The colon is divided into five sections.

• Caecum: It is a reservoir for the content of the small bowel. Measures 6 cm in length and can have a maximum diameter of 9 cm before it is considered abnormally enlarged. The vermiform appendix typically arises from the posteromedial surface, 2 cm inferior to the ileocecal valve  

  The cecum is covered by peritoneum, except posteriorly where it has a layer of loose connective tissue and it has a variable mesentery.

• The ascending colon starts at the top of the caecum and moves upward along the right side of the body until reaching the lower surface of the liver, where it makes a sharp left turn at the hepatic flexure. 
• Transverse colon begins from hepatic flexure forming a loose arch from right to left. It ends at the spleen, making a sharp downward turn at the splenic flexure, which  is in relation with the lower end of the spleen and the tail of the pancreas; the flexure is so acute that the end of the transverse colon usually lies in contact with the front of the descending colon. It lies at a higher level than, and on a plane posterior to, the right colic flexure, and is attached to the diaphragm, opposite the tenth and eleventh ribs, by a peritoneal fold, named the phrenicocolic ligament. The max diameter is  no more than 6 cm
• Descending colon measures around 25cm, runs retroperitoneally, inferiorly down the posterior abdominal wall passing anterior to the left kidney. The small intestine lies medial to the descending colon, while the left lateral paracolic gutter sits laterally. Upon reaching the left lower pelvis, the descending colon bends to the right and becomes the sigmoid colon.
• Sigmoid colon. Is a 25-40cm S-shaped segment, fixed at both its upper and lower ends, 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. It is completely surrounded by peritoneum, which forms a mesentery (sigmoid mesocolon), which gives it a considerable range of movement in its central portion. 
• Rectum 12 cm long; upper third covered by peritoneum; no peritoneum on lower third which is also called the rectal ampulla. About 8-10 cm of the rectum lies below the lower edge of the peritoneum (below the peritoneal reflection), outside the peritoneal cavity- In men, the reflection of the peritoneum to the posterior bladder forms the rectovesical pouch. In women, the reflection is from the rectum to the posterior cervix forming the rectouterine pouch, also known as the pouch of Douglas. There are also three lateral flexures, made by submucosal folds in the lumen called valves of Houston, often two on the left and one on the right.  The final portion of the rectum is the ampulla, which is an expanded segment that can strecht to accommodate faeces. The rectum transitions to the anal canal at the level of the levator ani muscle.

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The ascending and descending colon are mostly retroperitoneal, meaning they are located behind the peritoneum, while the transverse colon is suspended by a short mesentery attached to the posterior abdominal wall.

Anal canal

Area from the dentate line to the anal verge, based on embryology and mucosal histology. However, the “surgical” anal canal, extends from the anorectal ring to the anal verge, emphasizing surrounding muscles crucial in surgeries like low anterior resections and fistulotomies. This area includes the internal and external anal sphincters, as well as the puborectalis muscle, and can be identified by physical examination, ultrasound, or MRI.

The anal canal begins where the rectum passes through the pelvic hiatus, forming a “tube within a tube” structure. The inner layer consists of smooth muscle controlled by the autonomic nervous system, while the outer layer, made of somatic muscles, provides conscious control over continence.

The anal canal is lined with different types of epithelium, transitioning from the rectal columnar epithelium to squamous epithelium at the anal transition zone (ATZ), where human papillomavirus-related dysplasia is most often found. The dentate line, marked by the columns of Morgagni and anal crypts, divides the endodermal and ectodermal regions and is where most anorectal abscesses and fistulas originate. Below this, the anoderm begins, extending for 1.5 cm to the anal verge.

The internal anal sphincter (IAS) is a smooth muscle continuation of the rectum, around 2 mm thick and 35 mm long, while the conjoined longitudinal muscle lies between the IAS and the external anal sphincter (EAS), contributing to normal anorectal function. The EAS, composed of striated muscle, surrounds the IAS, extending beyond it, forming the intersphincteric groove. While once thought to have three distinct layers, modern studies show it to be a continuous muscle, but still often referred to as deep, superficial, and subcutaneous sections.

Hemorrhoids, commonly thought of as pathological, are normal anatomical features aiding in continence, composed of blood-filled cushions located above and below the dentate line. They receive blood supply from branches of the superior hemorrhoidal artery and are anchored to the internal sphincter and conjoined longitudinal muscle.