The large intestine is the terminal portion of the gastrointestinal tract and is derived from the midgut, hindgut, and cloaca. Derivatives of the midgut include the cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon. Derivatives of the hindgut include the descending colon, sigmoid colon, rectum, distal third of the transverse colon, and the portion of the anal canal above the pectinate line. The portion of the anal canal distal to the pectinate line is derived from the cloaca (ectoderm). The superior mesenteric artery supplies the midgut and the inferior mesenteric artery supplies the hindgut. The veins of the midgut and hindgut derivatives drain into the portal vein. The distal anal canal is supplied by the internal pudendal artery and drains into the inferior vena cava. The mesenteric lymph nodes receive lymphatic drainage from the colon. The internal iliac and inguinal lymph nodes drain lymph from the rectum and anal canal. The large intestine is innervated by the autonomic nervous system (mesenteric plexus), except for the distal anal canal, which receives somatic innervation from the pudendal nerve. The mucosa of the large intestine is columnar epithelium, except for the distal anal canal, which is lined with stratified squamous epithelium. The main functions of the large intestine are the absorption of sodium and water, excretion of potassium and bicarbonate, synthesis of vitamin K and group B vitamins (by the colonic bacteria), and the storage and expulsion of feces.
The large intestine is the terminal portion of the gastrointestinal tract and is approx. 1.5 m (5 ft) long. The large intestine is divided into the cecum and appendix, the ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.
Subdivisions of the large intestine
- U-shaped, sac-like structure located in the right iliac fossa
- The three teniae coli converge at the base of the cecum.
- Receives chyme through the ileocecal junction
Vermiform appendix 
- Blind tubular structure that arises from the base of the cecum
- Located in the right iliac fossa
The position of its free end is variable.
- The most common position is retrocecal
- Other positions include paracecal , preileal , postileal , and pelvic
- Connects the cecum proximally and the rectum distally
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
- Partially retroperitoneal; does not have a mesentery
- Extends from the S3 vertebra to the
- The peritoneum on the anterior rectal wall reflects anteriorly onto the bladder in males and the uterus in females, forming a blind pouch.
- Anorectal junction
- The terminal part of the gastrointestinal tract
- Approx. 4 cm long
- Extends from the anorectal junction at the pelvic diaphragm to the anal orifice
- Pectinate line (dentate line)
|Proximal to the pectinate line||Distal to the pectinate line|
|Embryologic origin|| |
|Venous drainage|| || |
- Internal anal sphincter
- External anal sphincter
- Anal glands lie in the intersphincteric groove between the internal and external anal sphincters.
Internal hemorrhoids occur above the pectinate line (superior rectal vein) and are not painful (visceral innervation). External hemorrhoids occur below the pectinate line and are painful because the pudendal nerve provides this area with somatic innervation.
Vasculature and innervation of the large intestine 
|Rectum and anal canal||Above pectinate line|
|Below pectinate line|| |
colon of the : 
- The splenic flexure (Griffiths point): junction between the SMA and IMA, located between the left colic artery and the marginal artery of Drummond
- Rectosigmoid junction (Sudeck point): junction between the IMA and internal iliac artery, located between the last sigmoid branch of the IMA and the superior rectal artery
The rectum is a site of portosystemic anastomosis. Rectal varices develop in portal hypertension due to the shunting of venous blood from the superior rectal vein (portal system) to the middle and inferior rectal veins (systemic or caval circulation).
- The large intestine is composed of the same four histological
- Teniae coli and haustra
|Cecum and colon|
|Rectum and anal canal||Above the pectinate line|| || |
|Below the pectinate line|| |
Ascending and transverse colon 
- Absorption of Na+ and Cl-
- Absorption of water along the osmotic gradient created by Na+ absorption
- Aldosterone increases the absorption of Na+ and water as well as the excretion of K+ and HCO3- from the colon.
- Synthesis of vitamin K and B vitamins by colonic bacteria
- Solidification of chyme into stool
- Lubrication for the passage of feces
Distal colon, rectum, and anal canal: storage, propulsion, and expulsion of feces
- Involuntary relaxation of the internal anal sphincter for anal sampling: prior to defecation, a small amount of rectal content passes into the anal canal to test consistency (gas, liquid, or solid)
- Voluntary relaxation of the external anal sphincter and puborectalis muscle and contraction of abdominal muscles or Valsalva maneuver expel stool from the anal canal
- Contraction of external anal sphincter to reestablish continence (closing reflex)
- Defecation process