Investigations One through Four introduced students to the nervous system, skeletal and muscular systems, respiratory system, and cardiovascular system. Students conducted experiments and used models to explore memory, reaction time, respiration, muscle-bone interactions, and heart function. In Investigation Five, students will explore the digestive system.

The digestive system is responsible for supplying nutrients to the rest of the body’s systems and tissues. The digestive system is a tube through the center of the body. Its role is to obtain and process food to supply the necessary nutrients to the blood for transport to the rest of the body. Food is digested through a series of mechanical and chemical processes.

Digestion begins in the mouth, where food is broken down into smaller particles by the teeth as the food is chewed. During chewing, the tongue turns the food and mixes it with saliva, as well as detects flavors which it transmits to the brain as nerve impulses. Tasting is the responsibility of the taste buds, which are located on the surface of the tongue. In the picture below, you can see the many taste buds on an outstretched tongue. The saliva moistens the food and supplies an enzyme that begins the process of chemical digestion.

Food is swallowed and moves across the pharynx into the esophagus. The esophagus is a muscular tube connecting the pharynx to the stomach. A series of contractions force the food into the stomach, where it is mixed with hydrochloric acid and other enzymes. Hydrochloric acid acts as a digestive aid as well as an important line of defense. Many bacteria and other disease agents are destroyed by hydrochloric acid. The stomach contracts to mix the food with the acid and enzymes, as well as to move food into the small intestine.

The small intestine is a very long tube and is the primary site of digestion and absorption. The inner surface of the small intestine is made even larger by the presence of finger-like projections called villi. The villi are lined with even smaller finger-like projections called microvilli. The villi and microvilli greatly increase the amount of surface area available for absorbing nutrients. Here many of the proteins, lipids, and simple carbohydrates in food are digested and absorbed. This is also the site of fat-soluble vitamin absorption. Many minerals are also absorbed from the small intestine. Food moves from the small intestine into the much shorter large intestine, where water is absorbed along with water-soluble vitamins and any minerals whose movements are associated with the movement of water. Any undigested food is then eliminated from the body as waste.