Small Intestine Anatomy: Duodenum, Jejunum, Ileum, and Absorptive Surface Design

Proximal-to-Distal Overview: Where the Small Intestine Fits

The small intestine begins at the pylorus and ends at the ileocecal junction. Anatomically, it is divided into three segments—duodenum, jejunum, and ileum—each with recognizable external features and internal surface adaptations that support digestion and absorption.

Duodenum: Parts, Peritoneal Status, and the Pancreatic “C-Loop” Relationship

Four parts of the duodenum (D1–D4)

The duodenum is the first segment of small intestine and is classically described in four parts. Learning it in order helps you predict what structures lie against it and where secretions enter the lumen.

• First (superior) part (D1): short segment immediately distal to the pylorus. Clinically important because it is a common site of peptic ulceration. It transitions from a more mobile proximal portion to more fixed distal portions.
• Second (descending) part (D2): descends along the right side of the vertebral column. This is the key segment for entry of bile and pancreatic secretions.
• Third (horizontal/inferior) part (D3): crosses the midline from right to left, typically at a lower level than D1–D2. It is relatively fixed and lies deep in the abdomen.
• Fourth (ascending) part (D4): ascends to meet the jejunum at the duodenojejunal flexure, a consistent transition point into the more mobile small bowel.

Peritoneal status (practical framing)

In practical anatomy, the duodenum is mostly fixed compared with jejunum and ileum. The proximal portion of D1 is more mobile, while the majority of the duodenum is relatively immobile and closely applied to posterior abdominal structures. This “fixed” nature is a useful clue during imaging interpretation and surgery: the duodenum does not drape and shift like the jejunal and ileal loops.

The C-loop and the pancreas: a spatial relationship you can visualize

The duodenum forms a C-shaped curve that embraces the head of the pancreas. Think of the pancreatic head sitting in the concavity of the duodenal C-loop. This relationship matters because the duodenum is not just a conduit—it is the receiving chamber for secretions that complete chemical digestion.

• D2 is the key receiving segment: bile and pancreatic enzymes enter the duodenum here.
• Pancreatic head adjacency: because the pancreatic head is nestled in the C-loop, pathology in one structure can affect the other (e.g., swelling near the pancreatic head can influence duodenal passage).

Entry point of bile and pancreatic secretions

Bile and pancreatic secretions enter the duodenum through the hepatopancreatic ampulla (ampulla of Vater) at the major duodenal papilla (typically on the medial wall of D2). The sphincter of Oddi regulates flow into the lumen.

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Step-by-step: tracing the path of secretions into the duodenum

1. Locate D2 conceptually as the descending segment of the duodenal C-loop.
2. Identify the medial wall of D2 as the typical site of the major duodenal papilla.
3. Follow the ducts: common bile duct and main pancreatic duct converge (commonly) to form the hepatopancreatic ampulla.
4. Pass through the sphincteric control: the sphincter of Oddi regulates entry into the duodenal lumen.
5. Result in the lumen: bile and pancreatic juice mix with chyme, setting up efficient absorption downstream.

Jejunum vs Ileum: How to Tell Them Apart Using Reliable Anatomical Criteria

Jejunum and ileum are continuous, but they differ in gross appearance and vascular pattern. When you are handed a segment of small bowel (in lab, surgery, or imaging), use multiple criteria together rather than relying on a single feature.

Comparison table: high-yield distinguishing features

Practical step-by-step: identifying jejunum vs ileum on a specimen

1. Look at the mucosal folds (if visible): prominent, tall plicae suggest jejunum; flatter/sparser folds suggest ileum.
2. Assess wall feel and thickness: a thicker, more robust wall favors jejunum; a thinner wall favors ileum.
3. Inspect the mesentery near the bowel: if fat stops short and you can see translucent “windows,” think jejunum; if fat approaches the bowel edge, think ileum.
4. Check the arterial pattern: fewer arcades with long straight vasa recta suggests jejunum; multiple arcades with short vasa recta suggests ileum.
5. Use at least two criteria before deciding: mixed features can occur near the transition zone.

Absorptive Surface Design: Plicae, Villi, and Microvilli as Anatomical Specializations

The small intestine is engineered to maximize contact between luminal contents and absorptive epithelium. Three nested surface structures increase surface area at different scales.

1) Plicae circulares (circular folds)

Plicae circulares are macroscopic, permanent folds that project into the lumen. They slow the movement of chyme and increase the surface available for absorption. They are most prominent in the jejunum and become less prominent toward the distal ileum.

• Practical implication: more prominent plicae correlate with a “busier,” more folded lumen on imaging or gross inspection in proximal small bowel.

2) Villi

Villi are finger-like projections of the mucosa that dramatically expand the absorptive surface. Each villus contains a core with blood capillaries (for absorbed sugars and amino acids) and a central lymphatic channel (lacteal) for absorbed lipids.

• Practical example: after a fatty meal, lipids are packaged and transported via lymphatics from villi, while glucose enters blood capillaries—two transport routes housed within the same villus structure.

3) Microvilli (brush border)

Microvilli are microscopic projections on the apical surface of absorptive epithelial cells. They form the brush border and provide an enormous increase in surface area, while also hosting enzymes and transporters that finalize digestion and enable uptake.

• Practical example: disaccharides are broken down at the brush border, and the resulting monosaccharides are transported into enterocytes via specific membrane transporters.

Step-by-step: linking structure to function during a meal

1. Chyme enters the duodenum and mixes with bile and pancreatic secretions.
2. Plicae circulares slow and swirl contents, increasing time and contact with the mucosa.
3. Villi provide a dense field of absorptive projections; nutrients cross into capillaries or lacteals.
4. Microvilli complete final digestion at the cell surface and drive absorption through membrane transport mechanisms.

Ileocecal Junction: The Transition Landmark to the Large Intestine

The ileocecal junction marks the end of the small intestine and the beginning of the large intestine at the cecum. It is a key anatomical landmark for orienting distal small bowel and for understanding where small-bowel contents enter the colon.

Key anatomical features

• Terminal ileum: the distal segment approaching the cecum; its external features often match the “ileum pattern” (thinner wall, more mesenteric fat near the border, more arcades with shorter vasa recta).
• Ileocecal valve: a mucosal/structural transition that regulates passage into the cecum and helps limit backflow from the large intestine into the ileum.
• Cecal entry point: the ileum enters the cecum on its medial aspect, providing a consistent regional landmark when identifying right lower quadrant anatomy.

Practical step-by-step: confirming you are at the ileocecal junction

1. Find the cecum as a large-intestine pouch in the right lower quadrant region.
2. Trace a small-bowel loop distally until it meets the cecum—this is the terminal ileum.
3. Look for the transition in caliber and wall appearance: small bowel to larger bowel with different external contouring.
4. Identify the valve region at the entry point, marking the ileocecal junction as the endpoint of the small intestine.