Unit 3: The digestive and respiratory systems                             1   2   3 

2.4. The large intestine

The large intestine is a thick tube which is located in the abdominal cavity,

surrounding the small intestine. It is about 1.5 m long and 6 cm in diameter.

It is differentiated in three portions:

       - Cecum. It is the initial portion, separated from ileum

         by a valve.  Jointed to it, there is the appendix,  

         a small blind prolongation.

       - Colon. It is the middle and largest portion. It is divided into:

         ascending, transverse, descending and sigmoid colon.

       - Rectum. It is de final portion which ends in the anus, a double sphincter

         that controls defecation.

The remains of the chyle which passes to cecum still contains a lot of water and mineral salts. These are absorbed meanwhile the remains goes through the colon forced by peristaltic movements of intestine wall.

As a result, it forms faeces by progressive compacting of chyle remains. This process is so efficient thanks to symbiotic bacteria which live in our colon (bacterial flora). They help in the absorption of water and mineral salts and produces vitamins K and B_12.

The wall of the large intestine is similar to other parts of the digestive tract but:

        - The mucosa has a large number of mucus secrete cells

           but does not have any villi.

        - The longitudinal muscle layer is limited to three bands (teniae coli)

           that create pouches (haustra) along the colon.

When faeces are formed, they must be expulsed (defecation).

The interior anal sphincter, under involuntary control, relaxes when faeces arrive to rectum (reflex act). By contrast, the external sphincter is under voluntary control.             

2.16. Why is it not recommendable to take antibiotics orally

          for a long time?

2.17. Listen and indicate if the following statements are true or false: