Intestinal chyme enters the colon from the small intestine through the ileocecal valve as discussed under intestinal motility. Within the colon, it undergoes the final stages of absorption and then the remaining waste material will be passed out as feces when appropriate. The colon therefore has two main functions – absorption of water and electrolytes and storage of the fecal matter until defecation. The type of peristalsis within the colon will assist with these functions.
Fluid chyme enters the colon at the cecum, travels up the ascending colon where the absorption of water and electrolytes converts it into a semi-fluid mush by the time it enters the transverse colon. As it passes through the transverse colon, further absorption of water and electrolytes transforms it into a semi-mush, almost approaching a solid consistency. The final stages of solidifying this material occurs within the descending colon.
In order to achieve this absorption and delay the transit of the colonic contents to a sufficient degree to enable the colon to store the fecal matter, the movement through the colon is fairly sluggish. Mass movements, however, increase the speed just prior to and during defecation. Similar to the small intestine, colonic movements can be divided into mixing movements and propulsive movements.
Mixing Movements in the Colon
The circular muscle in the colonic wall contracts thereby creating a constriction. Simultaneously, the longitudinal muscle arranged into three strips (teniae colli) contracts causes adjacent bulges in the colon in between the constrictions. The pattern of constriction and subsequent bulging, known as a haustration, is similar to the segmentation contractions of the mixing movements within the small intestine.
By churning the colonic contents in this manner, the fluid to mush-like chyme and even the semi-solid feces is dug into and exposed to the colonic mucosa from where water and electrolytes are absorbed. Once a haustration has ended, and the area of constriction has relaxed, a new haustration adjacent to it will occur within a few minutes.
Apart from the mixing action of the haustrations, it also plays a significant role in moving the colonic contents towards the anus albeit quite slowly. Unlike the propulsive movements mentioned below, mixing movements occur throughout the day although it may temporarily stop when propulsive movements take over for short episodes in a day.
Propulsive Movements in the Colon
The major propulsive movements that occur in the colon are known as mass movements. These motions allow the colonic contents to move towards the rectum at a much faster rate than the mixing movements mentioned above. However, mass movements only occur in 1 to 3 episodes within day, usually lasting about 10 to 30 minutes at a time. These are the main movements that are triggered by the defecation reflex and responsible for moving fecal matter into the rectum in preparation for defecation. If there is irritation of the colon, the mass movements may occur in many more episodes during the day or possibly persist throughout the day.
In a mass movement, a constriction first occurs usually just before (proximal) to a distention in the colon. Then a long strip of the colon immediately after (distal) to the constriction contracts thereby forcing the colonic contents forward. As the constriction relaxes, another constriction arises a short distance further down from the original constriction. Mass movements tend to occur a short while after eating and are therefore controlled to a large extent by the gastrocolic and duodenocolic reflexes.