Structure
The rectum is a hollow tube that is continuous with the sigmoid colon proximally and the anal canal distally. The junction with the sigmoid colon is about 15 cm from the external anal margin and is characterised by circumferential muscle instead of the three taeniae coli. The rectum has three prominent lateral curves (right/left/right from superior to inferior). The internal mucosa is pink to tan, and characterised by three folds which assist in anal continence.
The peritoneum is an important landmark. The upper third of the rectum is covered by peritoneum on its anterior and lateral aspects. The middle third is related to the peritoneum on its anterior aspect only. The lower third is completely retroperitoneal.
The mesorectum is an important surgical and radiological structure. It contains the superior rectal artery and vein, and numerous lymphatics as well as rectal branches of the inferior hypogastric plexus. It is bounded by the mesorectal fascia, a distinct layer which separates the rectal fat from other pelvic fat.
Relations
The prostate and seminal glands lie anteriorly, separated by the prostatic fascia. Above the peritoneal reflection (the rectovesical pouch), small bowel may intervene between the rectum and the prostate.
The rectum is related laterally with the levator ani muscles and the ischoanal fossa.
Posteriorly, the rectum is in close proximity to the sacrum and the coccyx, the piriformis muscle and the nerves of the lumbosacral plexus.
The anal canal and anococcygeal ligament are inferior.
The sigmoid colon lies anterior and superior, the proximal continuation of the rectum.
Microscopic Structure
The rectum is lined with columnar epithelium, with deep crypts that produce mucous. The muscular coat is arranged in three layers. The rectum rests in the loose connective tissue of the mesorectum.
Neurovascular Supply
Arterial Supply
The arterial supply of the rectum is complex. The superior part receives the majority of blood from the superior rectal artery, a terminal branch of the inferior mesenteric artery which also supplies the sigmoid and descending colon. The middle part receives supply from the middle rectal artery, a branch of the internal iliac artery. The inferior part receives some supply from the inferior rectal artery which is a branch of the internal pudendal artery, itself a terminal branch of the internal iliac artery[1].
Venous Drainage
Venous drainage of the rectum is likewise complex! Blood collects into an internal (between epithelium and muscularis propria) and external venous plexus (outside muscularis propria). The internal plexus mostly drains to the inferior rectal vein and thereby to the portal vein. The external plexus drains to the internal pudendal vein or internal iliac vein and thereby to the inferior vena cava.
Lymphatics
Lymphatics from the rectum drain to mesorectal nodes. From here, they may pass superiorly to nodes along the inferior mesenteric veins or to the internal iliac or presacral nodes.
Nerves
The rectum receives autonomic input from sympathetic (arising from lumbar plexus) and parasympathetic nerves (arising from the S2-S4).
