Intestines

The absorptive engine. Six meters of small intestine harvest nearly every nutrient molecule, while the colon recovers water and farms its microbiome.

Primary Role

Nutrient digestion and absorption

Metabolic Pathways

Brush-border hydrolysis, transporter-mediated uptake

Energy Budget

About 250 square meters of absorptive surface

Key Cell Types

Absorptive cells, Goblet cells, Enteroendocrine cells

Structure & Metabolism

Duodenum, jejunum, ileum, then colon — each with a niche. Villi and microvilli amplify the small bowel's surface to a tennis court; the ileum alone reabsorbs bile salts and absorbs B12; the colon's goblet-cell mucus and absorbing epithelium convert liquid chyme to stool. Renewal is the tissue's signature: the crypts rebuild the lining every few days.

Metabolic Pathways

• Terminal digestion: disaccharidases and peptidases at the brush border finish what trypsin began.
• Carrier-mediated uptake: SGLT1 and GLUT5 for sugars, PepT1 for peptides, bile-acid transporters in the ileum.
• Colonic fermentation: microbes convert fiber to short-chain fatty acids the colon itself burns.

Regulation

• Migrating motor complexes sweep between meals; segmentation mixes during them.
• GLP-1 and CCK from enteroendocrine cells pace secretion and satiety.
• Colon water absorption follows sodium gradients — the physiology cholera defeats.

Interactions

• Portal blood delivers everything absorbed straight to the liver.
• Its hormones are the incretin system's source — see insulin-glucose homeostasis.
• The microbiome is a metabolic organ resident here: vitamin K, B vitamins, fiber breakdown.

Clinical Relevance

• Celiac disease, Crohn, and short-bowel syndrome are the absorptive failures.
• Colorectal cancer from crypt stem cells is screening medicine's staple.
• Microbiome science — from fiber to fecal transplant — centers on this organ.
• Transit through the small bowel takes hours; through the colon, a day or more — the timing that defines constipation and diarrhea.
• The gut holds the body's largest immune mass: tolerance to food and flora is an active, learned process.
• Ileal resection predicts B12 deficiency years ahead — the absorption site cannot be bypassed.

Sources

• Wikipedia: Intestine
• Wikipedia: Small intestine