Last reviewed: 25 October 2026
The digestive system turns the food we eat into nutrients the body can use. It appears in upper primary science and returns in more detail in junior school Integrated Science. Questions often ask learners to label a diagram, trace the path of food and explain what happens at each stage. This guide covers all of that. Nutrition in animals, which covers digestion, is a sub-strand in KICD’s Grade 9 Integrated Science curriculum design.
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What Digestion Is
Digestion is the breakdown of large, insoluble food molecules into small, soluble ones that can pass into the blood. It happens in two ways: mechanical digestion, the physical breaking of food by teeth and churning by the stomach, and chemical digestion, the breakdown by enzymes.
The Mouth
Digestion starts in the mouth. Teeth cut, tear and grind food into smaller pieces. Saliva from the salivary glands moistens food and contains an enzyme (salivary amylase) that begins breaking starch down into sugar. The tongue rolls food into a ball, called a bolus, for swallowing.
Types of Teeth
- Incisors: chisel-shaped front teeth for cutting.
- Canines: pointed teeth for tearing.
- Premolars and molars: broad teeth at the back for grinding and chewing.
Adults usually have 32 permanent teeth, while children have 20 milk teeth. Brushing twice a day, limiting sugary snacks and visiting a dentist help prevent tooth decay.
The Oesophagus
The oesophagus (gullet) is a muscular tube that carries food from the mouth to the stomach. Waves of muscle contraction, called peristalsis, push food along, so you can swallow even when lying down.
The Stomach
The stomach is a muscular bag that churns food and mixes it with gastric juice. This juice contains hydrochloric acid, which kills many germs and provides the right conditions for enzymes, and an enzyme that starts digesting proteins. Food may stay in the stomach for a few hours and leaves as a thick liquid.
The Small Intestine
Most digestion and absorption happen in the small intestine, a long coiled tube. Two important organs help here:
- The liver makes bile, which is stored in the gall bladder. Bile breaks fats into tiny droplets so enzymes can work on them faster.
- The pancreas produces pancreatic juice containing enzymes that digest starch, proteins and fats.
The inner wall of the small intestine has millions of finger-like projections called villi. They give a huge surface area, and their thin walls and rich blood supply let digested food pass quickly into the blood. This is absorption.
The Large Intestine
Undigested material passes into the large intestine, where much of the remaining water is absorbed back into the body. The waste that remains, faeces, is stored in the rectum and removed through the anus. This removal is called egestion.
The Path of Food in Order
Mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus. A quick way to revise is to say the path aloud with one job for each organ.
Keeping the Digestive System Healthy
- Eat a balanced diet with fibre from vegetables, fruits and whole grains such as millet, sorghum or whole-grain maize flour, which helps prevent constipation.
- Drink plenty of clean water.
- Wash hands and food to avoid infections that cause diarrhoea and typhoid.
- Chew food well and eat at regular times.
Revision Tips
Draw and label the digestive system from memory, then check. Make a table of each organ and its function. Our cells, tissues and organs guide explains how organs form systems, and the Grade 7 Integrated Science papers give exam practice. Use the exam countdown and What to Revise report to track gaps. Full papers with answers are in the Monthly Pass (KSh 500 by M-Pesa).