Unit 3: The digestive and respiratory systems                             1   2   3 

2. The digestive system

a) Parts of the digestive system:

The digestive system is formed by:

- Digestive tract

It is a long tube differentiated in several parts: mouth, pharynx, oesophagus, stomach, small intestine, large intestine and anus.

It is formed by three layers of tissue:

- Mucosa (the internal layer, very folded and lined with glands)

- Muscularis (the middle layer formed by two layers of smooth muscle)

- Serosa (the connective tissue layer that joins the digestive tract to other

   organs)

- Digestive glands

They secrete digestive juices into the digestive tract.

They are salivary glands, gastric glands, intestinal glands, the liver and the pancreas.

b) Digestive functions:

    - Ingestion. It is the entry of food into the digestive tract through the mouth.

    - Digestion. It is the transformation of food into simple nutrients

     which can be absorbed and used by cells. We can distinguish two

     types of processes:

           - Mechanical digestion.

           It includes physical transformations (cutting, crushing, mixing and kneading)

           to reduce the size and make easier the chemical digestion.

           - Chemical digestion

           It is the transformation of complex nutrients that are contained in food

           into simple nutrients through a chemical reaction. These transformations

           are carried out by enzymes, which are contained in digestive juices.

           Enzymes act as chemical scissors to cut the large molecules into

           small molecules that can be absorbed and used by cells.

     - Absorption. It is the pass of nutrients from the digestive tube to the blood

      or to the lymph.

     - Defecation. It is the expulsion of non-digested or non-usable parts of food

      (faeces).

2.1. Where do different digestive glands secrete their juices?

2.2. Why do you think it is recommendable chew the food well

        before swallowing it?

2.3. What are the differences between chemical and mechanical

        digestion?

2.4. Listen and indicate what part of the digestive system is described: