Basically, the measurement of the internal diameter of the rectum is almost the same as the sigmoid colon; however, its distal end is more dilated, forming the rectal ampulla. The rectal ampulla is the actual storage site for the stool or feces until they are excreted through the anal canal going out to the anal orifice in a process called defecation.

As an ample amount of stool accumulates in the rectal ampulla, the walls of the rectum expands. This stimulates the stretch receptors, which causes the urge to defecate. However, sufficient intrarectal pressure is needed to push the feces towards the rectal canal.

For this to happen, the rectal ampulla must be filled with feces to increase the amount of intrarectal pressure exerted. And if there’s enough pressure in the rectum, the walls of the anal canal will surely pull apart. This allows the stool to pass through and out from the body through the opening of the anus.