Digestion is the mechanical and chemical breaking down of food into smaller components, to a form that can be absorbed, for instance, into a blood stream. Digestion is a form of catabolism; a break-down of macro food molecules to smaller ones.

In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva via the saliva glands. Then it travels down the esophagus into the stomach, where acid both kills most contaminating microorganisms and begins mechanical analysis of some foods and chemical alteration of some. After some period of time; the results go through the small intestine, through the large intestine, and are excreted during defecation.

Other organisms use different mechanisms to digest food.

Digestive systems take many forms. Some organisms, including nearly all spiders, simply secrete biotoxins and digestive enzymes; into the extracellular environment prior to ingestion of the consequent “soup”. In others, digestion can be conducted to a vesicle or a sac-like structure, through a tube(stomach), or through several specialized organs aimed at making the absorption of nutrients more efficient.
In most vertebrates, digestion is a multi-stage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:

1. Ingestion: placing food into the mouth (entry of food in the digestive system),

2. Mechanical and chemical breakdown: mastication and the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures,

3. Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and

4. Egestion: Removal of undigested materials from the digestive tract through defecation.

Underlying the process is muscle movement throughout the system through swallowing and peristalsis. Each step in digestion requires energy, and thus imposes an “overhead charge” on the energy made available from absorbed substances. Differences in that overhead cost are important influences on lifestyle, behavior, and even physical structures. Examples may be seen in humans, who differ considerably from other hominids (lack of hair, smaller jaws and musculature, different dentition, length of intestines, cooking, etc).

The major part of digestion takes place in the small intestine. The large intestine primarily serves as a site for fermentation of indigestible matter by gut bacteria and for resorption of water from digesta before excretion.

In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. Absorption occurs in the stomach and gastrointestinal tract, and the process finishes with defecation.