Chemical digestion is the process through which the body of a mammal reduces food to a size where the nutrients that it contains can be absorbed into the bloodstream. The process begins in the mouth, continues in the stomach and ends when the digested particles move through the small intestine for absorption. Proteins in the mouth, stomach and intestine called enzymes are the primary facilitators of chemical digestion.
In the mouth, chemical digestion begins when the food enters and encounters saliva and the enzymes that it contains. The teeth assist in this process by providing mechanical digestion. After the food has been sufficiently broken down, it is swallowed and travels to the stomach through the esophagus.
When the food reaches the stomach, different hormones trigger the body to begin releasing enzymes that further digest the food. Pepsin is the name of the enzyme that breaks down most of the proteins in the food. The digestion of carbohydrates, which began in the mouth with an enzyme called amylase, also continues here. This stage of digestion can take several hours depending on the species of animal, but in humans, it generally takes one to two hours, after which the food is moved into the small intestine.
After leaving the stomach, the food is now a thick liquid called chyme. The chyme moves through the small intestines, where it is exposed to several different liquids. The most important of these is called bile. Bile changes fats in the chyme into a form that allows their digestion. During this stage of the digestive process, the small intestine is also absorbing more nutrients from the chyme and introducing them into the bloodstream.
The very last stage of chemical digestion occurs in the final area of the small intestine, called the ileum. In this area, two enzymes called protease and carbohydrase are released, and they complete the digestion of proteins and carbohydrates. As what is left of the originally swallowed food leaves the small intestine, it enters into the large intestine, where the digestion process ends.
Although mostly mammals follow this exact process, certain other animals do employ chemical digestion to extract nutrients from their food. Depending on the actual type of animal, the process might employ different enzymes or occur within different internal organs. The exact nature of non-mammalian chemical digestion varies from species to species.