Topography and relation to the peritoneum
Loops of the small intestine project onto the umbilical and hypogastric regions. On three sides, they are bounded by the large intestine: superiorly by the transverse colon (colon transversum), to the right by the ascending colon (colon ascendens), and to the left by the descending colon (colon descendens) and sigmoid colon (colon sigmoideum). In terms of depth, the loops are arranged in layers: superficial ones contact the greater omentum and the anterior abdominal wall, while deep ones lie against the posterior abdominal wall.
The position of the loops directly depends on the length of the mesentery and the body type:
- In brachymorphs, they lie horizontally.
- In dolichomorphs, vertically.
- In mesomorphs, obliquely.
The intestine has two borders: the free border (margo liber) and the mesenteric border (margo mesenterialis). On the mesenteric border between the layers of the peritoneum, a narrow strip of the wall remains — the "bare area" (pars nuda). Its width is proportional to the thickness of the mesentery and varies from 0.2–0.5 cm at the beginning of the jejunum to 1.5 cm at the end of the ileum.
Surgical significance: sutures placed in the pars nuda zone lack a serosal covering. They are weak, prone to dehiscence, and require mandatory additional peritonealization.
Arterial blood supply and vascular arcades
The main source of nutrition is the superior mesenteric artery (a. mesenterica superior). It branches off the abdominal aorta at an acute angle at the level of the first lumbar vertebra (L1), 1–2 cm below the celiac trunk. Passing between the aorta and the left renal vein, the artery lies upon the uncinate process of the pancreas and crosses the horizontal part of the duodenum. Compression of the intestine by the vessel is possible in this zone, leading to superior mesenteric artery syndrome.
Within the thickness of the mesentery, the vessel forms an arch with its convexity facing left. Branches to the large intestine depart from the concave surface (right and superior), while 12–16 jejunal (aa. jejunales) and ileal (aa. ileales) arteries depart from the convex surface.
The mechanism of vascular network formation is unique:
- At a distance of 1–8 cm from the origin, each branch divides into an ascending and a descending branch.
- By anastomosing with neighboring branches, they form first-order arcades.
- New branches depart from them, forming 3 to 5 rows of arches whose caliber gradually decreases.
The last row forms a continuous marginal vessel 1–3 cm from the intestinal wall. Straight arteries run from it to the mesenteric border. If the straight vessels are damaged over a length of 3–5 cm, that segment of the intestine will undergo ischemia and require resection. At the same time, injuries to the mesentery itself in the arcade zone cause severe bleeding but do not lead to necrosis, thanks to rich collaterals.
Venous drainage, lymphatic drainage, and innervation
The venous system mirrors the course of the arteries. Straight veins collect into a system of venous arcades, forming the superior mesenteric vein (v. mesenterica superior). It runs within the root of the mesentery to the right of the corresponding artery, passes beneath the head of the pancreas, and, by merging with the splenic vein (v. lienalis), forms the portal vein.
The lymphatic system is represented by "lacteal" vessels forming two layers between the peritoneal leaves. They have a beaded appearance due to an abundance of valves. Lymph nodes are located in three rows:
- Along the mesenteric border of the intestine.
- At the level of intermediate vascular arcades.
- Along the course of the main branches of the superior mesenteric artery.
Clinical feature: some lymphatic vessels may bypass nodes and drain directly into the thoracic duct (in one-third of cases, the intestinal trunk — truncus intestinalis — is formed). This explains the rapid lymphogenic spread of infections and tumor metastases.
Innervation is provided by the superior mesenteric plexus (plexus mesentericus superior). It comprises parasympathetic fibers of the vagus nerve, postganglionic sympathetic fibers (from the ganglion mesentericum superius), and viscerosensory neural pathways.
Briefly: answers to topical questions
Which specific arteries branch off from the concave surface of the superior mesenteric artery toward the large intestine?
From the concave surface of the superior mesenteric artery, branches to the large intestine extend to the right and upward: the ileocolic artery (a. ileocolica), the right colic artery (a. colica dextra), and the middle colic artery (a. colica media). They supply the initial segment of the large intestine, the ascending colon, and the transverse colon, respectively.
The merger of which veins forms the hepatic portal vein?
The portal vein (v. portae) is formed behind the head of the pancreas by the merger of two main venous trunks.
- Superior mesenteric vein (v. mesenterica superior)
- Splenic vein (v. lienalis)
The inferior mesenteric vein (v. mesenterica inferior) also collects blood from the intestine, but it most frequently drains into the splenic vein, less often into the superior mesenteric vein, and only occasionally participates directly in forming the portal vein trunk.
What are the anatomical names of the groups of lymph nodes that collect lymph from the small intestine?
Lymph nodes draining the small intestine are located in the mesentery and form three rows: along the mesenteric border of the intestine, at the level of the intermediate vascular arcades, and along the course of the main branches of the superior mesenteric artery. Some lymphatic vessels may bypass the nodes; in 1/3 of cases, an intestinal trunk (truncus intestinalis) forms and empties into the origin of the thoracic duct.
What layers make up the wall of the small intestine?
The wall of the small intestine consists of the mucosa, submucosa, muscular layer, and serosa. The mucosa includes the epithelium, lamina propria, and muscularis mucosae.
What are the macroscopic differences between the jejunum and the ileum?
The jejunum and ileum have the following macroscopic differences:
| Sign | Jejunum | Ileum |
|---|---|---|
| Maximum diameter during peristalsis | About 4 cm | About 3 cm |
| Circular folds | High and frequent, circular in configuration, about 2 mm thick | Lower and sparser, acquiring a spiral shape, about 1 mm thick; may be absent in the expanded terminal part |
| Lymphoid apparatus | Single (solitary) follicles are present; aggregated follicles in the form of Peyer's patches are not specified | Aggregated lymphatic follicles — Peyer's patches — are present |
To easily remember the position of the loops depending on body type, use body shape logic: Dolichomorph (elongated body) — loops are elongated vertically. Brachymorph (broad body) — loops are spread out horizontally.
Class and exam questions
Why do foreign bodies most frequently get stuck in the terminal part of the small intestine?
The diameter of the intestine gradually decreases in the distal direction. While it is 3.5–4.8 cm in the initial segment, it narrows to 2.0–2.7 cm at the site of entry into the large intestine.
What are the risks of placing sutures on the mesenteric border of the intestine?
Along the mesenteric border lies the "bare area" (pars nuda), which lacks peritoneum. Sutures in this zone are not covered by serosa, turn out to be weak, and can dehisce, so they must be additionally peritonealized.
Will an injury to the mesentery in the zone of vascular arcades lead to intestinal necrosis?
No, an injury within the arcades will cause profuse bleeding, but ischemia will not occur thanks to the well-developed collateral network. Necrosis occurs only when straight arteries are damaged near the intestinal wall itself over a length of 3–5 cm.
Why can small intestine tumors cause rapid lymphogenous metastasis?
Some lymphatic vessels of the small intestine can bypass regional lymph nodes and drain directly into the thoracic duct, accelerating the systemic spread of malignant cells.
What else to study on this topic
- Congenital developmental anomalies (atresias, stenoses, malrotation)
- Surgical techniques for orientation in the wound when searching for loops
- Topography and branches of the ileocolic artery (a. ileocolica)
- Formation and topography of the portal vein
- Anastomoses between the superior and inferior mesenteric arterial systems (Arc of Riolan)
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