Lower GI Tract Anatomy

The gastrointestinal (GI), or digestive, tract extends from mouth to anus (see the image below). The division of the GI tract into upper and lower is a matter of some confusion and debate. On embryologic grounds, the GI tract should be divided into upper (mouth to major papilla in the duodenum), middle (duodenal papilla to midtransverse colon), and lower (mid-transverse colon to anus) according to the derivation of these 3 areas from the foregut, midgut, and hindgut, respectively.

Nevertheless, the GI tract is conventionally divided into upper (mouth to ileum) and lower (cecum to anus). From the point of view of GI bleeding, however, the demarcation between the upper and lower GI tract is the duodenojejunal (DJ) junction (ligament of Treitz); bleeding above the DJ junction is called upper GI bleeding, and that below the DJ junction is called lower GI bleeding.

 

The cecum lies in the right lower quadrant and is the most proximal and widest part of the lower gastrointestinal (GI) tract (see the image below). It is the blind end (pouch) of the ascending (right) colon, which lies below the ileocecal junction (opening). The terminal ileum opens in the cecum on the medial wall and the opening is guarded by an ileocecal valve.

The appendix (an appendage of cecum) is a tubular structure with a blind end. It also opens in the cecum at an opening that is usually situated at its bottom, inferior to the ileocecal opening. The base of the appendix is easily identified by the point of convergence of the 3 taeniae coli in the cecum. The cecum leads to the ascending (right) colon, which lies vertically in the most lateral part of the abdominal cavity. The ascending colon takes a right-angle turn just below the liver (the right colic or hepatic flexure) and becomes the transverse colon, which takes a horizontal course from right to left.

The transverse colon takes a right-angle turn just below the spleen (the left colic or splenic flexure) and becomes the descending (left) colon, which lies vertically in the most lateral part of the abdominal cavity. ^{[1, 2, 3, 4, 5, 6]}

The descending colon leads to the inverted V-shaped sigmoid colon, which becomes the rectum at the S3 level. The rectum descends along the curve of the sacrum and becomes the anal canal at the dentate line. The anal canal opens at the anus in the perineum.

The transverse colon and sigmoid colon have a mesentery (mesocolon), but the ascending colon, descending colon, and rectum are retroperitoneal, whereas the cecum borrows the mesentery of the ileum. Appendix also has a small mesentery of its own (mesoappendix). Soft tissues around the rectum are often referred to as 'mesorectum' by surgeons. Three longitudinal taeniae coli are present along the entire length of the cecum and colon; they are not present in the rectum. In the ascending and descending colon, they are present anteriorly and on posterolateral and posteromedial aspects. Appendages of fat known as omental appendages (appendices epiploicae) are attached to the colon.

Blood supply

The lower GI tract is supplied by the superior mesenteric artery through its ileocolic, right colic, and middle colic branches; by the inferior mesenteric artery through its left colic, sigmoid, and superior rectal (hemorrhoidal) branches; and by the internal iliac artery through its middle rectal and inferior rectal (branch of internal pudendal) branches. A continuing series of anastomoses between the distal branch of the proximal artery and the proximal branch of the distal artery runs along the mesenteric (inner) border of colon and is called the marginal artery of Drummond. The terminal branches of this arcade entering the wall of the large intestine are called the vasa recta.

The superior mesenteric vein (SMV) accompanies the superior mesenteric artery (SMA), but the inferior mesenteric vein (IMV) does not accompany the inferior mesenteric artery (IMA). The inferior mesenteric vein runs vertically upward to the left of the duodenojejunal junction (flexure) and joins the splenic vein or its junction with the superior mesenteric vein (to form the portal vein).

The lower gastrointestinal (GI) tract has the same 4 layers that are present in most parts of the GI tract, as follows:

• Mucosa, which includes a columnar epithelium with numerous mucus-secreting goblet cells (villi, present in the small intestine, are absent from the lower GI tract), lamina propria, and muscularis mucosa

• Submucosa, which contains the blood vessels and Meissner’s nerve plexus

• Muscularis propria, which contains the inner circular and outer longitudinal muscles and myenteric (Auerbach’s) nerve plexus; taeniae coli are formed by the outer longitudinal muscles

• Serosa (visceral peritoneum)

Malrotation of the gut results in the small intestine lying in the right half of the abdomen and the large intestine lying in the left half; cecum then lies in the epigastrium

Colonic atresia may occur. This consists of narrowing or even complete obliteration of the intestinal lumen, resulting in neonatal intestinal obstruction.

Anorectal anomalies are very common.

For the purposes of endoscopy, the upper GI tract includes the esophagus, stomach and duodenum (esophagogastroduodenoscopy [EGD] or upper GI endoscopy [UGIE]), and the lower GI tract includes the anus, rectum, colon, and cecum (anoproctocolonoscopy or lower GI endoscopy). The small intestine (jejunum and ileum) is relatively inaccessible to endoscopy; proximal jejunum can be examined by push enteroscopy at upper GI endoscopy while disatl ileum can be examined by retrograde ileoscopy at colonoscopy. Capsule endoscopy can visualize the entire small intestine.

The lower gastrointestinal (GI) tract can be evaluated by means of lower GI endoscopy (anoproctosigmoido colonoscopy), and the lower GI series can be evaluated with radiologic contrast media (eg, barium, diatrizoate meglumine and diatrizoate sodium [Gastrografin], and fluoroscopy). Late films of a barium follow-through study may also show the lower GI tract. Reconstruction of CT scans (CT colonography) gives as good an inside view of the colon as does colonoscopy (virtual colonoscopy). Magnetic resonance imaging (MRI) is very good for evaluation of the anorectum. Ultrasonography is not useful for evaluation of the lower GI tract, but endoscopic ultrasonography (EUS) is useful for evaluation of the anorectum.