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The sigmoid colon (or pelvic colon) is the part of the large intestine that is closest to the rectum and
anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, ...
. It forms a loop that averages about in length. The loop is typically shaped like a Greek letter sigma (ς) or Latin letter S (thus '' sigma'' + '' -oid''). This part of the colon normally lies within the pelvis, but due to its freedom of movement it is liable to be displaced into the abdominal cavity.


Structure

The sigmoid colon begins at the superior aperture of the
lesser pelvis The pelvic cavity is a body cavity that is bounded by the bones of the pelvis. Its oblique roof is the pelvic inlet (the superior opening of the pelvis). Its lower boundary is the pelvic floor. The pelvic cavity primarily contains the reprodu ...
, where it is continuous with the iliac colon, and passes transversely across the front of the sacrum to the right side of the pelvis. It then curves on itself and turns toward the left to reach the middle line at the level of the third piece of the sacrum, where it bends downward and ends in the rectum. Its function is to expel solid and gaseous waste from the gastrointestinal tract. The curving path it takes toward the anus allows it to store gas in the superior arched portion, enabling the colon to expel gas without excreting faeces simultaneously.


Coverings

The sigmoid colon is completely surrounded by
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
(and thus is not retroperitoneal), which forms a
mesentery The mesentery is an organ that attaches the intestines to the posterior abdominal wall in humans and is formed by the double fold of peritoneum. It helps in storing fat and allowing blood vessels, lymphatics, and nerves to supply the intesti ...
( sigmoid mesocolon), which diminishes in length from the center toward the ends of the loop, where it disappears, so that the loop is fixed at its junctions with the iliac colon and rectum, but enjoys a considerable range of movement in its central portion.


Nerve supply

Pelvic splanchnic nerves Pelvic splanchnic nerves or nervi erigentes are splanchnic nerves that arise from sacral spinal nerves S2, S3, S4 to provide parasympathetic innervation to the organs of the pelvic cavity. Structure The pelvic splanchnic nerves arise from th ...
are the primary source for parasympathetic innervation. Lumbar splanchnic nerves provide sympathetic innervation via the inferior mesenteric ganglion.


Relations

Behind the sigmoid colon are the external iliac vessels, ovary, obturator nerve, the left Piriformis, and left sacral plexus of nerves. In front, it is separated from the
bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine en ...
in the
male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...
, and the
uterus The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
in the
female Female ( symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Fema ...
, by some coils of the small intestine.


Clinical significance

Diverticulosis Diverticulosis is the condition of having multiple pouches ( diverticula) in the colon that are not inflamed. These are outpockets of the colonic mucosa and submucosa through weaknesses of muscle layers in the colon wall. Diverticula do not ...
often occurs in the sigmoid colon in association with increased intraluminal pressure and focal weakness in the colonic wall. It is a common cause of hematochezia. Volvulus occurs when a portion of the bowel twists around its mesentery, which can lead to obstruction and
infarction Infarction is tissue death ( necrosis) due to inadequate blood supply to the affected area. It may be caused by artery blockages, rupture, mechanical compression, or vasoconstriction. The resulting lesion is referred to as an infarct (from th ...
. Volvulus in the elderly commonly occurs in the sigmoid colon, whereas in infants and children it is more likely to occur in the midgut. This may correct itself spontaneously or the rotation may continue until the blood supply of the gut is cut off completely.


Additional images

Image:Dickdarm-Schema.svg, 1: Ascending colon
2: Transverse colon
3: Descending colon
4: Sigmoid colon
5: Rectum Image:Gray1076.png, Iliac colon, sigmoid or pelvic colon, and rectum seen from the front, after removal of pubic bones and bladder Image:WeiblichesBeckenMedian.gif, Sagittal section of the lower part of a female trunk, right segment


References


External links

* - "The large intestine." * {{Authority control Digestive system Large intestine