Human Anatomy - Kotsan I. Y. 2009

Splanchnology
Digestive system
Small intestine

The Small Intestine (intestinum tenue) is a derivative of the midgut. It is located between The Stomach and the Large Intestine. The upper boundary of the small intestine is the pylorus of the stomach, and the lower boundary is the ileocecal valve near its opening into the large intestine.

The length of the small intestine in a living person ranges from 2.2 to 4.4 m. It is longer in men than in women. In a cadaver, due to the relaxation of the intestinal musculature, the length of the small intestine is 5–6 m. The small intestine is shaped like a tube, with a cross-section of 4–5 cm in the proximal section and 2–3 cm in the distal section.

In the small intestine, food processed by saliva and gastric juice is exposed to the action of intestinal juice, Bile, and pancreatic juice; that is, its chemical Processing continues. Here, the products of Digestion are absorbed into Blood and Lymphatic vessels (capillaries), and the mechanical mixing and propulsion of food toward the large intestine also take place.

The small intestine is divided into three parts: the duodenum, the jejunum, and the ileum. Unlike the duodenum, the jejunum and ileum have a well-developed mesentery and are considered the mesenteric part of the small intestine.

The duodenum (duodenum) is the initial section of the small intestine; its length in a living person is 17–21 cm, and in a cadaver, 25–30 cm. It begins at the pylorus of the stomach, horseshoe-shapedly surrounds the HEAD of the Pancreas, and ends at the duodenojejunal flexure. Four parts are distinguished in the duodenum: superior, descending, horizontal, and ascending (Fig. 149).

The superior part (pars superior) begins at the pylorus of the stomach to the right of the XII thoracic or I lumbar vertebra, extends to the right, slightly posteriorly and superiorly, and ends at the superior duodenal flexure (flexura duodeni superior). The length of this part of the intestine is 4–5 cm, and the diameter is 3.5–4 cm. This is the widest part of the duodenum; based on its X-ray appearance, it is called the duodenal ampulla (bulb) (ampulla (bulbus) duodeni). Posterior to the superior part lie the portal vein and the common bile duct, and superiorly it is in contact with the quadrate lobe of the Liver.

The descending part (pars descendens) begins at the superior duodenal flexure at the level of the I lumbar vertebra and descends almost vertically downwards, where at the level of the III lumbar vertebra it sharply turns to the left, resulting in The formation of the inferior duodenal flexure (flexura duodeni inferior). The length of the descending part is 8–10 cm. To the right and posteriorly, the descending part of the duodenum is adjacent to the right Kidney, to the left—to the head of the pancreas and the common bile duct, and anteriorly—to The Liver and the mesentery of the transverse colon.

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Fig. 149. Duodenum and pancreas

1 — duodenojejunal flexure; 2 — superior mesenteric artery; 3 — superior mesenteric vein; 4 — ascending part of the duodenum; 5 — uncinate process of the pancreas; 6 — horizontal part of the duodenum; 7 — circular folds; 8 — minor duodenal papilla; 9 — major duodenal papilla; 10 — longitudinal fold of the duodenum; 11 — descending part of the duodenum; 12 — accessory pancreatic duct; 13 — superior duodenal flexure; 14 — superior part of the duodenum; 15 — pylorus; 16 — body of the pancreas; 17 — pancreatic duct; 18 — tail of the pancreas

The horizontal part (pars horizontalis) begins from the inferior duodenal flexure, extends horizontally to the left at the level of the body of the III lumbar vertebra, crossing the INFERIOR VENA CAVA anteriorly, and then turns superiorly and continues into the ascending part.

The ascending part (pars ascendens) runs obliquely from bottom to top and from right to left up to the level of the II lumbar vertebra, where, following a sharp duodenojejunal flexure (flexura duodenojejunalis), it transitions into the jejunum. The duodenojejunal flexure of the intestine is fixed to the Diaphragm by means of the suspensory Muscle of the duodenum (m. suspensorius duodeni). Posterior to the ascending part of the duodenum lies the Abdominal Aorta.

The wall of the duodenum is formed by three tunics.

