Unit 3: The digestive and respiratory systems                             1   2   3 

2.3. Intestinal digestion

Intestinal digestion takes place into small intestine thanks to several digestive juices: intestinal juice (produced by intestinal wall glands), pancreatic juice (produced by the pancreas) and bile (produced by the liver)

a) Small intestine

    The small intestine is a long tube which is located within the abdominal cavity.

    It is about 7 m long and 2.5 cm in diameter.

    It is differentiated in three portions:

        - Duodenum. It is the initial portion, about 25 cm long.

         It connects with the stomach.

         The pancreas and the liver pour their secretion in it.

        - Jejunum. It is the middle portion, about 3 m long.

        - Ileum. It is the final portion, around 3 m long.

         It connects with the large intestine.

    Its structure is similar to the rest of digestive tract but its mucosa has:

      - Intestinal glands that secrete intestinal juice.

       - Villi which are finger-like projections which increase the absorption surface area.

     a) Chemical processes

         Intestinal juice contains among other substances, enzymes to digest

         each type of nutrient (glucids, lipids and proteins)

     b) Mechanical processes

The small intestine has also peristalsis to mix chyme with digestive juices

and make advance food along it.

b) Pancreas

   It is an elongated, pinkish and spongy-shaped organ which is located underneath

   and behind the stomach.

   It is a gland, which produces two types of secretion:

      - Hormones which control the sugar levels in blood (insulin and glucagon).

      - Pancreatic juice which has digestive function.

   Pancreas pours pancreatic juice into the duodenum. This juice contains water and:

            - Enzymes to digest every type of nutrient.

            - Sodium bicarbonate (neutralizes chyme acid and makes pH very alkaline,

              about 9, providing an optimal environment to enzymes work)

c) Liver

   It is the largest gland in the body. The liver is a brown-red organ divided

   into several lobes which is located in the right side of stomach within the

   abdominal cavity.

   Liver has several important functions:

- Storage glycogen (polysaccharide formed by many molecules of glucose)

- Recycling red blood cells which non-useful parts are excreted (bile pigments)

- Detoxify blood.

- Produce bile (digestive juice)

   It is connected with the gall bladder which stores bile. Bile is continuously

   produced in the liver but it is only poured into duodenum when chyme is in it.

   Bile contains mainly:

         - Bile pigments (waste substances which are responsible for feces colour)

         - Bile acids (they emulsify lipids, break down large fat drops into small ones

           which are easier to attack by enzymes)

         - Cholesterol

Chyle is the result of intestinal digestion. It is a very liquid substance, which comes from the mixture of chyme with bile, pancreatic juice and intestinal juice, in which all nutrients are completely digested.

2.12. Give a reason to explain why:

a. meals rich in fats usually provoke a long and heavy digestion.

b. humans cannot feed only of vegetables and fruits.

c. Intestinal ulcers are produced in duodenum but neither in jejunum nor ileum.

2.13. Listen and complete the text: