The human body uses various kinds of food for energy and growth. To be used, however, food must be changed into a form that can be carried through the bloodstream. The body's process of extracting useful nutrients from food is called digestion. The digestive system is a group of organs that perform the process by which food, containing nutrients, is eaten and broken down into different components. This breakdown makes it possible for the digested material to pass through the intestinal wall into the blood stream. The digestive process contains many different steps that take place in many different organs .
Human digestion, or the change that food undergoes in the digestive system, takes place in a long tube like canal called the alimentary canal, or the digestive tract. There is good reason why the passageway used by food to travel through the body is called the alimentary canal. Just as canals are constructed to guide ships through waterways to their destinations, the alimentary canal guides food as it travels through the human body. The whole canal is lined with a mucous membrane. .
Ingestion, which occurs in the mouth, is the first step of the digestive process. After food enters the mouth, the food is cut and chopped by the teeth. The tongue helps mix the food particles with a digestive juice called saliva, which is secreted in the mouth. Saliva moistens the food so it can be swallowed easily. It also changes some starches into simple sugars. From the mouth the food is swallowed into a transport tube, named the oesophagus, or gullet. A flap called the epiglottis closes the windpipe while food is being swallowed. Peristalsis, a wavelike muscular movement of the oesophagus walls, forces food down the tube to stomach. .
Peristalsis takes place throughout the digestive tract. It is an automatic, or involuntary, action, carried out in response to nerve impulses set up by the contents of the tube.