Digestion

Digestion is the breakdown of food into small molecules that the body can absorb and use for energy, growth and repair. It takes place in the gastrointestinal tract, a continuous tube from the mouth to the anus that includes the esophagus, stomach and small and large intestines, with help from accessory organs: the teeth, tongue, salivary glands, liver, gallbladder and pancreas.

Digestion has two parts. Mechanical digestion breaks food into smaller pieces without changing it chemically, through chewing, the churning of the stomach and segmentation, the back-and-forth mixing movements of the small intestine. Chemical digestion uses enzymes to split large molecules by hydrolysis. Amylases break starch into sugars, pepsin from the stomach and the pancreatic enzymes trypsin and chymotrypsin break proteins down toward amino acids, and lipases split fats into fatty acids after bile from the liver has emulsified them into small droplets. Most chemical digestion, and nearly all absorption, happens in the small intestine, whose folds, villi and microvilli give it an enormous surface.

Waves of muscle contraction called peristalsis move the contents along. The large intestine absorbs much of the remaining water and houses gut bacteria. Nerves in the gut wall and hormones such as gastrin, secretin and cholecystokinin coordinate the whole process.