Digestion is the mechanical and chemical breaking down of food into smaller components that can be absorbed into a blood stream. Digestion is a form of catabolism: a break-down of larger food molecules to smaller ones.
Food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the oesophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food, and chemical alteration of some. The hydrochloric acid also has a low pH, which is great for enzymes. After some time (typically an hour or two in human) the results go through the small intestine, through the large intestine, and are excreted during defecation.
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