Section 3: Action of the Digestive System

The intake of food into the digestive system is called ingestion.

As food passes through the digestive system large molecules are broken down into smaller molecules. This is called digestion. Digestion is brought about by special chemicals called enzymes.

The small molecules (nutrients) can then pass through tiny holes in the wall of the intestine into the bloodstream. This is called absorption. The blood carries these small molecules to all parts of the body where the cells use them as raw materials for growth and respiration. The use of these nutrient molecules is called assimilation.

Food that the body cannot digest is eventually expelled through the anus. This is called egestion.

Mouth

The teeth cut up and chew food into pieces small enough to be swallowed. Chewing also mixes food with saliva, a fluid released from salivary glands into the mouth.

Saliva contains an enzyme called amylase that starts to break down (digest) large starch molecules in the food into smaller sugar molecules.

Saliva also lubricates the food to make it easier to swallow.

Swallowing

Muscles in the wall of the gullet contract (squeeze) to push food along. This is called peristalsis. Food then enters the stomach.

The action of peristalsis in the gullet