The digestive system is the pathway your food takes through your body -- from the moment you put it in your mouth to the moment waste leaves your body. Its job is to break food down into nutrients small enough for your blood to absorb and deliver to cells throughout your body. The main organs form a long tube: mouth, esophagus, stomach, small intestine, and large intestine. Helper organs -- the liver, pancreas, and gallbladder -- produce chemicals that aid digestion without food passing directly through them. The entire journey from eating to elimination takes about 24-72 hours.
Use a physical model or large diagram showing the digestive tract as one continuous tube (this surprises many students who think of the stomach as the endpoint). Trace the path of a sandwich through each organ, describing what happens at each stop. Use analogies: the stomach is like a blender with acid, the small intestine is like a sponge that absorbs nutrients, the large intestine is like a dryer that removes water. Have students estimate the length of the small intestine (about 20 feet) by measuring that distance in the classroom.
Every bite of food you eat begins a journey through a tube that stretches about 30 feet from end to end -- your digestive system. Its job is simple to state but complex to perform: break food down into pieces small enough for your blood to absorb, then deliver those pieces (nutrients) to every cell in your body.
The journey starts in your mouth, where your teeth grind food into smaller pieces while saliva begins breaking down starches chemically. When you swallow, food enters the esophagus, a muscular tube about 10 inches long that pushes food downward into your stomach through wave-like contractions called peristalsis -- this is why you can swallow even while standing on your head.
Your stomach is like a muscular bag that churns food and mixes it with strong acid and digestive enzymes. This turns your meal into a thick paste called chyme. The stomach's acid is strong enough to dissolve metal, but a thick lining of mucus protects the stomach wall from digesting itself. Food stays in the stomach for 2-4 hours before being released in small amounts into the next organ.
The small intestine is where the real action happens. Despite its name, it's the longest part of the digestive tract -- about 20 feet, coiled and folded to fit inside your abdomen. Its inner surface is covered with millions of tiny finger-like projections called villi, which create an enormous surface area (roughly the size of a tennis court) for absorbing nutrients. As the chyme moves through, nutrients -- sugars, amino acids, fatty acids, vitamins, minerals -- pass through the intestinal wall into your bloodstream.
After the small intestine has extracted the nutrients, whatever remains -- mostly water and fiber -- moves into the large intestine (about 5 feet long, wider in diameter). The large intestine's main job is absorbing water from the remaining material, turning it from liquid to solid waste. Helpful bacteria living in your large intestine also produce some vitamins and break down fiber. Finally, solid waste is stored in the rectum until it's eliminated.
The liver, pancreas, and gallbladder assist digestion without food passing through them directly. The liver produces bile to break down fats. The gallbladder stores bile until it's needed. The pancreas produces enzymes that break down all three macronutrients. These helper organs make the small intestine's absorption job possible.