutured to the wall of the duodenum in the area of the mouth.
Sphincterotomy is a dissection of the sphincter of Oddi, indicated for cicatricial changes, sclerosis of the sphincter muscle, and pinched stones. After duodenotomy, a section of the major duodenal papilla is excised in the form of a triangle (base at the mouth) and the mucous membrane of the duodenum is sutured to the mucous membrane of the common bile duct.
Duodenectomy - removal of the duodenum, is usually one of the stages of pancreatoduodenectomy, which is performed for cancer, as well as benign tumors of the duodenum. During the operation, a vesico-small intestinal anastomosis is performed, and the pancreatic duct is implanted into a loop of the small intestine. The patency of the gastrointestinal tract is restored by applying gastroenteroanastomosis.
Many operations involve the imposition of anastomoses between the duodenum and other organs of the digestive system. These include gastroduodenostomy - anastomosis between the stomach and duodenum (used, for example, for peptic ulcers), hepaticoduodenostomy - anastomosis between the common hepatic duct and the duodenum (produced with scar narrowing, damage or cancer of the common bile duct), hepatoduodenostomy - anastomosis between the intrahepatic bile duct and duodenum (used if hepaticoduodenostomy is not possible), choledochoduodenostomy - anastomosis between the common bile duct and the duodenum (performed when the distal part of the common bile duct is obstructed as a result of its scar changes, stones, cancer), cholecystoduodenostomy - anastomosis between the gallbladder and duodenum (used for obstruction of the common bile duct, for example due to trauma, malignant neoplasm, etc.).
All operations on the duodenum are performed under general anesthesia. Upper median laparotomy is used as access.
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General plan of structure and functions
The small intestine is about 6 m long. Approximately its first 30 cm are in the duodenum (Fig. 21 - 1). This intestine is almost entirely fixed and has no mesentery. It bends like a horseshoe around the head of the pancreas and passes into the next section of the small intestine, the jejunum (Fig. 21 - 1). The last part of the small intestine is called the ileum (Fig. 21 - 1).
Two main functions are carried out in the small intestine: 1) the digestion of food coming from the stomach is completed, and 2) the products of digestion are selectively absorbed into the blood and lymph. In addition, some hormones are produced in the intestine.
The structure of the small intestine is adapted to perform the functions of digestion and absorption. For convenience, we will first outline how its structures are adapted to absorption, and then we will describe its features associated with the digestion of food.
Structural features associated with absorption, folds, villi and microvilli
Rice. 21 - 32. Microphotograph (low magnification) of a longitudinal section of the wall of the jejunum of a dog, which shows two circular folds (Kerkring valves), cut transversely.
The folds are covered with villi of variable shape.
To effectively perform the absorption function, it is necessary that the small intestine has a large surface covered with epithelial cells that absorb substances. This large surface is due largely to the large length of the small intestine, but the increase in surface area through which absorption occurs is achieved in three other ways, namely:
- Starting approximately 2 - 3 cm after the pyloric sphincter, the mucosa forms circular or spiral folds, which are also called Kirkring valves (Fig. 21 - 32).
Rice. 21 - 33. Schematic three-dimensional representation of the lining of the small intestine.
Note that the villi are finger-like projections protruding into the intestinal lumen; their basis is formed by the lamina propria of the mucosa. Please also note that intestinal crypts are glands located in the thickness of the lamina propria. Particularly note the difference between villi and crypts in cross-section. 7 - villi, 2 - cross-section of the villi, 5 - core of the villi, formed by the lamina propria, 4 - surface of the mucous membrane, 5 - mouth of the crypt, b6 - cross-section of the crypt, 7 - muscular lamina of the mucosa, 8 - crypts, 9 - lamina propria mucous membrane.
These folds usually have a semilunar shape and occupy from half to two-thirds of the circumference of the lumen. Individual folds, however, can completely cover the circumference of the intestine or even form a spiral with 2 or 3 turns; the highest folds protrude into the lumen at a distance of up to 1 cm. The basis of all these folds is formed by the submucosa, and when the intestine is filled, these folds do not smooth out. At the proximal end of the small intestine, the circular folds are larger and located at a closer distance from each other (Fig. 21 - 32). In the upper part of the jejunum they become smaller and are located further apart. In the middle of the ileum or at its distal end they disappear.
2. The surface of the mucosa on the folds and between them is dotted with small protrusions in the form of a leaf, tongue or finger, the height of which varies from 0.5 to 1 mm or more. These formations are called intestinal villi (Fig. 21 - 33). Since they are protrusions of the mucous membrane, their basis is the lamina propria. The muscular plate of the mucosa and the submucosa, unlike circular folds, do not penetrate into them.