The mucous membrane (tunica mucosa) of the duodenum consists of an epithelial layer, lamina propria of the mucous membrane, muscularis mucosae, and submucosa, which separates the mucous membrane from the muscular layer. The mucous membrane of the superior part (ampulla) of the duodenum, like that in the Cytology/practical/109.html">Pyloric part of the stomach, has longitudinal folds (plicae longitudinales), whereas circular folds (plicae circulares) are found in all other PARTS OF THE intestine. This structural feature of the ampulla is related to the fact that the ampulla develops not from the midgut, like the entire duodenum, but from the foregut. On the inner surface of the medial wall of the descending part of the duodenum, the longitudinal fold of the duodenum (plica longitudinalis duodeni) is visible, which is formed by the pancreatic and bile ducts adjacent externally to the intestinal wall. At the distal end, this fold terminates in a rounded elevation—the major duodenal papilla (papilla duodeni major), at the apex of which the bile duct and the main pancreatic duct open with a common orifice. Slightly above the major papilla on the longitudinal fold lies the minor duodenal papilla (papilla duodeni minor), which contains the opening of the accessory pancreatic duct.

In addition to large glands (liver, pancreas), A large number of small glands secrete their product into the lumen of the duodenum. Some of them exhibit endocrine activity. Therefore, the duodenum is sometimes figuratively called the Pituitary Gland of the Abdominal cavity.

The mucous membrane of the duodenum, like that of the rest of the small intestine, forms finger-like projections on its surface—intestinal villi (villi intestinales), which give it a velvety appearance. The length of the villi ranges from 0.5 to 1.5 mm, and the thickness—from 0.2 to 0.5 mm. Muscle Cells are present within the thickness of the villus, so the villi function as pumps, facilitating the secretion of juice from the gland and the absorption of the breakdown products of food. Nerves, Blood Vessels, and Lymphatic vessels are located within the villus (Fig. 150). Fats are absorbed into the lymphatic vessels, and CARBOHYDRATES and Proteins into the blood vessels.

Along the entire length of the mucous membrane of the small intestine, in the intervals between the villi, open the mouths of numerous simple tubular glands called intestinal glands (glandulae intestinales) or crypts of Lieberkühn. They are located within the thickness of the mucous membrane and secrete intestinal juice (succus entericus) into the intestinal lumen. Intestinal juice breaks down fats, proteins, and carbohydrates into end products that are absorbed into the BLOOD AND Lymph. In the duodenum, predominantly in its upper half, another type of gland is located—duodenal glands (Brunner's glands) (glandulae duodenales (Brunneri)), which, unlike Lieberkühn's glands, are located in the submucosa. In their Structure, duodenal glands are similar to the Glands of the gastric pylorus and secrete a mucous fluid into the duodenal cavity.

The muscular coat (tunica muscularis) of the duodenum is formed by an outer longitudinal and an inner circular layer of smooth muscle cells.

Fig. 150. Diagram of The structure of a small intestinal villus (according to M. R. Sapin)

1 — intestinal epithelium; 2 — central lymphatic capillary (central lacteal); 3 — arterial vessel; 4 — venous vessel; 5 — blood capillaries

Externally, the duodenum is covered by the adventitia (tunica adventitia). The serous membrane (tunica serosa) is present only anteriorly, in those places where the duodenum is covered by the Peritoneum. In relation to the peritoneum, the duodenum is located mesoperitoneally, except for the ampulla, which is covered by the peritoneum on all sides.

The duodenum, together with the pancreas and liver, occupies a central place in digestive function. In the intestinal cavity, pancreatic juice breaks down proteins, fats, and carbohydrates, while bile promotes Fat Digestion and the absorption of Fatty acids. In addition, bile increases intestinal tone, enhances peristalsis, and participates in membrane (juxtacrine) digestion. The Cells of the duodenal mucosa produce BIOLOGICALLY ACTIVE SUBSTANCES that promote absorption processes and The regulation of general METABOLISM.

The duodenum is supplied with blood by the superior pancreaticoduodenal Arteries (Branches of the gastroduodenal artery) and the inferior pancreaticoduodenal artery (a branch of the superior mesenteric artery). Venous drainage is carried out through Veins OF THE same name that flow into the portal vein and its tributaries. Lymphatic vessels lead to the pancreaticoduodenal, mesenteric, and celiac Lymph Nodes.

The duodenum is innervated by branches of the vagus nerves and branches of the gastric, hepatic, and superior mesenteric plexuses.

