The human intestines are essential components of the digestive system, involved in the digestion, absorption of nutrients, and excretion of waste. They are divided into the small intestine and the large intestine, each with distinct functions. The small intestine, about 20 feet long, is where most digestion and absorption occur and is divided into three sections: the duodenum, jejunum, and ileum. In the duodenum, chyme from the stomach mixes with bile and pancreatic juices, aiding digestion. The jejunum, with its numerous folds and villi, is the primary site for nutrient absorption, while the ileum absorbs bile acids and vitamin B12. Digestion in the small intestine involves breaking down carbohydrates, proteins, and fats into simpler molecules, which are then absorbed into the bloodstream or lymphatic system.

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The large intestine, approximately 5 feet long, absorbs water and electrolytes from indigestible food matter and forms feces. It comprises the cecum, colon, and rectum. The colon, with its ascending, transverse, descending, and sigmoid parts, hosts a large bacterial population that aids in breaking down waste and producing vitamins like vitamin K and B vitamins. The rectum stores feces until expulsion through the anus. The movement of contents through the intestines is regulated by peristalsis and segmentation, coordinated by the enteric nervous system. Mucus secreted along the intestinal lining lubricates and protects it, while hormones like secretin and cholecystokinin regulate digestive processes.

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The large intestine's microbiota play a crucial role in digestion, immune function, and the production of short-chain fatty acids from undigested carbohydrates. Conditions affecting intestinal function include inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), malabsorption syndromes like celiac disease, and lactose intolerance, as well as colon cancer. Understanding the operation of the intestines is vital for maintaining gastrointestinal health and managing related diseases.