Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Doctrine of the Viscera (Splanchnology)
Digestive System
Small Intestine

The human Small Intestine begins at the pylorus of The Stomach, at the level between the bodies of the 12th thoracic and 1st lumbar vertebrae, and ends by opening into the cecum (Large Intestine) in the region of the right iliac fossa. The small intestine is subdivided into the duodenum, jejunum, and ileum. In living individuals, the length of the small intestine ranges from 2.2 to 4.4 m. In cadavers, where the intestinal Muscle tone is absent, its length reaches 5–6 m. The shortest and widest part is the duodenum, measuring 25–30 cm in length. Approximately ²/₅ of the small intestine's length belongs to the jejunum, and about ³/₅ to the ileum. The diameter of the small intestine does not exceed 3–5 cm. The jejunum and ileum form loops that are covered anteriorly by the greater omentum and bounded superiorly and laterally by the large intestine. The mucous membrane of the small intestine forms numerous circular folds and a vast number of villi, which significantly increases the absorptive surface area of the mucosa.

The duodenum is horseshoe-shaped, curving around the HEAD of the Pancreas. Its initial and terminal segments are covered by the Peritoneum almost entirely. For the remaining PARTS OF THE duodenum, the peritoneum covers only the anterior surface. The duodenum is divided into superior, descending, horizontal, and ascending parts. As it transitions into the jejunum, the duodenum forms a sharp duodenojejunal flexure, located to the left of the body of the 2nd lumbar vertebra.

In addition to circular folds, the mucous membrane of the duodenum forms a well-defined longitudinal fold running along the posteromedial wall of its descending part. Inferiorly, this fold terminates in an elevation known as the major duodenal papilla (papilla of Vater), at the apex of which the common Bile duct and the main pancreatic duct open.

The jejunum and ileum are covered by the peritoneum on all sides, meaning they are located intraperitoneally and possess a mesentery. Their mucous membrane forms 600–700 circular folds and a massive number of microscopic projections called villi (approximately 90 million). The surface of the mucous membrane, including the villi, is covered by a simple columnar epithelium consisting of enterocytes, a small number of intestinal endocrine Cells, and mucous cells (Fig. 48). The free (apical) surface of the enterocytes features a so-called striated (brush) border formed by a multitude of microvilli (up to 1,500–3,000 On the surface of each epithelial Cell). The presence of intestinal villi and microvilli on enterocytes vastly increases the absorptive surface area of these cells. Each villus receives 1 to 2 arterioles that branch into a capillary network. Simple sugars and Protein Digestion products are absorbed directly into the Blood. Located in the center of each villus is a wide, blind-ended lymphatic capillary (lacteal) responsible for the absorption of emulsified fats. The spaces between the villi receive the openings of intestinal crypts (crypts of Lieberkühn)—tubular invaginations of the lamina propria of the mucosa, measuring 0.25–0.55 mm in length.

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Fig. 48. Structure OF THE small intestine villi:

1 — enterocytes; 2 — goblet cells; 3 — central lymphatic sinus; 4 — arteriole; 5 — venule; 6 — blood capillaries

The lamina propria of the small intestine mucosa contains numerous solitary lymphoid follicles, 0.5–1.5 mm in diameter, as well as Aggregated lymphoid nodules (Peyer's patches). These are primarily located in the walls of the ileum.

The muscular coat, the function of which is to mix chyme within the intestinal lumen and propel it toward the large intestine, consists of an outer longitudinal layer and an inner circular layer.