Most of the gastrointestinal (G.I.) tract from the esophagus to the anus can be examined by endoscopy (endo, inside; scope, see; to see inside the body). The endoscope is a long and flexible tube and contains a light source, lens system for focusing and fiber optics to conduct light into the bowel. A picture of the bowel wall is sent back to a video camera and displayed on a monitor. The tube also contains a working channel through which small instruments can be passed for various uses. Colonoscopy enables the physician to examine the lining of the colon (large bowel), and is done by inserting the flexible endoscope (called a colonoscope) into the rectum and then into the entire colon.

Anatomy and Physiology

The large bowel absorbs 90% of the water content of the digested food it receives from the small intestine. It also propels the residue towards the rectum, where it is stored and expelled with a bowel movement. The large bowel is composed of:

• Colon. The colon averages 150 cms. (60 inches) in length. The colon is divided into four segments: the ascending colon, transverse colon, descending colon and sigmoid colon. There are two bends (flexures) in the colon. The hepatic flexure is where the ascending colon joins the transverse colon. The splenic flexure is where the transverse colon merges into the descending colon. (Figure 1)
  1. Cecum. This is the first portion of the large bowel and is joined to the small bowel at the ileocecal valve. (Figure 2) The appendix lies at the lowest portion of the cecum. (See Colectomy and Appendectomy)
  2. The ascending colon is about eight inches in length, extends upwards from the cecum to the hepatic flexure near the liver
  3. The transverse colon is usually over 18 inches in length and extends across the upper abdomen to the splenic flexure
  4. The descending colon, usually less than 12 inches long extends from the splenic flexure downwards to the start of the sigmoid colon
  5. The sigmoid colon is S-shaped and measures about 18 inches long. It extends from the descending colon to the rectum
• Rectum. The rectum is a curved pouch that lies in the hollow formed by the sacrum and connects with the anal canal at its lower end
• Normal colon wall is seen in Figure 3