The Human Digestive System

The digestive system is the series of organs that break down food into small pieces our body cells can absorb and use. Food enters at the mouth and leaves as waste at the anus, and the whole journey takes about 24 to 72 hours — depending on the food and the person.

The seven main parts of the digestive system, in order:

1. Mouth — where digestion begins. Teeth chew the food (mechanical digestion). Saliva from the salivary glands moistens the food and contains the enzyme amylase, which starts breaking down starch into simpler sugars.
2. Oesophagus (food pipe) — a muscular tube about 25 cm long that pushes food from the throat to the stomach using wave-like contractions called peristalsis.
3. Stomach — a J-shaped muscular bag. It mixes food with gastric juice (containing hydrochloric acid and the enzyme pepsin) to break down proteins. The acid also kills most germs in the food.
4. Small intestine — about 6 metres long. This is where MOST digestion and absorption happens. Food is mixed with bile from the liver (which breaks fat into tiny droplets) and pancreatic juice (containing enzymes that complete the digestion of starch, protein and fat). Tiny finger-like projections called villi line the walls and absorb the digested nutrients into the bloodstream.
5. Large intestine (colon) — about 1.5 metres long. It absorbs water from the leftover material and forms solid waste (faeces).
6. Rectum — stores the waste before it leaves.
7. Anus — the opening through which faeces leave the body.

Helper organs that don't carry food but produce digestive juices:

• Liver — produces bile, which breaks fat into smaller droplets. Bile is stored in the gall bladder until needed.
• Pancreas — produces pancreatic juice containing enzymes that digest starch, protein and fat.
• Salivary glands — produce saliva in the mouth.

Two types of digestion:

• Mechanical digestion — the physical breaking of food into smaller pieces by chewing in the mouth and churning in the stomach.
• Chemical digestion — the breaking of food at the molecular level by enzymes in saliva, gastric juice, bile and pancreatic juice.

Where each nutrient is digested:

• Starch (carbohydrates) — digestion begins in the mouth (saliva), completes in the small intestine.
• Proteins — digestion begins in the stomach (pepsin in acidic conditions), completes in the small intestine.
• Fats — digestion happens almost entirely in the small intestine, helped by bile from the liver.

Common digestive problems Kenyan students learn about:

• Constipation — caused by not drinking enough water or eating enough fibre (vegetables, whole grains). Faeces become hard and difficult to pass.
• Diarrhoea — caused by germs in unclean water or food. Loose, watery faeces; if untreated it can dehydrate and kill a person, especially a young child. Treat with Oral Rehydration Salts (ORS).
• Tooth decay (cavities) — caused by sugar feeding bacteria on teeth. Prevent by brushing twice a day and limiting sweet drinks.
• Stomach ulcers — sores in the stomach lining; can be caused by bacteria (H. pylori) or excessive stress. Treated with antibiotics.

Common student mistakes to avoid:

• Thinking food enters the lungs. Food goes down the oesophagus; air goes down the trachea (windpipe). The epiglottis covers the trachea when you swallow.
• Confusing the small intestine (where digestion + absorption happens) with the large intestine (where mainly water is absorbed).
• Forgetting that the liver doesn't digest food directly — it produces bile, which then helps in the small intestine.
• Saying digestion starts in the stomach. It actually starts in the mouth (saliva on starch).

CBC Grade 4 introduces healthy eating and the basic parts of digestion; Grade 5–6 covers the full digestive system, enzymes and absorption; Grade 7–9 Integrated Science extends to nutrient deficiency, balanced diets and digestive disorders — material that appears in KPSEA and KJSEA.