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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 from the salivary glands. Then it travels down the esophagus into the stomach, where acid both kills most contaminating microorganisms and begins mechanical break down of some food (eg denaturation of protein), and chemical alteration of some. After some time (typically an hour or two in humans, 4-6 hours in dogs, somewhat shorter duration in house cats), the results go through the small intestine, through the large intestine, and are excreted during defecation.Other organisms use different mechanisms to digest food.