General characteristics and parts
The small intestine begins directly at the pylorus of the stomach and extends to the point of transition into the large intestine. The total length of the organ reaches 5 meters, while the lumen of the intestinal tube gradually narrows: whereas in the initial segments the diameter is 4–6 cm, closer to the end it decreases to 2.5–3 cm.
Anatomically, the small intestine is divided into three consecutive parts:
- Duodenum (duodenum);
- Jejunum (jejunum);
- Ileum (ileum).
Most of the organ has its own mesentery (mesenterium) and is covered by the peritoneum on all sides. The only exception is the duodenum, which is located predominantly retroperitoneally (behind the peritoneum).
Mesenteric part of the small intestine
The mesenteric part (pars mesenterica intestini tenuis) includes the jejunum and ileum. It occupies the lower part of the abdominal cavity, lying below the transverse mesocolon.
This section begins at the duodenojejunal flexure (flexura duodenojejunalis), which is localized to the left of the body of the 2nd lumbar vertebra. The mesenteric part ends in the right iliac fossa at the level of the body of the 4th lumbar vertebra, where it transitions into the large intestine. The loops lie intraperitoneally and have pronounced mobility due to the mesentery. Two borders are distinguished on the intestine itself: the mesenteric border (site of fixation) and the free antimesenteric border.
The mesenteric part includes two segments:
- Jejunum (jejunum) — makes up the proximal ⅔ of the length. Its loops lie mainly in the upper and left parts of the abdominal cavity. The diameter at the beginning is 4.8–5.0 cm.
- Ileum (ileum) — occupies the distal ⅓. The loops descend into the lower and right parts, reaching the pelvic cavity. The diameter of the distal segment narrows to 2.7–3.0 cm.
It is important to note that there is no clear anatomical or histological boundary between the jejunum and the ileum.
Structure of the intestinal wall
The wall of the mesenteric part of the small intestine has a classic layered structure and consists of three coats:
- Serous coat (tunica serosa) — visceral peritoneum covering the organ externally (except for a narrow strip along the mesenteric border).
- Muscular coat (tunica muscularis) — represented by smooth muscle tissue, divided into an outer longitudinal layer (stratum longitudinale) and an inner circular layer (stratum circulare).
- Mucous membrane (tunica mucosa) — forms structures that significantly increase the absorption surface area.
The relief of the mucous membrane is formed by circular folds (plicae circulares), intestinal villi (villi intestinales), and crypts (glandulae intestinales). The total number of folds ranges from 500 to 1200. They are most pronounced in the jejunum, while toward the ileum their height and number noticeably decrease. The villi in the mesenteric part are shorter and thinner than in the duodenum, and their density also decreases in the distal direction.
In the submucosa (tela submucosa), there are solitary lymphoid nodules (noduli lymphoidei solitarii). A specific feature of the ileum is the presence of aggregated lymphoid follicles — Peyer's patches (noduli lymphoidei aggregati).
Transition to the large intestine
The small intestine ends at the site where the ileum empties into the cecum. This junction is called the ileocecal opening (ostium ileale).
The opening is surrounded by prominent folds of the mucous membrane that form a special ileocecal valve apparatus. It performs a crucial valvular function, allowing chyme to pass in one direction (into the large intestine) and preventing its retrograde reflux.
The parts of the small intestine are easy to remember with the phrase: Duodenal, Jejunal, Ileal.
Class and exam questions
Where is the boundary between the jejunum and the ileum?
There is no clear anatomical boundary between them. Conventionally, the proximal ⅔ of the mesenteric part belongs to the jejunum, and the distal ⅓ belongs to the ileum.
How does the mucous membrane change along the mesenteric part?
In the proximal segments (jejunum), circular folds and villi are more numerous and higher. In the distal direction (toward the ileum), their number and height decrease, whereas large aggregated lymphoid nodules appear in the submucosa.
How is the small intestine covered by the peritoneum?
The duodenum lies predominantly retroperitoneally. The jejunum and ileum are covered by the peritoneum on all sides (intraperitoneally) and have a mesentery.
What else to study on this topic
- Anatomy and topography of the duodenum
- Structure of the mesentery of the small intestine and its root
- Histological structure of intestinal villi
- Ileocecal valve (ileocecal junction)
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