FUNCTIONS OF THE DIGESTIVE SYSTEM
Every cell of the body requires nutrients, yet most cells can-not leave their position in the body and travel to a food source. Therefore, the digestive system must help deliver food to them. The digestive system (figure 16.1), with the assistance of the circulatory system, is like a gigantic “meals on wheels,” serving over 100 trillion customers the nutrients they need. It also has its own quality control and waste disposal methods. Food is taken into the digestive system, where it is broken down into smaller and smaller particles. Enzymes in the digestive system break the particles down into very small molecules, which are absorbed into the circulation and transported all over the body. There, those molecules are broken down by other enzymes to release energy or are assembled into new molecules to build tissues and organs.
The functions of the digestive system include the following:
1. Ingestion of food. Food and water enter the body throughthe mouth.
2. Digestion of food. During the process of digestion, food isbroken down from complex particles to smaller molecules that can be absorbed.
3. Absorption of nutrients. The epithelial cells that line the lumenof the small intestine absorb the small molecules of nutrients (amino acids, monosaccharides, fatty acids, vitamins, minerals, and water) that result from the digestive process.
4. Elimination of wastes. Undigested material, such as fiberfrom food, plus waste products excreted into the digestive tract are eliminated in the feces.