Unit 3: The digestive and respiratory systems                             1   2   3 

2.2. The stomach: the gastric digestion

The stomach is a J-shaped organ, located in the abdominal cavity beneath the diaphragm. The stomach is about 30 cm long and 15 cm wide at its widest point. It is very elastic and it can be up to 2 litres of capacity in an adult.

The stomach has two openings. Both are sphincters. A sphincter is a circular muscle that acts as a valve, controlling the pass of substances from one place to another.

        - Cardia connects with the oesophagus and prevents that food goes back.

        - Pylorus separates the stomach from the duodenum and prevents

          that non-digested food passes into the small intestine.

The structure of the stomach wall is similar to the rest of the digestive tract, but it has three muscular layers (circular, longitudinal and an additional oblique muscle layer).

Gastric digestion includes mechanical and chemical processes:

a) Chemical processes

They are made by gastric juice. This juice is produced by gastric glands that are located on the stomach mucosa (its innermost layer). The gastric juices contains mainly:

             - Hydrochloric acid (HCl) 

- It helps to disaggregate the food. 

- It activates pepsinogen because reduce the pH (about 2),

   making the chemical environment very acid.

   It is transformed in pepsin (active enzyme)

- It destroys any bacteria or germs that could have entered with food.

              - Pepsinogen (inactive enzyme or precursor)

              - Mucin (protects the stomach from acid)

              The main function of gastric digestion is beginning the digestion of proteins.

              Pepsine breaks down long chains of amino acids (proteins) into short chains

              (peptides)

b) Mechanical processes

Stomach wall has peristaltic movements that mix food with gastric juice and grind and move it. The contraction waves close the pylorus when arrive to it. Only when food is very liquid and the gastric chemical digestion has finished, a very small portion of the stomach's content passes into the duodenum in each train of waves.

Chyme is the result of gastric digestion. It is an acid semi-liquid substance, which comes from the mixture of food bolus with saliva and gastric juice, and it is formed by partly digest glucids and proteins.

2.8. Why is pepsine secreted as an inactive enzyme (pepsinogen)?

2.9. What food will be more time in stomach gherkins in vinegar or natural

        gherkins? Why?

2.10. Listen and find the six mistakes in the text: