To obtain nutrients the body needs to break down food and then transport the nutrients it provides to cells. Digestion breaks down food. Absorption brings the nutrients into the blood, so they can be transported to the cells where they are needed.
The digestive system is responsible for both digestion and absorption. The main part of this system is the gastrointestinal tract, also called the GI tract. This hollow tube starts at the mouth, where chewing breaks food into small pieces. From there, food passes down the esophagus into the stomach, and then on to the small intestine.
Rhythmic contractions of the smooth muscles that line the GI tract help mix food and move it along. Digestive enzymes in the gastrointestinal tract break down the nutrients in food. The digestive system also secretes hormones into the blood that help regulate GI activity.
Most of the digestion and absorption of nutrients occurs in the small intestine. Anything that is not absorbed passes into the large intestine. Here, water and small amounts of some other nutrients can be absorbed, and waste is prepared for elimination.
Once inside body cells, carbohydrates, lipids, and proteins are involved in chemical reactions that break them down to provide energy or use them to build other substances that the human body needs.
The sum of these chemical reactions that occur inside body cells is called metabolism. The chemical reactions of metabolism can synthesize the molecules needed to form body structures such as muscles, nerves, and bones.
The reactions of metabolism also break down carbohydrates, lipids, and proteins to yield energy in the form of ATP (adenosine triphosphate). ATP is a high-energy molecule that is used by cells as an energy source to do work, such as to pump blood, contract muscles, or synthesize new body tissue.
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