Fisting & Anatomy: the Colon...

The Sigmoid Colon…
Lying above the rectum is the colon. The two are separated by a curving junction called the Sigmoid. The sigmoid colon (pelvic colon) is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about 40 cm in length, and normally lies within the pelvis, but on account of its freedom of movement it is liable to be displaced into the abdominal cavity. When feces exit the colon into the rectum, the pressure triggers the rectal reflex, at this point, the internal sphincter automatically relaxes. So also when we put something in, on this moment the inner part of the ass, the internal sphincter, the rectum and sigmoid colon relaxes, in a way.
Understanding the curve of this junction becomes important in more advanced forms of anal play such as fisting and penetration with large toys. When feces exit the colon into the rectum, the pressure triggers the rectal reflex, At this point, the internal sphincter automatically relaxes.

The large Intestine…
Essentially, the large intestine (or bowel, colon) is one continuous tube, approximately 1.80 meters to 2 meters long, comprising five sections ascending colon, transverse colon, descending colon, sigmoid and rectum: and all ending in the anus. Its purpose is to absorb vitamins, minerals and water, as well as to move digestive waste from the small intestine to the anus, where it’s expelled.  
The large intestine takes about 16 hours to finish the digestion of the food. It removes water and any remaining absorbable nutrients from the food before sending the indigestible matter to the rectum. The colon absorbs vitamins which are created by the colonic bacteria - such as vitamin K (especially important as the daily ingestion of vitamin K is not normally enough to maintain adequate blood coagulation), vitamin B12, thiamine and riboflavin. 
It’s characterized by folds and curves. The folds give the large intestine a great degree of expansion much like an accordion.

The large intestine is particularly efficient at liquid absorption, which turns the liquid waste of the small intestine into solid feces. (This absorptive quality, coupled with the delicacy of the anal tissue, is what makes the rectal region particularly susceptible to infections transmitted via body fluids, such as HIV and gonorrhea.) The smooth muscles of the intestinal walls combined with a lubricating lining of mucosal tissue, ensure the easy movement of bulky, solid waste trough the large intestine.