Structure and Function of the Digestive System

The digestive system consists of the organs that make up the alimentary canal, or digestive tract, from the mouth to the anus (Fig. 16-1). The digestive tract is about 30 feet long. It is not a sterile system like the other internal organs because it is open to the outside environment at both ends.

The main function of the digestive system, also called the gastrointestinal system, is to break down food to a form that can be used by body cells. The digestive process includes transportation of food and wastes, physical and chemical breakdown, absorption of digested food, and final elimination of wastes (Table 16-1). It also helps to maintain the proper amount of water, electrolytes, and other nutrients in the body.

Food enters the alimentary canal at the mouth (ingestion). The teeth bite and chew the food to begin its physical breakdown. The tongue aids in tasting, chewing (mastication), and swallowing (deglutition) of food. The hard palate is the anterior roof of the mouth. Unlike the hard palate, the soft palate tissue is not attached to bone on the posterior portion of the mouth. The uvula is a small piece of tissue at the rear of the mouth that prevents food from entering the nasal cavity during swallowing.

As food is chewed it is mixed with saliva. Three salivary glands secrete an enzyme (amylase) that begins the chemical portion of the digestive process. An enzyme is a protein that increases the rate of a chemical activity in the body. The three salivary glands are the parotid, sublingual, and submandibular (Fig. 16-2). Amylase starts the transformation of starch to sugar. The portion of food mixed with saliva that is swallowed is called a bolus.

Small Intestine

From the stomach, the food enters the small intestine (Fig. 16-3). The small intestine is longer and narrower than the large intestine. It is lined with tiny threadlike projections of tissue called villi (singular, villus) that increase the area for absorption of nutrients. The three sections of the small intestine are the duodenum, jejunum, and ileum. The small intestine produces juices to aid the digestive process. Most absorption of digestive products occurs in the small intestine. When digestion is completed, carbohydrates have been reduced to sugar (monosaccharide and disaccharide). Protein has been changed to amino acid and dipeptide. Fats have been reduced to fatty acid and glyceride.

Brain Byte

Large amounts and cool fluids empty from the stomach and are absorbed more quickly by the intestines than warm fluids. Fluids with high solutes empty slowly. After exercise, large amounts of cool, dilute fluids are best to replace what has been lost.

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