Single-celled organisms can directly take in nutrients from their outside environment. Multi-cellular animals, with most of their cells removed from contact directly with the outside environment, have developed specialized structures for obtaining and breaking down their food.
The human digestive system is a complex series of organs and glands that processes food. It is a coiled, muscular tube (6-9 meters long when fully extended) extending from the mouth to the anus. Inside this tube is a lining called the mucosa. In the mouth, stomach, and small intestine, the mucosa contains tiny glands that produce juices to help digest food. This tube also includes the pharynx, esophagus, large intestine, and anus.
The digestive system is responsible for converting the food we eat into energy for our bodies to use. In order to use the food we eat, our body has to break the food down into smaller molecules that it can process; it also has to excrete waste. In other words, the food we eat consists of large lumps of material. We must bite off small pieces and chew them up into even smaller ones before swallowing them.
There are two types of digestion: Mechanical and chemical. The teeth carry out mechanical digestion. There are 4 types of teeth:
Incisors: 8 front teeth 4 on the top and 4 on the bottom. Shaped for biting and cutting.
Canines 4 teeth located on either side of the incisors. 2 on the top and 2 on the bottom. Shaped for tearing food.
Pre molars: 8 located behind cuspids. 4 on the top and 4 on the bottom. Shaped for crushing food.
Molars: 8 - Double rooted teeth with bumpy chewing surfaces. 4 on the top and 4 on the bottom. Shaped for grinding food.
The Process of Digestion:
The process starts at the mouth. Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starch into smaller molecules). The tongue... [continues]
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