The villi of the duodenum are wider than in other areas, and many leaf-shaped villi can be found here. In the upper part of the jejunum, the villi are usually tongue-shaped. Even further they become finger-shaped. The shape of the villi, however, varies among individuals. A more important role is played by the length and surface area of the villi. Typically, length and surface area are greatest at the beginning of the small intestine (i.e., just behind the pylorus), decreasing gradually and reaching a minimum in the ileum just anterior to the ileocecal valve (Figs. 21 - 34). At first glance, it may seem that the size of the villi varies depending on the intensity of the absorption process. However, the large size of the villi in the duodenum appears to be determined by both local factors and those associated with the stomach and pancreas - when the duodenum is connected to the terminal ileum, so that the secretion evenly enters both intestines - ileal villi the intestines become higher and the duodenum lower than normal (Altmann G., 1976; Leblond S., Cheng N., 1976). 
Rice. 21 - 34. Microphotographs of villi from various parts of the small intestine of a rat (with the kind permission of G. Altmann, S. Leblond).
From left to right: the beginning of the duodenum, the jejunum, the border of the jejunum and ileum, the middle of the ileum and the terminal ileum. Note the gradual decrease in the height of the villi from the pylorus to the ileocecal valve, and also that the villi are located very close to each other (much closer than shown in Fig. 21 - 33).
3. The absorption surface becomes even more significant due to the presence of microvilli on the free surfaces of epithelial cells - microvilli were described in detail in Chapter. 5 and shown in Fig. 5 - 7 and 21 - 37.
Structural features associated with the digestion of food, glands and their enzymes
To perform the second main function (complete the digestion of food coming from the stomach), the small intestine needs a large amount of digestive enzymes and mucus. Digestive enzymes are produced by glands, while mucus is supplied not only by special glands, but also by numerous goblet cells located in the mucous membrane among the cells that perform the function of absorption. The glands that provide the production of digestive juices and mucus necessary for the function of the small intestine are located mainly in three areas: 1) outside the intestine, but connecting to it through ducts; 2) in the submucosa and 3) in the lamina propria of the mucous membrane.
The microscopic structure of the pancreas and liver, two glands located outside the small intestine and secreting their secretory products into it, will be discussed in Chapter. 22. Here we will discuss only the effect of their products on the digestive process. The ducts of these glands usually open together into the duodenum at a distance of about 7 cm from the pylorus (see Fig. 21 - 1). The secretion of the exocrine part of the pancreas, which enters the duodenum in this area, has an alkaline reaction (which helps neutralize acidic gastric juice) and contains enzymes that take part in the digestion of proteins, carbohydrates and fats. Apparently, the pancreas secretes several enzymes that carry out various stages of protein digestion. Enzymes are inactive until they enter the intestinal lumen, where they become active. Together, these enzymes are able to break down proteins into amino acids; it is in this form that proteins are absorbed. Pancreatic juice also contains enzymes that break down starch into sugars. In order for some sugars, such as maltose, to be absorbed, they must be further acted upon by enzymes produced by the epithelial cells of the villus, which decompose these sugars into monosaccharides. Pancreatic juice also contains lipolytic enzymes that emulsify fats and break them down into free fatty acids and monoglycerides. The action of these enzymes is facilitated by the presence of bile, a product of the secretory activity of the liver.
The second group of glands to consider is located in the submucosa. In this place, the glands are found only in the duodenum. These are complex tubular glands called Brunner's glands (Fig. 21 - 35). As a rule, they are more numerous in the proximal part of the duodenum and are found in smaller numbers (and then completely disappear) in its distal parts.
The secretory sections of the Brunner's glands have an appearance characteristic of the terminal sections of the mucous glands (Fig. 21 - 35), and are located mainly in the submucosa. Their ducts pass through the muscular plate of the mucous membrane (Fig. 35) and secrete their contents (mucosal secretion) into the Liberian crypts, which will now be discussed.
The third type of glands: intestinal crypts (glands), or Lieberkühn's crypts. They are depressions starting between the villi and reaching almost the muscular plate of the mucous membrane (see Fig. 21 - 21, and also Fig. 21 - 36, A). Their mouths on the surface of the intestinal mucosa are shown schematically in Fig. 21 - 33, but in reality these holes are very difficult to see, since they are tightly closed during life. Of the various enzymes secreted in the small intestine, one produced exclusively in the crypts is lysozyme, a bactericidal enzyme produced by Paneth cells (described below). 
Rice. 21 - 35. Microphotograph of part of the wall of the human duodenum - x 100 (with the kind permission of S. Leblond).
Note the pale-colored Brunner's glands (mucus-producing glands) located in the submucosa (D). They pass through the muscular plate (II) of the mucosa into the lamina propria (III), which lies under the single-layer columnar epithelium (IV), which also contains goblet cells. The arrow shows the place where the duct of Brunner's gland opens into the intestinal crypt. The broad leaf-shaped villi seen at the top left are characteristic of this part of the small intestine.
Most of the enzymes produced by the small intestine appear, however, on the surface of the microvilli of columnar cells and remain associated with their striated (brush) border, as will be explained below.