The rectum is the concluding part of the large intestine that terminates in the anus.

The average length of the human rectum may range between 10 and 15 cm. Its diameter can be compared to that of the sigmoid colon (the part of the large intestine nearest the rectum) at its onset. However, it becomes larger near the anus, where it forms the rectal ampulla.

The key role of the rectal ampulla is to act as a temporary storehouse for feces. The expansion of the rectal walls causes the stretch receptors within the walls to stimulate the urge to defecate. If the defecation process is delayed, it may result in constipation. When the storage site becomes full, the intrarectal pressure causes the anal canal walls to dilate and expand. This results in the feces entering the canal.

A rectal exam may be conducted to diagnose certain diseases. Certain types of cancers may be diagnosed by performing an endoscopy in the rectum. An endoscopy is a procedure where a doctor uses an endoscope — a small, flexible tube with a camera and light — to examine areas inside the body. Body temperature may also be obtained from the rectal area. In case of infants and babies, this is generally the most accurate method for determining actual body temperature.