Unit 3: The digestive and respiratory systems                             1   2   3 

2.1. The mouth: the beginning of digestion

The food digestion begins in the mouth.

There, mechanical and chemical processes take place.

a) Mastication (Chewing)

It is the crushing of food by teeth, helped by the tongue which changes the food from place to place.

Teeth are hard structures compounded by calcium and fluorine, which are located within dental alveolus (tooth sockets) in the border of both mandibles. Their function is chewing food.

There are four types of teeth. Each one of them has a particular function and shape. Their characteristics are summarised in the chart below:

Humans have two dentitions along their lives. Children have a set of teeth that fall out and are replaced by adult teeth (beginning 6 years old). These are called deciduous, baby or milk teeth. These teeth are smaller than adult teeth and do not have roots. In addition, in children, molars are missing.

b) Insalivation

It is the mix of food with saliva.

Saliva is a digestive juice secretes by salivary glands. Humans have three pairs of them:

parotid, submandibular and sublingual glands.

Saliva is a liquid compounded by water, mineral salts, mucin (mucous substance) and enzymes (mainly salivary amylase or ptyalin, and maltase)

Functions of saliva are:

     - To start the digestion of glucids (thanks to its enzymes)

     - To make easier swallow the food (thanks to its water and mucin)

     - To be disinfectant (thanks to its antibacterial compounds)

Food bolus is each portion of food chewed, mixed with saliva and prepared to be swallowed.

c) Deglutition (Swallowing) 

It is the pass of food bolus from the mouth to the oesophagus and then, to the stomach.

We can distinguish three steps in this process:

1st) The tongue pushes food bolus into the pharynx (voluntary action)

2nd) When food bolus is in the pharynx, it activates a reflex (an involuntary action) that:

       - Closes the choanae (openings that connect the mouth and the nasal

          passages with the uvula.

       - Closes the glottis (opening that communicates the pharynx with the larynx)

          with the epiglottis.

3rd) Food bolus passes into the oesophagus. It progresses through it thanks to peristaltic movements, circular contraction waves that progress from top to botton of the oesophagus and force the food bolus into the stomach (reflex act).

2.5. How many teeth have a human adult? And a child?

2.6. What is the function of the tongue in chewing?

2.7. Listen and indicate what digestive process carried out in the mouth

        is described: