Gut

The gut, or gastrointestinal tract, is the tubular system that digests food, absorbs nutrients, and eliminates waste. In humans it extends from the mouth to the anus and includes the esophagus, stomach, small intestine, and large intestine, with related organs such as the liver, pancreas, and gallbladder connected to it but not part of the tract itself.

Anatomy and function: The lining of the gut comprises layers that support digestion and protection. The small intestine—the duodenum, jejunum, and ileum—primarily handles digestion and nutrient absorption, while the large intestine—the cecum, colon, and rectum—absorbs water and forms solid feces. The gut wall contains the enteric nervous system, often called the second brain, and a rich supply of immune tissue.

Digestion and absorption: The gut performs mechanical processing through movements such as peristalsis and segmentation, and chemical digestion with enzymes, acids, and bile. Nutrients, water, and electrolytes are absorbed mainly in the small intestine, while the large intestine concentrates and reabsorbs water. The mucosal barrier protects against pathogens while coordinating immune responses.

Gut microbiota and signaling: The gut hosts a complex microbial community, the microbiota, which metabolizes dietary components, produces vitamins and short-chain fatty acids, and modulates immune function. Microbial activity interacts with the nervous system via the gut-brain axis, influencing digestion, metabolism, and potentially mood.

Clinical relevance: Disorders affecting the gut include gastroesophageal reflux, irritable bowel syndrome, inflammatory bowel disease, coeliac disease, and infections. Diet, medications, antibiotics, and lifestyle can alter gut function and microbiota composition, with implications for overall health.