Learning Objectives

• Identify the vertebral level (S3) and flexures that define the rectum’s course.
• Distinguish the rectum from the colon by the absence of haustra, teniae coli, and omental appendices.
• Understand the peritoneal reflections and the formation of pelvic pouches (Douglas vs. Rectovesical).
• Master the triple arterial supply and its significance in portocaval anastomoses.
• Correlate rectal anatomy with the Digital Rectal Examination (DRE).

Anatomical Structure & Flexures

The rectum is the final 12–15 cm of the large intestine. It lacks the “sacculated” appearance of the colon, appearing as a smooth-walled tube.

• Sacral Flexure: Follows the hollow of the sacrum (anterior concavity).
• Anorectal Flexure: An 80° posterior angulation maintained by the puborectalis muscle. This is a critical mechanism for fecal continence.
• Ampulla: The dilated terminal part that stores feces until defecation.

Peritoneal Coverings & Pouches

The rectum’s relationship with the peritoneum changes in thirds (“Rule of 1/3s”):

• Upper 1/3: Covered anteriorly and laterally.
• Middle 1/3: Covered anteriorly only.
• Lower 1/3: Entirely extraperitoneal (subperitoneal).

Neurovascular Supply

The rectum is a high-yield transition zone between the Portal and Systemic venous systems.

• Arterial Supply:
  1. Superior Rectal A.: From the Inferior Mesenteric Artery (Hindgut supply).
  2. Middle Rectal A.: From the Internal Iliac Artery.
  3. Inferior Rectal A.: From the Internal Pudendal Artery.
• Venous Drainage: The Superior Rectal V. drains into the Portal system. The middle and inferior veins drain into the Systemic system. This creates a portocaval anastomosis.
• Innervation: Parasympathetic (S2-S4 Pelvic Splanchnic) and Sympathetic (Lumbar Splanchnic).

Clinical Relevance: Digital Rectal Exam (DRE)

The rectum’s anterior relations make it a “window” for clinicians to palpate adjacent organs:

• In Males: The Prostate Gland and Seminal Vesicles are anterior to the rectum.
• In Females: The Cervix and Vagina are anterior.
• In Both: The coccyx and sacrum are posterior; the ischial spines are lateral.

Activity