Gastrointestinal tract

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Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large 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. This is called mastication. Then it travels down the esophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food (e.g., denaturation of protein), 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 humans, 4–6 hours in dogs, somewhat shorter duration in house cats, ...), the resulting thick liquid is called chyme. Chyme will go through the small intestine, where 95% of absorption of nutrients occurs, through the large intestine, and are excreted during defecation.[1]

Other organisms use different mechanisms to digest food.

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