Human Anatomy and Physiology (with Age-Related Features of the Child's Body) - Sapin M.R., Sivoglazov V.I. 2002

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 is divided into the duodenum, jejunum, and ileum. The length of the small intestine in a living person ranges from 2.2 to 4.4 m (5–6 m in a corpse). The shortest and widest part is the duodenum, with a length not exceeding 25–30 cm. About ²/₅ of the length of the small intestine belongs to the jejunum and about ³/₅ to the ileum. The diameter of the small intestine does not exceed 3–5 cm. The small intestine forms loops that are covered anteriorly by the greater omentum and bounded above and laterally by the Large Intestine. The mucous membrane of the small intestine forms numerous circular folds and a vast number of villi, thereby increasing the absorptive surface area of the mucosa.

The horseshoe-shaped duodenum curves around the HEAD of the Pancreas and is located mostly retroperitoneally. Only its initial and terminal segments are covered by the Peritoneum on almost all sides. The peritoneum adheres only to the anterior surface of the remaining PARTS OF THE duodenum. The superior, descending, horizontal, and ascending parts of the duodenum are distinguished. As it transitions into the jejunum, the duodenum forms a sharp bend to the left of the body of the 2nd lumbar vertebra.

In addition to circular folds, the mucous membrane of the duodenum forms a longitudinal fold running along the posteromedial wall of its descending part. This fold terminates in an elevation known as the major duodenal papilla (ampulla of Vater), upon the summit of which the common Bile duct and the main pancreatic duct open.

The jejunum and ileum are covered by the peritoneum on all sides, positioned intraperitoneally, and possess a mesentery. Their mucous membrane forms 600–700 circular folds and a vast number of microscopic projections called villi (9×10⁷). The surface of the villi is covered by simple columnar epithelium, which contains intestinal epithelial Cells (enterocytes), mucus-secreting cells, and a small number of enteroendocrine cells (Fig. 43). The apical surface of the enterocytes features a brush border formed by a huge number of microvilli (150–3000 On the surface of each Cell), which significantly increase the absorptive surface of these cells.

In the center of each villus lies a wide, blind-ended lymphatic capillary (lacteal) into which emulsified fats are absorbed. Each villus receives 1–2 arterioles that branch out into capillaries. Simple sugars and Protein Digestion products are absorbed directly into the Blood.

The openings of the intestinal crypts (Crypts of Lieberkühn)—tubular invaginations of the lamina propria of the mucous membrane with a length of 0.25–0.55 mm—open into the lumen between the villi.

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

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

Age-related Features of the small intestine

The small intestine of a newborn has a length of 1.2–2.8 m. At 2–3 years of age, its length averages 2.8 m. By the age of 10, the length of the intestine reaches its adult size (5–6 m). The diameter of the intestine by the end of the first year of life is 16 mm, and at 3 years of age, it is 23 mm.

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Fig. 43. Diagram of The Structure of small intestinal villi:

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

The duodenum in a newborn is ring-shaped. Its beginning and end are located at the level of the 1st lumbar vertebra. By the age of 7, its descending part descends to the 2nd lumbar vertebra. The duodenal glands are poorly branched. Intensive growth of the glands is observed During the first years of a child's life.

In the newborn's jejunum and ileum, the folds are poorly expressed and the glands are underdeveloped, although numerous villi are already present. The muscular layer is poorly developed. Intensive growth of all structures of the small intestine is noted up to 3 years of age, after which growth slows down and intensifies again at 10–15 years.