Rectum

Function

The rectum acts as a temporary storage site for feces. The rectum receives fecal material from the descending colon, transmitted through regular muscle contractions called peristalsis. 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 pass feces, a process called defecation.

An internal and external anal sphincter, and resting contraction of the puborectalis, prevent leakage of feces (fecal incontinence). As the rectum becomes more distended, the sphincters relax and a reflex expulsion of the contents of the rectum occurs. Expulsion occurs through contractions of the muscles of the rectum.

The urge to voluntarily defecate occurs after the rectal pressure increases to beyond 18 mmHg; and reflex expulsion at 55 mmHg. In voluntary defecation, in addition to contraction of the rectal muscles and relaxation of the external anal sphincter, abdominal muscle contraction, and relaxation of the puborectalis muscle occurs. This acts to make the angle between the rectum and anus straighter, and facilitate defecation.

Clinical significance

Examination

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. Forms of medical imaging used to examine the rectum include CT scans and MRI scans. An ultrasound probe may be inserted into the rectum to view nearby structures such as the prostate.

Colonoscopy and sigmoidoscopy are forms of endoscopy that use a guided camera to directly view the rectum. The instruments may have the ability to take biopsies if needed, for diagnosis of diseases such as cancer. A proctoscope is another instrument that is used to visualise the rectum.

Body temperature can also be taken in the rectum. Rectal temperature can be taken by inserting a medical thermometer not more than 25 mm (0.98 in) 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 (97 to 100 °F) and is about 0.5 °C (0.9 °F) above oral (mouth) temperature and about 1 °C (1.8 °F) above axilla (armpit) temperature. Availability of less invasive temperature-taking methods including tympanic (ear) and forehead thermometers has facilitated reduced use of this method.

Route of administration

Some medications are also administered via the rectum (Latin: per rectum). By their definitions, suppositories are inserted, and enemas are injected into the rectum. Medications might be given via the rectum to relieve constipation, to treat conditions near the rectum, such as fissures or haemorrhoids, or to give medications that are systemically active when taking them by mouth is not possible. People do not tend to like medications administered by this route because of both cultural issues, discomfort, and issues that may affect the medication working, such as leakage.

Constipation

One cause of constipation is faecal impaction in the rectum, in which a dry, hard stool forms. Constipation is most commonly due to dietary and lifestyle factors such as inadequate hydration, immobility, and lack of dietary fibre, although there are many potential causes. Such causes may include obstruction because of narrowing, local disease (such as Crohn's disease, fissures or haemorrhoids), or diseases affecting the neurological control of the bowel, or slow bowel transit time, including spinal cord injury and multiple sclerosis; use of medications such as opioids, and conditions such as diabetes mellitus, as well as severe illness. High calcium levels and low thyroid activity may also cause constipation.

Testing may be carried out to investigate the cause. This may include blood tests such as biochemistry, calcium levels, thyroid function tests. A digital rectal examination may be performed to see if there is stool in the rectum, and whether there is an obstruction. When symptoms such as weight loss, bleeding through the rectum, or pain are present, additional investigations such as a CT scan may be ordered. If constipation persists despite simple treatments, testing may also include anal manometry to measure pressures in the anus and rectum, electrophysiological studies, and magnetic resonance proctography.

In general however, constipation is treated by improving factors such as hydration, exercise, and dietary fibre. Laxatives may be used. Constipation that persists may require enemas or suppositories. Sometimes, use of the fingers or hand (manual evacuation) is required. Although peristalsis in the colon delivers material to the rectum, laxatives such as bisacodyl or senna that induce peristalsis in the large bowel do not appear to initiate peristalsis in the rectum. They induce a sensation of rectal fullness and contraction that frequently leads to defecation, but without the distinct waves of activity characteristic of peristalsis.

Inflammation

• Proctitis is inflammation of the anus and the rectum.
• 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 2014, the cause is unknown.

Cancer

• Rectal cancer, a subgroup of colorectal cancer specific to the rectum.

Other diseases

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
• In the context of mesenteric ischemia, the upper rectum is sometimes referred to as Sudeck's point and is of clinical importance as 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. Sudeck's point is often referred to along with Griffith's point at the splenic flexure as a watershed region.

Society and culture

Sexual stimulation

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, the rectum is an erogenous zone and its stimulation or penetration can result in sexual arousal.

History

Etymology

English rectum is derived from the Latin intestinum rectum 'straight gut', a calque 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 is the Latinization of ἀπευθυσμένον and euthyenteron has a similar meaning (εὐθύς 'straight). Much of the knowledge of the anatomy of the rectum comes from detailed descriptions provided by Andreas Vesalius in 1543.