11 de febrero de 2026

Anatomy of the Rectum and Anal Canal

Overview of the Rectum

• The rectum is approximately 15 centimeters long and terminates at the anal canal, which measures about 3 centimeters. This section will cover both anatomical and surgical aspects of the anal canal.

Structure and Function

• The rectal wall consists of common layers: an inner circular layer, an outer longitudinal layer, mucosa, and serosa. It features three important valves known as Houston's valves that help in fecal formation during defecation.

• Houston's valves are mucosal folds with muscular layers that increase surface area for stool formation, facilitating elongated shapes for easier passage during defecation. There are two on the left side and one on the right side.

Anatomical Landmarks

• The rectum ends at a structure called the dentate line (or pectinate line), formed by Morgagni columns (6 to 10 in number) that contain venous plexuses related to internal hemorrhoids. This marks a transition from cylindrical epithelium in the rectum to stratified epithelium in the anal canal.

• At this junction, there is a histological change where the epithelium transitions from cylindrical to stratified due to its proximity to skin-like tissue in the anal region.

Sphincter Mechanism

• The internal anal sphincter is a specialization of the circular muscle layer at the distal end of the rectum; it is smooth muscle and involuntary, while the external anal sphincter is striated muscle and voluntary. Understanding this distinction is crucial for recognizing issues like hypertonicity affecting control over bowel movements.

Blood Supply and Innervation

• The upper third of the rectum receives blood supply from superior hemorrhoidal arteries (branches of inferior mesenteric artery), while middle and lower thirds receive blood from middle hemorrhoidal arteries (from hypogastric artery) and inferior hemorrhoidal arteries (from internal pudendal artery). Drainage patterns differ significantly between these sections concerning liver relations through portal circulation versus systemic circulation via vena cava.

• Tumors located in different parts of the rectum have varying metastatic potentials; those in upper regions may metastasize via portal circulation to liver, whereas those in lower regions may spread systemically bypassing liver involvement entirely through caval routes leading potentially to lungs or bones instead.

Anatomy and Innervation of the Recto-Anal Region

Innervation Overview

• The recto-anal region receives both sympathetic and parasympathetic innervation. Sympathetic innervation is provided by lumbar sympathetic plexuses, while parasympathetic innervation comes from sacral roots S2, S3, and S4. This dual innervation supports peristalsis and fecal continence.

Anal Canal Structure

• The anal canal features two main venous plexuses: an internal plexus that can dilate to form hemorrhoids, and an external plexus. Internal hemorrhoids are more common and often associated with chronic constipation.

Dimensions of the Anal Canal

• The anal canal measures approximately 3 centimeters in length and is lined with stratified epithelium. It contains both internal and external anal sphincters which play a crucial role in maintaining continence. Blood supply to the anus is primarily through the internal pudendal artery, with venous drainage mirroring this route.

Cancer Risks in the Anal Region

• Cancers in the anal area may not be adenocarcinomas due to its cylindrical epithelium; instead, they are more likely to be squamous cell carcinomas (epidermoid). These types of cancers can metastasize extrahepatically outside of the liver due to their epithelial characteristics.

Clinical Relevance

• Understanding the anatomy, blood supply, and nerve innervations of the recto-anal region is essential for clinical practice as it informs treatment strategies for conditions like hemorrhoids and potential malignancies in this area. Emphasis on irrigation and venous drainage highlights significant clinical implications for patient care.