Next Lesson - Gastrointestinal Motility and the Enteric Nervous System
Core
The hindgut gives rise to the distal third of the transverse colon, the descending colon, the sigmoid (S-shaped) colon, the rectum and the upper anal canal. All three colon sections lie in the left half of the abdominal cavity.
The transverse colon is an intraperitoneal structure (it is wrapped in peritoneum, the thin lining of the abdominal cavity) and has its own mesentery (a fold of peritoneum that attaches it to the back wall of the abdomen), the transverse mesocolon.
The descending colon is usually a retroperitoneal structure (it lies behind the peritoneum). It lies anterior to the left kidney, passing over the kidney’s lateral (outer) border. Its arterial supply is from the left colic artery (colic means relating to the colon), which branches off the inferior mesenteric artery (IMA; mesenteric means relating to the mesentery). Its venous drainage is into the left colic vein then into the inferior mesenteric vein (IMV). Lymphatic drainage is into the inferior mesenteric lymph nodes.
The sigmoid colon has its own mesentery (the sigmoid mesocolon) and is an S-shaped section of the large intestine proximal to the rectum. Its arterial supply is from the sigmoid arteries, which branch off the IMA. Its venous drainage is into the sigmoid veins then into the IMV. Lymphatic drainage is into the inferior mesenteric lymph nodes.
The colon is much wider and shorter than the small intestine, and it has crypts (small pits in the lining) but no villi (finger-like projections that increase the surface area). The external longitudinal muscle of the colon is incomplete: it is made up of teniae coli (bands of muscle) which cause the formation of haustra (sacculations, or pouches) when they contract. Haustra can be told apart from the plicae circulares (permanent circular folds in the lining of the small intestine) because the plicae circulares traverse the whole diameter of the lumen (the space inside the gut), and the haustra do not.
The main functions of the colon are final water and electrolyte absorption, and the formation and temporary storage of faeces prior to defaecation (passing faeces). Sodium is absorbed from the lumen into the enterocytes (the absorptive cells lining the gut). In the distal colon this happens partly through epithelial sodium channels (ENaC). Sodium and chloride are also absorbed together by a different carrier route that is found throughout the colon.
Diagram - The anatomy of the large intestine
Public Domain Source by Mikael Häggström and B jonas, edited by Dr. Laura Hansell [Public domain]
The rectum has a curved shape and sits anteriorly to the sacrum in the pelvis. It is distinguishable from the sigmoid colon because it has a continuous band of longitudinal muscle instead of the separate bands (teniae coli) found in the colon.
Peritoneal covering varies with rectal level: the upper rectum is covered anteriorly and laterally, the middle rectum anteriorly, and the lower rectum has no peritoneal covering. This describes the peritoneal covering only. The levels do not mark out separate vascular territories (areas of blood supply).
The superior rectal artery, the terminal continuation of the IMA, is the principal supply to the upper rectum and communicates with other rectal arteries. The superior rectal vein drains through the IMV towards the portal system. Lymphatic drainage is into the pararectal lymph nodes (nodes next to the rectum) then into the inferior mesenteric nodes.
The middle rectal arteries arise from the internal iliac arterial system (the main blood supply of the pelvis), often through its branches, and make a variable contribution to the middle and lower rectum. The middle rectal veins drain into the internal iliac veins. Lymphatic drainage is a combination of the upper and lower rectal drainage: some lymph goes to the inferior mesenteric nodes and some goes to the internal iliac nodes.
The inferior rectal arteries, branches of the internal pudendal arteries (arteries that supply the perineum, the area around the anus and genitals), mainly supply the distal anorectal region and communicate with the other rectal arteries. The inferior rectal veins drain through the internal pudendal veins into the internal iliac veins. Lymphatic drainage of the lower rectum is into the internal iliac lymph nodes. Lymph from the anal canal below the pectinate line drains to the superficial inguinal nodes (in the groin) instead, as described in the section on the anal canal.
The rectal venous plexus connects the superior rectal veins of the portal system with the middle and inferior rectal veins of the systemic circulation. These communicating veins form a portosystemic anastomosis (a connection between the portal and systemic veins), rather than separate upper- and lower-third drainage compartments. In portal hypertension (raised pressure in the portal venous system), for example from cirrhosis, these collateral channels (alternative routes for blood) can enlarge. The enlarged veins are called anorectal varices (dilated veins) and may cause painless rectal bleeding.
The function of the rectum is the temporary storage of faeces prior to defaecation. Stretching of the rectum is what stimulates the urge to open the bowels.
The anal canal is the distal end of the gastrointestinal tract. It begins at the terminal rectum and ends at the anus.
The canal is divided into two sections by a line known as the pectinate line (pectinate means comb-like) or the dentate line (dentate means toothed). Above this line, the anal canal is developed from the hindgut. Below this line, the anal canal is developed from the proctodeum, an embryological structure formed by the folding of the embryo and derived from ectoderm (the outer layer of the embryo, which also forms the skin). For more information, please see our article Development of the Gastrointestinal Tract.
Above the pectinate line, the anal canal’s arterial supply is the superior rectal artery, which branches off the IMA. Its venous drainage is into the superior rectal vein then into the IMV. This means that the superior portion of the anal canal drains into the portal system. Lymphatic drainage is into the internal iliac lymph nodes. The epithelium of the anal canal above the pectinate line is columnar (glandular, made of tall column-shaped cells) epithelium. It has a visceral nerve supply (the type of nerve supply that serves the internal organs; S2-S4) that allows for a sense of stretch.
Below the pectinate line, the anal canal’s arterial supply is the inferior rectal artery, which branches off the internal pudendal artery. Its venous drainage is into the inferior rectal vein then into the internal pudendal vein. This means that the inferior portion of the anal canal drains into the systemic circulation. Lymphatic drainage is into the superficial inguinal lymph nodes. The epithelium of the anal canal below the pectinate line is stratified squamous epithelium (several layers of flat cells). It has a somatic nerve supply (S2-S4) that allows for a sense of pressure, pain, and temperature.
The functions of the anal canal are faecal continence (holding faeces in until it is time to pass them) and defaecation. Both are controlled by sphincters: the internal anal sphincter is made of smooth muscle (involuntary), and the external anal sphincter is voluntary muscle.
Diagram - The anatomy of the anal canal
Public Domain Source by U.S. Government National Institutes of Health [Public domain]
Connect this region to the integrated physiology: use Gastrointestinal Motility and the Enteric Nervous System for colonic mixing, mass movement (strong waves of contraction that push faeces along the colon) and defaecation control. Use Gastrointestinal Secretion, Digestion and Absorption for water and electrolyte handling across the gut. Use Liver, Biliary and Exocrine Pancreatic Physiology for the bile-acid cycle, which sits upstream of the colon: most bile acids are reabsorbed in the ileum (the last part of the small intestine) and return to the liver in the portal blood.
Edited by: Dr. Maddie Swannack
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
- 2001
