A colostomy is an opening made in the large bowel or colon. Most commonly, colostomies are made in the sigmoid colon or the descending colon, but can also be made from the transverse colon. The resulting opening is called a stoma from the Greek word meaning "mouth". After surgery, the stoma tends to be quite swollen due to the handling of the bowel. The size may decrease, however, this will occur over a period of time often taking up to six weeks.

The bowel is not unlike the inside of our mouths. The stoma is red, soft, and moist. It is not as critical that a colostomy stoma protrude like an ileostomy stoma because of stool consistency and most colostomy stomas tend to be quite flush. The stoma can be either a loop or an end portion of the colon. Through the opening the bowel is turned back on itself almost like a cuff and stitched to the skin. The loop colosotmy will have two openings - the top opening called the proximal opening where stool is passed and the distal opening or bottom opening that is connected to the resting portion of bowel. A colostomy may be temporary or it may be permanent. Colostomies are usually located in the left lower quadrant of the abdomen.

The stool from a colostomy can vary slightly in consistency, depending upon how much of the colon has been removed or bypassed. For most colostomies (in the sigmoid or descending colon), the stool will be formed because the water, salt and potassium has been absorbed by the time the stool enters into the sigmoid portion of the and used by the body. How quickly it returns to a formed bowel movement is dependent upon how quickly you return to a regular diet. Initially you might find that the stoma will function for small amounts of stool, either pasty or formed, several times a day. However, once you return to your regular habits of diet and exercise, you may find that stool function becomes somewhat more regulated, less often (once or twice a day) and perhaps even predictable.