Kids Research Express

. If a human adult’s digestive tract were stretched out, it would be 6 to 9 m (20 to 30 ft) long. In humans, digestion begins in the mouth, where both mechanical and chemical digestion occur. The mouth quickly converts food into a soft, moist mass. The muscular tongue pushes the food against the teeth, which cut, chop, and grind the food. Glands in the cheek linings secrete mucus, which lubricates the food, making it easier to chew and swallow. Three pairs of glands empty saliva into the mouth through ducts to moisten the food. Saliva contains the enzyme ptyalin, which begins to hydrolyze (break down) starch—a carbohydrate manufactured by green plants. Once food has been reduced to a soft mass, it is ready to be swallowed. The tongue pushes this mass—called a bolus—to the back of the mouth and into the pharynx. This cavity between the mouth and windpipe serves as a passageway both for food on its way down the alimentary canal and for air passing into the windpipe. The epiglottis , a fl...

Digestive System, series of connected organs whose purpose is to break down, or digest, the food we eat. Food is made up of large, complex molecules, which the digestive system breaks down into smaller, simple molecules that can be absorbed into the bloodstream. The simple molecules travel through the bloodstream to all of the body's cells, which use them for growth, repair, and energy. All animals have a digestive system, a feature that distinguishes them from plants. Plants produce their own food in a process called photosynthesis, during which they use sunlight to convert water and carbon dioxide into simple sugars. But animals, including humans, must take in food in the form of organic matter, such as plants or other animals. Digestion generally involves two phases: a mechanical phase and a chemical phase. In the mechanical phase, teeth or other structures physically break down large pieces of food into smaller pieces. In the chemical phase, digestive chemicals called enzymes b...