Describe the main parts of the digestive system and explain how food is broken down and absorbed

What this dot point is asking

This dot point wants you to describe the main parts of the digestive system and explain how the food we eat is broken down and absorbed into the body. The big idea is that the food we eat is too big and complex for the body to use straight away, so it must be broken down into tiny, soluble pieces. These small pieces can then pass into the blood and be carried to where they are needed. The digestive system is the set of organs that does this job.

The answer

Why we need to digest food

The food we eat is made of large pieces that the body cannot use directly. Digestion is the breaking down of food into very small, soluble pieces that are tiny enough to pass into the blood. Only then can the body use the food for energy, growth and repair.

The main parts of the digestive system

Food travels through a long tube called the gut, passing through several organs in order:

• The mouth: where food enters and digestion begins.
• The gullet (oesophagus): a tube that carries food from the mouth down to the stomach.
• The stomach: a muscular bag that mixes food with digestive juices.
• The small intestine: a long tube where digestion is completed and most food is absorbed.
• The large intestine: where water is taken back and undigested waste is formed.

How food is broken down

Digestion happens in two ways. First, food is broken down physically, mainly by the teeth in the mouth, which chew it into smaller pieces. Smaller pieces have a larger surface for the digestive juices to act on, which speeds up digestion.

Second, food is broken down chemically by digestive juices in the mouth, stomach and small intestine. These juices contain special chemicals that break the food into smaller and smaller pieces until it is small enough to be absorbed. The stomach also makes acid that helps break down food and kills many germs.

Absorption into the blood

By the time food reaches the small intestine, it has been broken down into tiny, soluble pieces. Here the food passes through the wall of the gut into the blood. This passing of digested food into the blood is called absorption.

The small intestine is well suited to this job because it is very long and its inner wall is covered in tiny finger-like folds (called villi). Together these give a very large surface area, so a lot of food can be absorbed quickly. The blood then carries the food around the body to where it is needed.

Getting rid of waste

Not all food can be digested. The parts the body cannot break down, such as fibre, pass into the large intestine. Here, water is absorbed back into the body, and the remaining solid waste (faeces) is stored and later removed from the body. This is why fibre in the diet helps keep the gut healthy and moving.

Examples in context

Example 1. Chewing your food well. People are told to chew food slowly and well before swallowing. This is good science: chewing breaks food into smaller pieces with a larger surface, so the digestive juices can act faster and digestion is easier on the stomach. Bolting food in big lumps makes the job harder.

Example 2. Fibre in fruit and vegetables. Fruit and vegetables contain fibre, which the body cannot digest. Although it is not absorbed, fibre is still important: it adds bulk in the large intestine and helps move waste through the gut, keeping digestion regular and the gut healthy. This is why a diet with enough fibre is recommended.

Try this

• Cue. Name, in order, the parts food passes through from the mouth to the large intestine. Mouth, then gullet, then stomach, then small intestine, then large intestine.

• Cue. Explain why food has to be broken down before it can be absorbed. It must be made into tiny, soluble pieces small enough to pass through the wall of the small intestine into the blood; large pieces cannot pass through.

• Cue. State one feature of the small intestine that helps it absorb food, and why it helps. It is very long with a large inner surface area (folds and villi), so a lot of digested food can pass into the blood quickly.