Day 18 of 30
Digestive system: overview
Lessons are AI-drafted and being reviewed and improved over time.
Learning Objective
Describe the four main steps of digestion and identify the organs food passes through and the organs that help from the side.
The food you eat is made of large molecules that your cells cannot use directly. The digestive system breaks food down into small nutrients that can enter the blood. It does this in four steps: ingestion (taking food in), digestion (breaking it down), absorption (moving nutrients into the blood), and elimination (removing what cannot be used).
Digestion happens in two ways. Mechanical digestion is physical breaking and mixing: your teeth chop and grind, and your stomach churns. Chemical digestion uses substances such as enzymes and acid to break food into its building blocks. You will study chemical digestion in detail tomorrow.
Food travels through one long, winding tube called the gastrointestinal (GI) tract, which is many meters long. It begins in the mouth, where teeth tear and grind food and saliva moistens it. When you swallow, the food passes through the pharynx and into the esophagus, a muscular tube that pushes it to the stomach. The stomach is a stretchy muscular bag. It churns food with strong acid and enzymes, and it is lined with mucus that protects the stomach wall from its own acid. After a few hours, food leaves the stomach as a thick liquid and enters the small intestine, where most chemical digestion is finished and most nutrients are absorbed. What is left moves into the large intestine, which absorbs water and forms solid waste. The waste is stored in the rectum and leaves through the anus. From mouth to anus, the journey usually takes one to three days.
Some organs help digestion without food passing through them. The salivary glands make saliva. The liver makes bile, which helps digest fats; the gallbladder stores the bile. The pancreas makes digestive enzymes and sends them into the small intestine, as you learned in the endocrine lessons.
Food does not move through the tube by gravity alone. Smooth muscle in the wall of the tube contracts in waves called peristalsis, pushing food forward. This is why you can swallow while upside down, and why digestion keeps working while you sleep.
Common mistakes: thinking that digestion happens only in the stomach, and thinking food passes through the liver or pancreas. Most digestion and absorption happens in the small intestine, and the liver, gallbladder, and pancreas are helpers beside the tube. Next lesson: chemical digestion.
Breaking food down into small nutrients the body can absorb.
The physical breaking and mixing of food, such as chewing and churning.
Breaking food into building blocks using enzymes and acid.
Waves of smooth muscle contraction that push food along the digestive tract.
The long tube from the mouth to the anus through which food passes.
A fluid made by the liver that helps digest fats.
Instructions
Materials Needed
- Long sock or stocking
- Tennis ball or small ball
Name the four main steps of food processing in the body.
What is the difference between mechanical and chemical digestion? Give an example of each.
Put these in order: stomach, esophagus, large intestine, mouth, small intestine.
Name two helper organs and say what each contributes to digestion.
How do the muscles in the tube move food forward?
Teaching Tip
Students often believe food passes through the liver. Use the helper-organ arrows in the activity to show that the liver, gallbladder, and pancreas send products to the tube. Link peristalsis back to smooth muscle from Day 6.
Go Deeper
Research how the stomach protects itself from its own acid, or what causes heartburn.
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