Digestion starts before the first bite, with the sight of food
Your stomach begins preparing for a meal the moment you see or smell it, releasing juices before anything reaches your mouth. From there, food travels a tube roughly nine metres long, squeezed along by rhythmic muscle waves, until what remains is waste and nearly everything useful has been absorbed.
Anatomists divide the system into the gastrointestinal tract itself and a set of helpers outside it: the tongue, salivary glands, liver, gallbladder and pancreas, with the teeth and the epiglottis sometimes counted too. The tract runs from mouth to anus, and rings of muscle called sphincters, such as the pyloric sphincter at the stomach's exit, control the flow between sections.
The work happens in three phases. The cephalic phase is triggered in the head: sight and smell prompt gastric glands to secrete, and in the mouth chewing breaks food up while saliva adds enzymes, including amylase for starch and lingual lipase for fat. The result is a soft ball called a bolus, which is swallowed down the oesophagus. In the gastric phase the stomach churns it with gastric juice, produced by millions of glands in its lining, while mucus protects the organ from its own acid. The intestinal phase begins in the duodenum, where pancreatic enzymes finish the chemical breakdown.
Movement relies on peristalsis, waves of contraction that start in the oesophagus and continue through the gut, together with segmentation, which mixes the contents. Food becomes a slurry called chyme, and the small intestine does most of the digesting and absorbing, even passing some material into the lymphatic system as a milky fluid named chyle. Oddly, the small intestine is the longest section of the tract yet narrower than the large intestine that follows. The large intestine recovers water and minerals and stores the leftovers until they are expelled, which is why it also counts as part of the excretory system.
Even the mouth is more specialised than it looks. It has a vestibule between the teeth and cheeks and an inner oral cavity, lined with a membrane whose mucus is built from a glycoprotein called mucin. That slippery, clinging coating lubricates food and helps guard teeth against decay.
Source: Human digestive system