The jejunum and ileum are collectively referred to as the mesenteric part of the small intestine (intestinum tenue mesenteriale). Unlike the duodenum, this entire section is intraperitoneal—covered by peritoneum on all sides—and attached to the posterior abdominal wall by the mesentery. The long mesentery of the small intestine (mesenterium) allows for significant mobility of the jejunum and ileum, enabling them to form loops within the abdominal cavity. Approximately 2/5 of the mesenteric small intestine belongs to the jejunum, and 3/5 to the ileum, with no sharp anatomical boundary between them.

The jejunum begins at the duodenojejunal flexure. Its loops are located predominantly in the upper left quadrant of the abdominal cavity. The jejunum gets its name (meaning "empty") from the fact that it is typically found empty of food in cadavers. Although slightly shorter than the ileum, the jejunum has a larger digestive surface area due to its wider diameter and more prominent, closely spaced circular folds.

As a continuation of the jejunum, the ileum (ileum) occupies the lower right part of the ABDOMINAL CAVITY AND terminates in the right iliac fossa at the ileocecal orifice (ostium ileocaecale), where it empties into the large intestine.

The wall of the mesenteric part of the small intestine exhibits the typical STRUCTURE OF THE gastrointestinal tract, consisting of the mucosa, submucosa, muscular layer, subserosa, and serosa.

The mucous membrane (tunica mucosa) of the mesenteric small intestine contains circular folds, intestinal villi, intestinal glands, and lymphatic follicles—all structures (except for duodenal glands) previously described in the Anatomy of the duodenum (Fig. 151). The structural differences in these elements within the mesenteric small intestine are as follows: circular folds are fewer in number compared to the duodenum, and their quantity gradually decreases from the jejunum toward the ileum, disappearing entirely in the terminal segment. Similarly, the height of the circular folds decreases from the oral to the anal end of the small intestine. Intestinal villi in the mesenteric part are thinner and slightly shorter than those in the duodenum, with their number, length, and thickness decreasing in the oral-to-anal direction. In addition to numerous solitary lymphatic follicles (folliculi lymphatici solitarii) scattered throughout the mucosa of the entire small intestine, the mucosa of the ileum (and occasionally the jejunum) contains large aggregations of lymphoid tissue forming elongated plaques, 2 to 10 cm long and 0.8 to 1.0 cm wide, known as Aggregated lymphoid follicles (Peyer's patches) (noduli (folliculi) lymphatici aggregati) (Fig. 152). Their longitudinal axis aligns with the long axis of the intestine; because they are located on the side opposite the mesenteric attachment, examining the intestine for Peyer's patches requires opening it along the mesenteric border. The surface of Peyer's patches typically lacks both intestinal villi and Lieberkühn crypts. The total number of Peyer's patches ranges from 20 to 30.

Fig. 151. Mucous membrane of the small intestine (segment of the jejunum)

1 — mesentery; 2 — circular folds; 3 — serous membrane; 4 — muscular layer; 5 — mucous membrane; 6 — submucosa; 7 — muscular layer (longitudinal layer); 8 — intestinal artery

The muscular layer (tunica muscularis) of the mesenteric small intestine, reflecting the tubular structure of the organ, consists of two layers of smooth muscle cells: an outer longitudinal layer and an inner circular layer, with the circular layer being better developed. Contractions of the small intestine musculature provide Two Types of movement: pendular movements (resulting from the alternate rhythmic contraction of the longitudinal and circular layers) and peristaltic movements (driven by the coordinated contraction of both layers). Additionally, a continuous tonic contraction of the intestinal wall musculature is observed.

The serous membrane (tunica serosa) covers the jejunum and ileum on all sides, leaving only a narrow strip free along the mesenteric border where the two layers of the mesentery diverge to embrace the intestinal wall.

Fig. 152. Mucous membrane of the small intestine (segment of the ileum)

1 — mesentery; 2 — solitary lymphatic follicles; 3 — aggregated lymphatic follicles (Peyer's patch); 4 — circular folds

The Blood supply to the mesenteric part of the small intestine is provided by 15–20 jejunal and ileal arteries (branches of the superior mesenteric artery). Venous blood drains via corresponding veins into the portal vein. Lymphatic vessels carry lymph to the mesenteric and ileocolic lymph nodes.

The mesenteric part of the small intestine is innervated by branches of the Vagus nerve and the superior mesenteric plexus.