Small intestine

Anatomy and regions

The organ is conventionally divided into three continuous regions with distinct roles: a short proximal segment, the duodenum, followed by the middle jejunum, and the distal ileum. The duodenum receives acidic stomach contents and secretions from the pancreas and liver that neutralize acid and provide digestive enzymes and bile. The jejunum is specialized for rapid absorption of many nutrients, while the ileum completes absorption and reclaims specific molecules such as vitamin B12 and bile salts.

Structure that enables function

The inner surface is highly folded and covered with villi and microvilli, microscopic projections that enormously expand surface area and create a brush border of enzymes and transporters. These adaptations turn the lining into an efficient interface for moving digested molecules into blood and lymph. Scattered areas of immune tissue, including Peyer’s patches, monitor gut microbes and contribute to local immune responses.

• Major functions: chemical digestion, nutrient and water absorption, immune surveillance.
• Physical features: long tubular shape, narrow diameter relative to the large intestine, mucosal folds, villi and microvilli.

During embryonic development the small intestine arises mainly from the midgut and undergoes a complex rotation and elongation to reach its adult position. Disorders of development, inflammation (for example coeliac disease or Crohn’s disease), mechanical blockage, infections or surgical loss can impair its function and lead to malabsorption or obstruction.

Compared with the large intestine, the small intestine is longer but narrower, has a much larger absorptive surface due to villi, and normally contains fewer bacteria. Its central role in nutrition, fluid balance and host defence makes it a frequent focus of medical diagnosis, dietary intervention and surgical treatment.