The big idea
The food you eat is made of big molecules, like starch, proteins and fats. They are far too large to get into your blood. Digestion is the process of breaking big food molecules into small ones that can pass into your blood and travel to every cell. Your digestive system is a long tube, about 8–9 metres in an adult, with helper organs along the way.
Let's explore
Follow a cheese sandwich on its journey:
- Mouth – teeth cut and grind the food into smaller pieces. This is physical digestion. Saliva wets the food and contains an enzyme that starts breaking starch into sugar. An enzyme is a protein that speeds up a chemical reaction without being used up. This is chemical digestion.
- Oesophagus – a muscular tube (the gullet) that squeezes the food down to the stomach.
- Stomach – a muscular bag that churns food for a few hours. It adds hydrochloric acid, which kills many harmful microbes, and an enzyme that starts to break down protein.
- Small intestine – a narrow tube about 6 metres long. Juices from the pancreas add more enzymes, and bile from the liver breaks fat into tiny droplets. Here digestion is finished, and the small molecules are absorbed into the blood. The inner wall is covered in millions of tiny finger-like bumps called villi, which give it a huge surface area for absorption.
- Large intestine – wider but shorter. It absorbs water from the leftover material.
- Rectum and anus – the undigested waste (faeces) is stored, then pushed out.