The first portion of food digestion actually happens in the mouth. A person’s teeth and saliva both work together to begin the process of breaking down food. They also generally help get everything down to a size and consistency that can be easily handled by the rest of the system. After that, muscles push the food through the esophagus into the stomach, where acids and other substances turn the food into a mush called chyme. The small intestine takes the resulting mush created in the stomach and absorbs nutrients from it, after which it is passed into a person's large intestine where it is turned into feces before being expelled from the anus.
Some scientists compare the human body to a machine, and digestion could be considered the way it processes energy. Food is full of energy that can potentially be used to keep the body running, but that energy is not readily accessible without turning it into something that can be easily absorbed. People digest different nutrients at different speeds using slightly different processes, and each nutrient is used in its own way. Protein is primarily used to repair the body while carbohydrates and fat are used primarily as fuel.
The chewing process is more organized than many people realize, and it is generally a crucial part of food digestion. The saliva itself has many chemicals in it that can digest food, and it also helps moisten the food so that it can be easily swallowed. When people chew, they actually form food into small chunks that are perfectly sized to enter the stomach easily. When the food enters the stomach, food digestion is already well underway, partly because it is saturated with the digestive chemicals of saliva before the stomach acid even begins working on it.
A person’s stomach has muscles that squeeze food and churn it, which is vital to the food digestion process. Foods that take longer to digest, such as protein, tend to stay in the stomach for much longer than faster-digesting foods like carbohydrates. The natural digestion process of the stomach causes this to happen without any special mechanisms. The stomach also has the capacity to absorb some nutrients, although that is not its primary job.
The small intestine is fairly long and shaped like a tube. It helps begin the process of forming food into something that can be expelled later. It’s also the main place where nutrients are absorbed.
Several important digestive juices reach the small intestine from organs like the liver and pancreas, and these help finish the food digestion process. Once food exits the small intestine, it is basically through with digesting. It simply needs to be packaged for expulsion as feces.