Overview of Small Intestine Anatomy and the Three Divisions
The small intestine measures approximately 20 feet long in adults, though length varies between individuals. It starts at the pyloric sphincter of the stomach and ends at the ileocecal valve where it meets the large intestine.
The Three Segments
The small intestine divides into three distinct regions:
- Duodenum: shortest at 10 inches
- Jejunum: middle section at approximately 8 feet
- Ileum: longest at approximately 12 feet
Structural Support and Surface Area
The mesentery is a fold of tissue that anchors the small intestine to the posterior abdominal wall. It allows movement while keeping the intestine properly positioned.
All three segments have simple columnar epithelium with specialized cells for absorption and secretion. The intestinal lining forms villi and microvilli that dramatically increase surface area. The total absorptive surface area reaches approximately 250 square meters when accounting for all microscopic structures.
Embryological Origins
The duodenum develops from foregut endoderm, while the jejunum and ileum develop from midgut endoderm. This distinction matters clinically. Meckel's diverticulum, a common birth defect, occurs in the ileum due to incomplete midgut development.
The Duodenum: Structure, Function, and Clinical Significance
The duodenum is the first and shortest portion of the small intestine. Its name comes from Latin meaning twelve, referring to its 12-finger-width length.
The Four Parts
The duodenum has four sections that form a C-shape wrapping around the pancreas:
- Superior (first) part - mobile, covered by peritoneum
- Descending (second) part - fixed retroperitoneal position
- Horizontal (third) part - retroperitoneal
- Ascending (fourth) part - retroperitoneal
Primary Functions
The duodenum receives chyme (partially digested food) from the stomach. This segment focuses on chemical digestion facilitated by pancreatic enzymes and bile salts. It has minimal absorptive capacity compared to the jejunum and ileum.
Key Anatomical Features
The major duodenal papilla (ampulla of Vater) is located in the descending portion's medial wall. This is where the pancreatic duct and common bile duct enter. Obstruction at this site can cause pancreatitis.
Brunner's glands in the duodenal lining secrete alkaline mucus. This mucus neutralizes stomach acid and protects the duodenal tissue from acid damage. These glands are unique to the duodenum.
The superior mesenteric artery and branches from the gastroduodenal artery provide rich blood supply. This vascular connection is important during pancreatic surgery.
The Jejunum: Structural Characteristics and Absorption Functions
The jejunum is the middle segment, comprising about 40 percent of the small intestine's length. Its Latin name means empty, referring to how it appeared in early cadaver studies.
Distinctive Structural Features
The jejunum starts at the duodenojejunal flexure at approximately the L2 vertebra level. It has a wider diameter and thicker walls than the ileum. The plicae circulares (circular folds) are larger and more numerous, increasing surface area for absorption.
The jejunum's mesentery is thicker with larger blood vessels and fewer branches. The vascular arcade (branching pattern) is simpler compared to the ileum. This creates a distinct appearance on X-ray imaging.
Lymphoid Tissue Differences
The jejunum contains fewer Peyer's patches and lymphoid aggregates compared to the ileum. However, lymphoid tissue remains present for immune function.
Primary Absorption Site
The jejunum absorbs amino acids, monosaccharides, and fats most efficiently. Tall, columnar epithelial cells with well-developed microvilli contribute to superior absorptive capacity. This is where most nutrient uptake occurs.
The Ileum: Terminal Segment and Immune Function
The ileum is the longest segment, comprising 50 to 60 percent of the small intestine's total length. It extends from the jejunum to the ileocecal valve at the colon junction.
Structural Characteristics
The ileum has a narrower diameter and thinner walls than the jejunum. The circular folds are smaller and less prominent, decreasing in frequency near the ileocecal valve.
The ileum's mesentery is longer and more mobile. It contains complex vascular arcades with multiple branching tiers and smaller terminal arteries. This creates the ileum's greater flexibility.
The Ileocecal Valve
The ileocecal valve marks the junction with the large intestine. It prevents colonic contents from refluxing back into the ileum. Thickened mucosa and circular muscle form this valve structure.
Abundant Immune Tissue
The ileum contains extensive Peyer's patches, organized as aggregated lymphoid follicles. These function as part of GALT (gut-associated lymphoid tissue) for immune surveillance. The ileum's abundance of lymphoid tissue reflects its role in monitoring intestinal contents.
Specialized Absorption
The ileum absorbs vitamin B12 and bile salts specifically. No other intestinal segment effectively compensates for these functions. The terminal ileum's receptor-mediated uptake mechanisms are unique.
The ileum is a common site for Crohn's disease, making understanding this anatomy clinically important. The wall includes all four layers: mucosa, submucosa, muscularis, and serosa.
Histological Features, Vascularization, and Clinical Study Considerations
All three intestinal segments share the same basic four-layer structure but have significant microscopic differences.
Shared Wall Structure
Each segment contains:
- Mucosa: epithelial tissue, lamina propria, muscularis mucosae
- Submucosa: connective tissue and glands
- Muscularis: circular and longitudinal smooth muscle
- Serosa: peritoneal visceral layer
Histological Distinctions
The duodenum has Brunner's glands exclusively in its submucosa. These decrease in frequency through the segments and disappear entirely in the ileum.
The jejunum and ileum lack Brunner's glands but contain intestinal glands (crypts of Lieberkühn). These produce alkaline secretions containing enterokinase and other enzymes.
All segments contain multiple epithelial cell types: absorptive enterocytes, goblet cells (producing mucus), Paneth cells (secreting antimicrobial peptides), and enteroendocrine cells (producing regulatory hormones).
Blood Supply Pattern
The superior mesenteric artery is the primary blood source. It divides into branches supplying each segment. The duodenum receives additional blood from the celiac trunk via the gastroduodenal artery, creating anastomotic networks.
The jejunum and ileum receive blood through arcading arteries within the mesentery. Jejunal branches create fewer arcades than ileal branches.
Venous Drainage and Lymphatics
Venous drainage follows arterial patterns, flowing into the superior mesenteric vein and portal circulation. Lacteals in the villi absorb dietary fats into mesenteric lymph nodes. Lymph eventually reaches the cisterna chyli.
Neural Control
The myenteric plexus controls peristalsis. The submucosal plexus regulates secretion. Both intrinsic and extrinsic nervous systems coordinate intestinal function.