smallintestinal

The small intestine, or small intestinal tract, is the longest part of the digestive system, extending from the pyloric sphincter to the ileocecal valve and measuring about 6 meters in adults. It comprises three regions: the duodenum, the jejunum, and the ileum. The duodenum receives chyme from the stomach along with bile and pancreatic secretions; the jejunum is the principal site of nutrient absorption, while the ileum reabsorbs bile acids and vitamin B12.

Anatomy and histology: the mucosal surface is highly folded and lined with villi and microvilli, increasing surface area for digestion and absorption. Enterocytes line the epithelium and bear brush-border enzymes that complete carbohydrate and peptide digestion. The small intestine also houses immune tissue, including Peyer's patches, contributing to gut immunity.

Physiology: enzymatic digestion continues with pancreatic enzymes and brush-border enzymes. Absorption involves transport of monosaccharides, amino acids, fatty acids and monoglycerides, vitamins, and minerals into enterocytes, then into either the portal circulation or the lymphatic system. The ileum uniquely reabsorbs bile acids and vitamin B12 bound to intrinsic factor.

Motility and circulation: peristalsis and segmentation move chyme along; the intestinal wall is supplied by branches of the superior mesenteric artery and drained via the portal venous system. The small intestine hosts the gut microbiota in the distal segments, influencing digestion and immunity but is less colonized than the large intestine.

Clinical relevance: diseases affecting the small intestine include malabsorption syndromes (such as celiac disease), Crohn's disease, and surgical resections leading to short bowel syndrome. Diagnostic approaches include endoscopy, imaging, biopsy, and tests of nutrient absorption.