3.2 Digestive system
Summary: You may sometimes notice your stomach making noises. This is a sign that you need to eat. You know that food contains nutrients. But how does your body get nutrients out of food? Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe the process of digestion
- describe the function of the main structures in the digestive system.
Think back
- What is the function of the digestive system?
- Name some organs in the digestive system.
- Name the part of a balanced diet that adds bulk to food.
Key idea The digestive system is a group of organs in the body. The function of these organs is to break down food into small nutrient Molecules, which can then be absorbed.
Key words digestive system, digestion, gullet, stomach, small intestine, rectum, anus, large
You may sometimes notice your stomach making noises. This is a sign that you need to eat. You know that food contains nutrients. But how does your body get nutrients out of food?
What is the digestive system?
The digestive system is a group of organs that work together to break down food. The nutrients in most of the food you eat are large molecules, like lipids and proteins. During digestion, these large molecules are broken down into small nutrient molecules. These nutrients can then pass into the blood where they are used by the body. Figure 1 shows this process.
Figure 1The small nutrient molecules produced in digestion pass into the blood.
A Compare the size of food molecules before and after digestion.
Structures in the digestive system
Your digestive system digests (breaks down) the food that you eat. Figure 2 shows the digestive system and its different parts.
Figure 2Structures in the digestive system. Your digestive system is also called your gut. Full text alternative to the content
B List the organs in the order that food passes through them in the digestive system.
Moving through the digestive system
Fibre in your food is not digested. However, fibre adds bulk (roughage) to the food. Muscles push against this, forcing food along the gut. Figure 3 shows this process. Eating lots of foods containing fibre, such as vegetables and wholemeal bread, helps prevent constipation.
Figure 3Muscles in the wall of the gut push food along – a bit like pressing down on a tube of toothpaste.
C Describe how food moves along the gut.
Passing into the blood
The small nutrient molecules produced during digestion pass into the blood through the wall of the small intestine. They then move around the body.
The small intestine needs to absorb (take in) the nutrients quickly, before the undigested food passes out of the body. The small intestine is adapted to (designed for) this function. The wall of the small intestine is thin. It is also covered with tiny structures called villi (see Figure 4). Villi stick out of the wall and give it a big surface area. They also contain blood capillaries to carry away the absorbed food molecules.
Figure 4Villi in the small intestine increase the surface area. This allows more nutrients to be absorbed.
Literacy skills Make a wordbank by listing all the scientific terms on digestion. You can refer to your wordbank as you progress through this topic.
Stretch zone
- Match the organs to their role in digestion. stomach food is chewed and mixed with saliva small intestine water is absorbed back into the body large intestine food is mixed with acid and digestive juices rectum faeces are stored here until they pass out of the body mouth small molecules of nutrients are absorbed into the blood
- Describe the process of digestion using a named nutrient.
Stretch zone
- Explain the adaptations of the small intestine.