Drawing of colon seen from front
(rectum coloured red)
Anatomy of the anus and rectum
|Artery||Superior rectal artery (first two-thirds of rectum), middle rectal artery (last third of rectum)|
|Vein||Superior rectal veins, middle rectal veins|
|Nerve||Inferior anal nerves, inferior mesenteric ganglia|
|Lymph||Inferior mesenteric lymph nodes, pararectal lymph nodes, internal iliac lymph nodes, Deep inguinal lymph nodes|
The rectum (from the Latin rectum intestinum, meaning straight intestine) is the final straight portion of the large intestine in some mammals, and the gut in others. The human rectum is about 12 centimetres (4.7 in) long, and begins at the rectosigmoid junction (the end of the sigmoid colon), at the level of the third sacral vertebra or the sacral promontory depending upon what definition is used. Its caliber is similar to that of the sigmoid colon at its commencement, but it is dilated near its termination, forming the rectal ampulla. It terminates at the level of the anorectal ring (the level of the puborectalis sling) or the dentate line, again depending upon which definition is used. In humans, the rectum is followed by the anal canal, before the gastrointestinal tract terminates at the anal verge.
The rectum is a component of the lower gastrointestinal tract. The rectum is a continuation of the sigmoid colon, and connects to the anus. The rectum follows the shape of the sacrum, and ends in an expanded section called the rectal ampulla. Unlike other portions of the colon, the rectum does not have taeniae coli.  :397
Supports of the rectum include:
- Pelvic floor formed by levator ani muscles.
- Fascia of Waldeyer
- 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
The rectum intestitium acts as a temporary storage site for feces. As the rectal walls expand due to the materials filling it from within, stretch receptors from the nervous system located in the rectal walls stimulate the desire to defecate. If the urge is not acted upon, the material in the rectum is often returned to the colon where more water is absorbed from the feces. If defecation is delayed for a prolonged period, constipation and hardened feces results.
When the rectum becomes full, the increase in intrarectal pressure forces the walls of the anal canal apart, allowing the fecal matter to enter the canal. The rectum shortens as material is forced into the anal canal and peristaltic waves propel the feces out of the rectum. The internal and external sphincter allow the feces to be passed by muscles pulling the anus up over the exiting feces.
For the diagnosis of certain ailments, a rectal exam may be done. These include faecal impaction, prostatic cancer and benign prostatic hypertrophy in men, faecal incontinence, and internal haemorrhoids.  :179-180
A colonoscopy or sigmoidoscopy are forms of endoscopy that use a guided camera to view the rectum. These may have the ability to take biopsies if needed, and may be used to performed to diagnose diseases such as cancer.
Body temperature can also be taken in the rectum. Rectal temperature can be taken by inserting a medical thermometer not more than 25 mm (1 inch) into the rectum via the anus. A mercury thermometer should be inserted for 3 to 5 minutes; a digital thermometer should remain inserted until it beeps. Normal rectal temperature generally ranges from 36 to 38 °C (96.8 to 100.4 °F) and is about 0.5 °C (1 °F) above oral (mouth) temperature and about 1 °C (2 °F) above axilla (armpit) temperature.
- Rectal temperature is the closest to core body temperature and in young children, accuracy is critical.
- Younger children frequently do not cooperate when having their temperature taken by mouth (oral), which is recommended for children ages 6 and above as well as adults.
In recent years, the introduction of non-invasive temperature taking methods including tympanic (ear) and forehead thermometers, and changing attitudes on privacy and modesty have led some parents and doctors to discontinue taking rectal temperatures.
Route of administration
The rectum may also be used as a method to deliver drugs. This method is called a suppository.
One cause of constipation is faecal impaction in the rectum, in which a dry, hard stool fills the rectum. Manual evacuation is the use of a gloved finger to evacuate faeces from the rectum, and, after the application of stool softeners, is utilised in acute constipation.  :914 It is also in the long-term management of neurogenic bowel, seen most frequently in people with a spinal cord injury or multiple sclerosis. Digital rectal stimulation, the insertion of one finger into the rectum, may be used to induce peristalsis in patients whose own peristaltic reflex is inadequate to fully empty the rectum.
Other diseases of the rectum include:
- Rectal prolapse, referring to the prolapse of the rectum into the anus or external area. This is commonly caused by a weakened pelvic floor after childbirth.
- Ulcerative colitis, one form of inflammatory bowel disease that causes ulcers that affect the rectum. This may be episodic, over a person's lifetime. These may cause blood to be visible in the stool. As of 2013[update] the cause is unknown.
Society and culture
Due to the proximity of the anterior wall of the rectum to the vagina in females or to the prostate in males and the shared nerves thereof, rectal stimulation or penetration can result in sexual arousal.
'English' rectum is derived from the full Latin expression intestinum rectum. The English name straigh gut  truly expresses the literal meaning of this expression, as Latin rectum means straight, and intestinum means gut. This Latin expression is a translation of Ancient Greek ἀπευθυσμένον ἔντερον, derived from ἀπευθύνειν, to make straight, and ἔντερον, gut, attested in the writings of Greek physician Galen. During his anatomic investigations on animal corpses, Galen observed the rectum to be straight instead of curved as in humans. The expressions ἀπευθυσμένον ἔντερον and intestinum rectum are therefore not appropriate descriptions of the rectum in humans. Apeuthysmenon  can be considered as Latinization of ἀπευθυσμένον ἔντερον and euthyenteron has a similar meaning (εὐθύς = straight).
Median sagittal section of male pelvis, showing arrangement of fasciae
Organs of the female reproductive system
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- Blue Histology
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- Cross section at UV pelvis/pelvis-e12-15
- SUNY Anatomy Image 7808
- Anatomy photo:43:11-0101 at the SUNY Downstate Medical Center