Ingestion, absorption, assimilation, and excretion are the four main processes of the digestive system in humans and other animals.
Ingestion is the process of taking food into the body through the mouth. This food is then mechanically and chemically broken down in the mouth, stomach, and small intestine through the process of digestion.
Absorption is the process of taking the nutrients from the digested food and moving them into the bloodstream. This occurs in the small intestine, where the nutrients are absorbed into the villi, which are small finger-like projections that line the walls of the small intestine.
Assimilation is the process of using the absorbed nutrients to build and repair tissues, produce energy, and carry out other bodily functions. The absorbed nutrients are transported by the bloodstream to the liver, where they are processed and distributed to the rest of the body.
Excretion is the process of removing waste products from the body. This includes undigested food, fiber, and other waste materials that are not absorbed by the body. The waste products are eliminated from the body through the anus as feces.
For example, when a person eats a sandwich, the process of ingestion begins in the mouth, where the sandwich is chewed and mixed with saliva. The food is then swallowed and travels down the esophagus to the stomach, where it is mixed with stomach acid and enzymes to break it down further.
The partially digested food then moves to the small intestine, where the nutrients are absorbed into the bloodstream. The nutrients are then transported to the liver, where they are processed and distributed to the rest of the body.
The undigested food and waste products then move to the large intestine, where water is absorbed and the feces are formed. Finally, the feces are eliminated from the body through the anus as waste.