Large intestine

Mnemonic

There is a very simple way to remember these features specific to the large intestine. The mnemonic ' Eva Has Ten Socks' stands for:

• Epiploic appendages
• Haustra (Sacculation)
• Tenia coli
• Semilunar folds

The large intestine consists of eight parts; the cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. The middle four sections (ascending to sigmoid parts) form the colon.

An overview is provided in the following study unit:

Cecum

The cecum is the first part of the large intestine, lying in the right iliac fossa of the abdomen. The cecum is intraperitoneal with various folds and pockets (retrocecal peritoneal recesses) surrounding it.

The terminal ileum joins the cecum at the ileocolic junction. The ileocecal orifice is marked by the ileal papilla, which consists of two folds called ileocecal lips (superior, inferior). The folds fuse together around the orifice, forming the frenula of the ileal orifice, which prevents reflux of cecal contents into the ileum. An ileocecal valve regulates the passage of intestinal contents from the small into the large intestine. The functions of the cecum involve temporary storage of chyme, and fluid and electrolyte reabsorption.

Test your knowledge on the large intestine with this quiz.

Appendix

The vermiform appendix is a blind lymphoid pouch located in the right iliac fossa which arises from the cecum. These two parts of the large intestine are connected by the meso-appendix. The appendix has a role in the maintenance of gut flora and mucosal immunity. 

Are you curious to find out more about the cecum and appendix, and how they can be located inferior to the liver?  Details are provided below:

Ascending colon

The portion of the large intestine located between the cecum and rectum is termed the colon. It consists of four parts; ascending, transverse, descending, and sigmoid. The main functions of the colon include fluid and electrolyte reabsorption. In addition, the microflora generates energy through a process called fermentation.

The ascending colon travels through the right iliac fossa, right flank, and right hypochondriac region. It ends at the right colic (hepatic) flexure. The ascending colon is retroperitoneal and it is connected to the posterior abdominal wall by the Toldt’s fascia. A deep vertical groove or recess (right paracolic gutter) lies between the ascending colon and the lateral abdominal wall. The ascending colon is heavily involved in fluid and electrolyte reabsorption, gradually forming fecal matter.

Transverse colon

The transverse colon is the second major part of the colon. It extends between the right and left colic (splenic) flexures, spanning the right hypochondriac, epigastric and left hypochondriac regions of the abdomen. The greater curvature of the stomach and gastrocolic ligament are superior to the transverse colon, while the greater omentum hangs over and extends inferiorly to it. 

The transverse colon is intraperitoneal. A peritoneal mesentery (transverse mesocolon) attaches it to the posterior wall of the omental bursa. This forms two abdominal compartments called supracolic and infracolic compartments.

Descending colon

The descending colon extends between the left colic flexure and sigmoid colon. It travels through the left hypochondriac region, left flank and left iliac fossa. The left paracolic gutter is located between the descending colon and the lateral abdominal wall. 

This part of the colon is retroperitoneal. Toldt’s fascia fixes the descending colon to the posterior abdominal wall. 

Sigmoid colon

The S-shaped sigmoid colon travels from the left iliac fossa until the third sacral vertebra (rectosigmoid junction). This part of the colon is intraperitoneal. It is connected to the pelvic wall by the sigmoid mesocolon.

Rectum

The rectum stretches between the rectosigmoid junction and the anal canal. The typical characteristics of the large intestine (taenia coli, haustra, epiploic appendages) change or even terminate at the rectum. The roles of the rectum include temporary storage of fecal matter and defecation.

The rectum has a characteristic S-shape marked by several bends or turns; sacral, anorectal and lateral flexures. The latter correspond with three infoldings called transverse rectal folds. The rectum ends at a dilated ampulla.

The rectum is partially intraperitoneal since the inferior third is subperitoneal. The peritoneum reflects from the rectum towards the bladder in males (rectovesical pouch) and the vaginal fornix in females (recto-uterine pouch or pouch of Douglas). The spaces around the rectum are potential spaces for infections, abscess formation, and many other pathologies.

Therefore, you need to have a crystal clear understanding of their anatomy.  Use our additional resources:

Anal canal

The anal canal forms the terminal part of the gastrointestinal tract. It extends from the anorectal junction to the anus. The latter represents the external orifice of the entire digestive system.

The mucosa of the superior half of the anal canal contains ridges called anal columns. The inferior portions of these columns contain anal valves, which are surrounded by anal sinuses. The latter are responsible for secreting lubricating mucus during defection. The anal valves also form the pectinate line which makes the distinction between the superior and inferior parts of the anal canal. They differ in terms of neurovascular supply and lymphatic drainage.

The internal and external anal sphincters surround the anal canal. Respectively, they involuntarily and voluntarily control the release of stool. Both sphincters are tonically contracted to prevent the uncontrolled release of fecal matter or flatus.

Arteries

The large intestine receives arterial blood predominantly from the superior and inferior mesenteric arteries. The former supplies the midgut derivatives, such as the cecum, appendix, ascending colon and the proximal two-thirds of the transverse colon via three main branches: ileocolic, right colic, and middle colic arteries.

The inferior mesenteric artery supplies the hindgut derivates, namely the posterior third of the transverse colon, descending colon, sigmoid, colon, rectum and the upper part of the anal canal via three branches: left colic, sigmoid, and superior rectal arteries. These branches are super easy to remember if you just learn the mnemonic ' Lesley Sings Songs'. The middle and inferior rectal arteries, which stem from the internal iliac artery, also supply hindgut derivates.

The superior and inferior mesenteric branches supplying the colon anastomose to form the marginal artery of Drummond. This runs along the inner margin of the large intestine, within the mesentery, and gives off direct arterial branches to the large intestine.

Veins

Deoxygenated blood from the large intestine flows mostly into the superior and inferior mesenteric veins. The midgut derivates drain first into the colic veins, which in turn empty into the superior mesenteric vein. Hindgut derivates flow directly into the inferior mesenteric vein. 

The rectum has a special venous drainage. Middle rectal and inferior rectal veins drain this part of the gastrointestinal tract into the internal iliac and internal pudendal veins, respectively. 

For more detail about the blood vessels of the large intestine and their trajectories, take a look at the explanations provided by our renowned anatomy geeks.