Digestion is the way that an organism changes a substance into nutrients. This happens in the gastrointestinal system. Humans start digesting food in the mouth. Food is chewed by the teeth. Food is swallowed, which means it goes through the esophagus. It goes into the stomach, where it is mixed with acid.
Digestion occurs in three phases. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces which can be got at by digestive enzymes. In chemical digestion, enzymes break down food into the small molecules the body can use. Finally, the nutrients are absorbed into the blood stream. Once in the blood stream the nutrients are taken to the liver, which is a kind of chemical factory for the body.
After we swallow food, it travels down a muscular tube to the stomach. There, it is mashed into a mixture like soup. The mixture passes into the small intestine, where tiny bits of food pass into the bloodstream. The food that is still left goes into the large intestine. Finally, waste products leave the body. Digestion usually takes about 18 hours. Food stays in the stomach for about three hours. If uncoiled, the small intestine would be about six meters (20 feet) long. Many digestive tracts are about as long as a bus.
Food slowly enters the small intestine from the stomach. This is where nutrients are taken into the blood. It then enters the large intestine. Water is taken away from it. The food that is left is called feces. The feces are stored in the rectum until the waste can leave the body through the anus.
When we eat, our body has to “digest” the food so we can get the vitamins, minerals, and other nutrients that are in food. We digest our food in the “gastro-intestinal system”. Digestion occurs in three main ways:
- We chew our food, and our teeth break down the food into smaller pieces. Also, the liquid in our mouths (saliva) causes a chemical reaction that starts to digest the food.
- In our stomach, chemicals called “enzymes” change the food into smaller molecules that the body can use.
- After the food is changed into small molecules, the blood carries the molecules to other parts of the body.
Now let’s follow the food all through the body:
- We chew the food, and it changes into smaller pieces.
- The saliva in our mouths contains molecules called “enzymes” that also start to break down the food into smaller pieces.
- We swallow the food and it goes to the stomach.
- In the stomach, chemical reactions change the food into liquid (like thick soup) for one or two hours, and this liquid flows into the small intestine.
- In the small intestine, our bodies take (“absorb”) the nutrients from our food, so they can travel around the body. The small intestines are about 6 meters long, so nutrients are absorbed for a long time. The things that are not absorbed are called “solid waste material”; this waste material goes to the large intestine.
- The large intestine mixes the solid waste material with water so we can easily eliminate it from our bodies.
- The solid waste material stays in the rectum until we go to the toilet. Then, this material leaves our bodies through the anus.