Digestive system
Digestive systemDigestive System
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 of humans and other higher animals is the group of organs that changes food--carbohydrates, fats, and proteins--into soluble products that can be used by the body. Both mechanical action and chemical action are necessary to change food into products that are usable by the body.
Human digestion, or the change that food undergoes in the digestive system, takes place in a long tubelike 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.
Digestion begins in the mouth. Here 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.
It is important to chew food thoroughly to mix it well with saliva. Thorough chewing cuts food into small pieces that are more easily attacked by digestive juices. Food should not be washed down with quantities of liquid to avoid chewing.
From the mouth the food is swallowed into a transport tube, named the esophagus, or gullet. A flap called the epiglottis closes the windpipe while food is being swallowed. Peristalsis, a wavelike muscular movement of the esophagus walls, forces food down the tube to the 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. When digestion is working normally, a person is unaware of the movements of the gullet, stomach, and most of the intestine. Swallowing is a voluntary muscular action.
At the end of the esophagus there is a muscular valve, or sphincter, through which food enters the stomach. This sphincter muscle keeps food in the stomach from being forced back into the esophagus. Peristalsis in the stomach churns the food and mixes it with mucus and with gastric juices, which contain enzymes and hydrochloric acid. These gastric juices are secreted from millions of small glands in the lining of the upper stomach walls. These glands pour about three quarts of fluid into the stomach daily. Similar glands in the small intestine...
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