rectum
Meaning of rectum: Storage rather than digestion is its job, and because it lies within reach it is one of the few bowel segments examined directly.
Definition of rectum
The rectum is the final segment of the large intestine, running from the end of the sigmoid colon to the anal canal. It works as a holding chamber: its walls stretch as stool arrives, and that stretch is what produces the urge to have a bowel movement. Despite the name, the human rectum is curved rather than straight.
What does the rectum actually do?
Digestion and absorption are effectively finished by the time material arrives here, so the rectum stores rather than processes. Its walls are elastic and accommodate a rising volume without a matching rise in pressure, which is why the urge to go builds, then eases if it is not acted on, then returns more strongly. Stretch receptors in the wall report how full it is, and that signal travels to the spinal cord and brain. Two rings of muscle guard the exit: an inner one that relaxes automatically as the rectum fills and an outer one under voluntary control. Continence depends on that outer ring, on the angle at which the rectum meets the anal canal, and on the ability to sense the difference between gas, liquid and solid.
Is it really straight?
No, and the name misleads. Rectum comes from the Latin for straight, a description inherited from the dissection of animals in which the segment does run straight. In humans it follows the forward curve of the sacrum and takes two or three side-to-side bends as well. Those curves have practical consequences: an instrument passed from below has to be steered rather than pushed, and positions along the segment are described by distance from the anal opening rather than by any straight line.
How is it examined, and what arises there?
| Method | How far it reaches |
|---|---|
| Digital rectal examination | The lower rectum within a finger's reach, plus the prostate in men |
| Anoscopy or proctoscopy | The anal canal and lower rectum under direct view |
| Flexible sigmoidoscopy | The rectum and the sigmoid colon above it |
| Colonoscopy | The rectum and the entire length of the colon beyond it |
Because the segment lies within reach, conditions arising in it are often identified earlier than those higher in the bowel. Hemorrhoids are enlarged vascular cushions at the junction with the anal canal. Proctitis is inflammation of the rectal lining, whether from infection, from radiation or as part of inflammatory bowel disease. Tumors also arise here, and a persistent change in the pattern of a bowel movement or blood passed with stool are among the findings that lead to examination of the segment.
Used in a sentence
A digital rectal exam was performed to evaluate for abnormalities within the rectum.
Good to know
Despite a name meaning straight, the human rectum follows the curve of the sacrum, and it is reachable during a digital rectal examination because it sits immediately above the anal canal rather than because it runs a straight course.
Word origin
Where “rectum” comes from
From Latin rectum intestinum, meaning straight intestine, a description taken from animals in which the segment does run straight.