The rectum is divided into thirds beginning at the anorectal ring at 3.5 cm: lower rectum 3.5 cm to 7.5 cm; midrectum 7.5 cm to 12 cm; upper rectum 12 cm to 16 cm.

The most proximal rectum is intraperitoneal.

The middle and lower rectum is partially or wholly extraperitoneal.

Refers to final straight portion of the large intestine in humans.

In the adult it is about 12 centimetres (4.7 in) long.

The rectum begins at the rectosigmoid junction.

It begins at the end of the sigmoid colon, at the level of the third sacral vertebra or the sacral promontory.

The rectal caliber is similar to that of the sigmoid colon at its beginnings, but widens near its termination, forming the rectal ampulla.

The rectum terminates at the level of the anorectal ring or the dentate line.

The rectum is followed by the anal canal which is about 4 centimetres (1.6 in) long, before it and the gastrointestinal tract terminates at the anal verge.

It lies in front of the sacrum, and behind the bladder in males, and the vagina and uterus in females.

It connects to the anus, and follows the shape of the sacrum and ending in an expanded section called the rectal ampulla.

In the ampulla of the rectum feces is stored before their release via the anal canal.

The rectum does not have taeniae coli.

The rectum connects with the sigmoid colon at the level of S3.

The rectum connects with the anal canal as it passes through the pelvic floor muscles.

Supports of the rectum include:

Pelvic floor formed by levator ani muscles.

Waldeyer’s fascia.

Lateral ligaments of rectum which are formed by the condensation of pelvic fascia

Rectovesical fascia of Denonvillers, which extends from rectum behind to the seminal vesicles and prostate in front.

Pelvic peritoneum

Perineal body

Blood vessels of the rectum and anus

The inner diameter of the rectum near rectal/sigmoid junction measures on average 5.7 cm (range 4.5-7.5 cm).

It acts as a temporary storage site for feces.

As the rectal walls expand stretch receptors from the nervous system located in the rectal walls stimulate defecation.

If the urge to defecate is not acted upon, the stool in the rectum is often returned to the colon where more water is absorbed from the feces that may result in constipation and hardened feces.

Rectal fullness results in the increase in intrarectal pressure force and the walls of the anal canal part, allowing the fecal matter to enter the canal.

As material is forced into the anal canal, the rectum shortens, and peristaltic waves propel the feces out of the rectum.

The internal and external rectal sphincters allow the feces to be passed by muscles pulling the anus up over the exiting feces.

The rectal examination may help in the diagnosis of fecal impaction, prostate cancer and benign prostatic hypertrophy in men, stool incontinence, and internal hemorrhoids.

Colonoscopy or sigmoidoscopy have the ability to take biopsies if needed

Body temperature can be taken in the rectum.

Normal rectal temperature generally ranges from 36 to 38 °C (96.8 to 100.4 °F).

Rectal temperature is about 0.5 °C (1 °F) above oral temperature and about 1 °C (2 °F) above axillary temperature.

Suppositories are inserted into the rectum , and enemas are injected, via the rectum.

Both of these may be used for the delivery of drugs or to relieve constipation.

Enemas are also used for a variety of other purposes, medical and otherwise.

One cause of constipation is fecal impaction in the rectum, in which a dry, hard stool forms.

Manual evacuation from the rectum, and, after the application of stool softeners, is utilized in acute constipation.

Manual evacuation is also used in the long-term management of neurogenic bowel, in people with a spinal cord injury or multiple sclerosis.

Digital rectal stimulation may be used to induce peristalsis in patients whose own peristaltic reflex is inadequate to fully empty the rectum.

Diseases of the rectum include:

Proctitis is inflammation of the anus and the rectum

Rectal cancer,

Rectal prolapse.

Ulcerative colitis,

The upper rectum is a watershed region between the inferior mesenteric artery circulation and the internal iliac artery circulation via the middle rectal artery, and thus prone to ischemia.

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