1. What are the major functions of the colon?

• Absorption of water: the most important

• Absorption of minerals: predominantly sodium. There is, however, net secretion of potassium and bicarbonate

• Expulsion of faeces

• Indirect role: bacterial flora in the colon are able to synthesise vitamin K and some of the B vitamins. They also produce some important fatty acids

2. What types of contraction does the colon have in common with the small bowel?

• Segmentation: this mixes the contents of the colon, facilitating absorption

• Peristalsis: propelling the contents distally

3. What type of contraction is peculiar to the colon?

Mass action contraction. There is simultaneous contraction of the smooth muscle over a very long length. This moves material from one portion of the colon to another in one movement. It occurs between 1–3 times per day.

4. Identify one way in which the basic electric rhythm of the colon differs from that of the small bowel.

Unlike in the small bowel, the frequency of the wave of contraction increases along the colon. At the ileocaecal valve it is 2 per minute, and in the sigmoid colon, up to 6 per minute.

5. What is the gastro-colic reflex?

This occurs after a meal enters the stomach, leading to an increase in the motility of the proximal and distal colon, together with an increase in the frequency of mass movements.

6. Outline the events that occur during defecation.