ScienceThe human digestive system
The journey of a roti from your mouth to your blood, organ by organ.
The food we eat cannot be used by the body as it is. A piece of roti is far too big to enter the blood. It must be broken down into very small, simple substances. This breaking down of food is called digestion, and the organs that do it form the digestive system. Together they make a long tube, called the food pipe or alimentary canal, running from the mouth to the anus.
Mouth
Digestion begins in the mouth. The teeth cut and grind the food into small pieces. The tongue mixes it with saliva, a watery juice that makes the food soft and begins to break down starch. Chew a piece of roti for a long time and it starts to taste sweet: saliva has started changing starch into sugar. This is why we should chew food well.
Food pipe
When we swallow, the food passes into the food pipe, or oesophagus. Its walls squeeze the food downwards, a little like toothpaste being pushed along a tube. It takes only a few seconds to reach the stomach.
Stomach
The stomach is a bag of muscle. It churns the food and mixes it with digestive juices. These juices contain a mild acid that kills many germs and substances that start digesting proteins. After a few hours the food becomes a thick paste.
Small intestine
The paste moves into the small intestine, a narrow coiled tube that is several metres long. Most digestion is completed here. Juices from the liver (bile) and the pancreas pour in and help break down fats, proteins and carbohydrates. The digested food is now simple enough to pass through the thin wall of the intestine into the blood. This is called absorption. The blood carries the food to every part of the body, where it gives energy and helps us grow.
Large intestine
What is left, mostly fibre and water, moves into the large intestine. Here most of the water is absorbed back into the body. The remaining solid waste is stored for a while and then passed out through the anus.
Keeping the system healthy
- Eat at fixed times and do not overeat.
- Chew slowly; do not swallow in a hurry.
- Eat fruits, vegetables and whole grains for fibre, and drink enough water.
- Wash your hands before eating, and eat clean, fresh food.
- Do not run or play hard right after a meal.
Key terms
- Digestion
- breaking food into simple substances the body can use
- Saliva
- the watery juice in the mouth
- Oesophagus
- the food pipe from the mouth to the stomach
- Absorption
- digested food passing into the blood
- Bile
- the digestive juice made by the liver
Try it yourself
Chew a small piece of plain roti or boiled rice slowly, thirty times, without swallowing. Notice how the taste changes and becomes slightly sweet. Saliva has begun to turn starch into sugar.
Quick questions
Answer in your head or on paper first, then tap to check.
Where does digestion begin?
In the mouth.
What is the work of saliva?
It softens food and begins the digestion of starch.
In which organ is digested food absorbed into the blood?
The small intestine.
What happens in the large intestine?
Water is absorbed and the solid waste is stored before leaving the body.
This is a simple explanation to help you understand. For examinations, read the same chapter in your prescribed textbook. Last checked: 11 November 2026.