Small Intestine and Mesentery

Position: Predominantly occupies the left upper quadrant/umbilical region, beginning at the duodenojejunal flexure (ligament of Treitz); constitutes roughly the proximal two-fifths of the small intestine distal to the duodenum.

• Wall features: Thicker, more muscular wall than the ileum; mucosa displays tall, densely packed circular folds (plicae circulares/valvulae conniventes), giving a characteristic "feathery" appearance on contrast imaging; relatively larger luminal diameter.
• Mesenteric features: Mesentery is comparatively thin, with less fat deposition, allowing the vessels within it to be more easily seen/palpated ("translucent" mesenteric windows).
• Vascular arcades: Fewer arcade tiers (typically only 1, occasionally 2), a simpler, less complex arcade pattern.
• Vasa recta: Long, giving each arcade a wider reach to the bowel wall before dividing.

• Position: Predominantly occupies the right lower quadrant/pelvis, continuing from the jejunum to the ileocaecal junction; constitutes roughly the distal three-fifths of the small intestine distal to the duodenum.
• Wall features: Thinner wall than the jejunum; circular folds are shorter, sparser, and eventually disappear in the terminal ileum; smaller luminal diameter; contains Peyer's patches (aggregated lymphoid follicles), most prominent in the terminal ileum, situated on the antimesenteric border.
• Mesenteric features: Mesentery is thicker, with more abundant fat deposition, making the vessels less readily visualised through the mesenteric leaves ("opaque" mesenteric windows).
• Vascular arcades: More numerous arcade tiers (typically 3-4 or more), a more complex, highly branched arcade pattern.
• Vasa recta: Short, closely spaced, reflecting the higher-order arcade branching reaching closer to the bowel wall before the final vessels arise.
• Peyer's patches: Aggregated lymphoid nodules, part of the gut-associated lymphoid tissue (GALT), most concentrated in the terminal ileum on the antimesenteric border, immunologically important for sampling luminal antigens (via specialised M cells), and clinically relevant as a site of typhoid fever-related ulceration/perforation and as a potential lead point for intussusception.

• The double-layered peritoneal fold suspending the jejunum and ileum from the posterior abdominal wall, carrying the superior mesenteric vessels, lymphatics, nerves, and a variable amount of fat between its two layers.
• Fan-shaped, with a long, folded intestinal border (attached along the entire length of the jejunoileal loops) and a short root.

• The fixed line of attachment of the mesentery to the posterior abdominal wall, approximately 15 cm long, running obliquely from the duodenojejunal flexure (upper left, at approximately L2) down to the ileocaecal junction (lower right, at approximately the right sacroiliac joint).
• This oblique orientation is a key anatomical fact: it explains the characteristic distribution of structures crossed by the root, and is clinically relevant to the pattern of mesenteric injury/rotation in volvulus.

• Given its oblique course from upper left to lower right, the root of the mesentery crosses, in sequence: the third and fourth parts of the duodenum, the abdominal aorta, the inferior vena cava, the right ureter, and the right psoas major muscle (and right gonadal vessels), a frequently tested sequence, since these are all major retroperitoneal structures that could theoretically be affected by pathology tracking along or compressing the root.

• Arises from the abdominal aorta at approximately L1 (just below the coeliac trunk), passing anterior to the third part of the duodenum before entering the root of the mesentery.
• Supplies the entire midgut: distal duodenum, jejunum, ileum, caecum, appendix, ascending colon, and proximal two-thirds of the transverse colon.
• Gives off, in sequence: inferior pancreaticoduodenal artery, multiple jejunal and ileal branches (forming the arcades), the middle colic artery (to the transverse colon), right colic artery (to the ascending colon, variable origin), and ileocolic artery (its terminal branch, supplying the terminal ileum, caecum, appendix via the appendicular artery, and the beginning of the ascending colon).

• Drains the same territory supplied by the SMA, running to the right of the SMA within the mesenteric root.
• Joins the splenic vein posterior to the neck of the pancreas to form the portal vein: a critical anatomical landmark.
• The inferior mesenteric vein typically drains into the splenic vein (rather than joining the SMV/portal vein confluence directly), though anatomical variation exists (sometimes draining into the SMV or the angle of confluence itself).

• Jejunal and ileal arteries: arise from the left side of the SMA, running within the mesentery to form the arcade system, giving rise to the vasa recta supplying the bowel wall directly.
• The progressive increase in arcade complexity (jejunum: 1-2 tiers; ileum: 3-4+ tiers) reflects an increasing redundancy/collateral density moving distally along the small intestine.

• Intestinal lymphatics (lacteals, specialised for fat absorption within each villus) drain into mesenteric lymph nodes arranged in three tiers along the vessels: juxtaintestinal nodes (near the bowel wall), intermediate mesenteric nodes (along the arcades), and superior mesenteric (central) nodes (near the SMA origin), draining to the intestinal lymphatic trunk, which contributes to the cisterna chyli.

• Parasympathetic: Vagus nerve, via the superior mesenteric plexus: increases motility and secretion.
• Sympathetic: Lesser and least splanchnic nerves (T10-T12), synapsing in the superior mesenteric ganglion: decreases motility, mediates vasoconstriction; also carries visceral afferent (pain) fibres, producing the classic periumbilical referred pain pattern characteristic of midgut pathology (reflecting the T10 dermatomal correlation with the umbilicus, and the midgut's shared embryological/vascular origin).

Mesenteric ischaemia:

• Acute occlusion of the SMA (embolism, thrombosis) or, less commonly, the SMV (venous thrombosis), compromises perfusion to the midgut territory.
• Given the progressively richer arcade collateralisation toward the distal small bowel, proximal jejunal ischaemia (fewer collaterals) can be more severe/rapid in onset than more distal ischaemia, though clinical presentation and severity depend heavily on the specific level and completeness of occlusion.
• Classically presents with pain out of proportion to physical examination findings (early ischaemia, before frank peritonism develops from full-thickness bowel wall necrosis).

Volvulus:

• Twisting of a segment of bowel (and its mesentery) around its own axis, causing both luminal obstruction and, critically, compromise of the mesenteric blood supply at the twist point, a surgical emergency given the risk of rapid bowel ischaemia/infarction.
• Midgut volvulus (a rotation around the SMA axis, classically associated with congenital malrotation and a shortened/narrow-based mesenteric root, predisposing the midgut to twist) is a distinct, particularly dangerous entity typically presenting in infancy, given the abnormally narrow root of mesentery attachment in malrotation compared to the normal long oblique root.

Internal herniation:

• Protrusion of a loop of small bowel through a peritoneal or mesenteric defect (congenital, or acquired post-surgically, e.g., after bariatric or other bowel resection surgery creating mesenteric defects) into a confined retroperitoneal or paramesenteric space.
• Carries a risk of bowel obstruction and strangulation given the confined, often narrow-necked nature of these defects, similar in principle to an external abdominal wall hernia but occurring entirely within the abdominal cavity, which can make diagnosis more challenging (no visible/palpable external bulge).