Rectum

The rectum is the terminal portion of the large intestine, extending from the sigmoid colon to the anal canal. Despite its name (from Latin "rectus" meaning straight), the rectum has a gentle curvature and plays a critical role in the temporary storage of feces before defecation. Structurally and functionally distinct from the rest of the colon, it is specially adapted for accommodation and controlled emptying of solid waste.

Structure

The rectum measures approximately 12–15 cm in length and is subdivided into three parts: upper, middle, and lower thirds. It begins at the level of the third sacral vertebra (S3) and ends at the anorectal junction.

The rectum has no haustra, teniae coli, or epiploic appendages, unlike the rest of the colon. Its wall includes the following layers:

• Mucosa: Contains simple columnar epithelium with numerous goblet cells. Longitudinal folds known as rectal columns (columns of Morgagni) are present near the anorectal junction.
• Submucosa: Rich in blood vessels and lymphatics.
• Muscularis externa: Has inner circular and outer longitudinal layers; thickens to form the internal anal sphincter at the lower end.
• Adventitia/Serosa: Serosa only covers the upper rectum; the rest is surrounded by adventitia anchoring it to pelvic structures.

Valves of Houston

The inner surface of the rectum contains 2–3 transverse mucosal folds known as rectal valves (valves of Houston), which help support the weight of feces and prevent premature emptying into the anal canal.

Location

The rectum is situated in the posterior part of the pelvic cavity and follows the curve of the sacrum and coccyx. It is located:

• Anterior to the sacrum and coccyx
• Posterior to the bladder (in males) or vagina and uterus (in females)
• Inferior to the sigmoid colon and superior to the anal canal

Its anatomical relationships vary by sex, as shown below:

Function

The rectum has several important functions in the lower gastrointestinal system:

• Fecal storage: Temporarily stores feces until an appropriate time for defecation.
• Absorption: Limited water and electrolyte absorption occurs here, although less than in the colon.
• Sensory role: Specialized receptors detect stretch and chemical composition, triggering defecation reflexes.
• Defecation control: Coordinates with the anal sphincters and pelvic floor muscles to allow voluntary or involuntary bowel movements.

Clinical Significance

The rectum is involved in several common and clinically important conditions:

• Hemorrhoids: Enlarged venous plexuses in the rectal mucosa may cause bleeding, discomfort, or prolapse. Internal hemorrhoids arise above the pectinate line and are usually painless; external ones are painful.
• Rectal prolapse: Involves descent of the rectal wall through the anal canal; often seen in elderly or chronically constipated patients.
• Rectal cancer: A common site of colorectal malignancy; surgical resection often requires total mesorectal excision due to lymphatic spread within the mesorectum.
• Digital rectal exam (DRE): A valuable clinical tool for examining rectal tone, detecting tumors, or assessing prostate size in males.
• Radiologic landmark: The rectum’s fixed position makes it a reliable reference point in pelvic imaging.

The rectum’s unique anatomical location and functional capacity require precise knowledge for both medical assessment and surgical planning, particularly in colorectal oncology, proctology, and pelvic floor dysfunction.