General Characteristics and Parts
The small intestine begins directly at the pylorus of the stomach and extends to the junction with the large intestine. The total length of the organ reaches 5 meters, while the lumen of the intestinal tube gradually narrows: from 4–6 cm in the proximal parts to 2.5–3 cm near the distal end.
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 lies primarily 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 abdominal cavity, inferior to the transverse mesocolon.
This section begins at the duodenojejunal flexure (flexura duodenojejunalis), which is located 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 4th lumbar vertebra, where it transitions into the large intestine. The loops lie intraperitoneally and possess significant mobility due to the mesentery. Two borders are distinguished on the intestine: the mesenteric border (attachment site) and the free antimesenteric border.
The mesenteric part consists of two segments:
- Jejunum (jejunum) — makes up the proximal ⅔ of its length. Its loops lie mainly in the upper and left regions of the abdominal cavity. The initial diameter is 4.8–5.0 cm.
- Ileum (ileum) — occupies the distal ⅓. Its loops descend into the lower and right regions, 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 distinct 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 consisting of three tunics:
- Serous coat (tunica serosa) — visceral peritoneum covering the organ externally (except for a narrow strip along the mesenteric border).
- Muscular coat (tunica muscularis) — composed of smooth muscle divided into an outer longitudinal layer (stratum longitudinale) and an inner circular layer (stratum circulare).
- Mucous membrane (tunica mucosa) — forms structures that greatly increase the absorptive 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 prominent in the jejunum, while their height and number noticeably decrease toward the ileum. Villi in the mesenteric part are shorter and thinner than in the duodenum, and their density also decreases in the distal direction.
The submucosa (tela submucosa) contains 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 where the ileum empties into the cecum. This junction is called the ileal orifice (ostium ileale).
The orifice is surrounded by prominent folds of the mucous membrane that form a specialized ileocecal valve apparatus. It performs a crucial valve function, allowing chyme to pass in one direction (into the large intestine) while preventing backflow.