Chemical digestion is the process by which food is broken down and has most of its nutrients extracted. It is distinct from mechanical digestion, which is the physical breakdown of food as it is chewed.
Chemical digestion begins when food comes into contact with saliva. A specialized protein, salivary amylase, is present in saliva and acts to break down the sugars in food. Amylase enzymes work only in alkaline environments, so the initial chemical breakdown of sugars largely stops as chewed food passes into the stomach.
The stomach maintains an acidic environment, which is ideal for a second enzyme, pepsin, to go to work catalyzing the breakdown of proteins in the food via hydrolysis. Food remains in the stomach for a few hours while the acid and stomach enzymes reduce it to a thick soup of partially decomposed nutrients.
This material then passes out of the stomach and into the duodenum, where it is bathed in bile secreted by the gallbladder. Bile is alkaline, which deactivates the pepsin and other stomach enzymes, but permits a second exposure to amylase, this time from the pancreas, to work on the remaining sugars and starches in the food. As the mostly digested food passes into the small intestine, its components are small enough to be taken up by the hepatic portal vein and carried to the rest of the body.